Plant productivity responses

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

Perhaps the best-known consequence of the rise in atmospheric CO2 is the stimulation of plant productivity. This growth enhancement occurs because carbon dioxide is the primary raw material utilized by plants to produce the organic matter out of which they construct their tissues. Consequently, the more CO2 there is in the air, the better plants grow. Over the past decade, the Center for the Study of Carbon Dioxide and Global Change, has archived thousands of results from hundreds of peer-reviewed research studies conducted by hundreds of researchers demonstrating this fact. The archive is available free of charge at http://www.co2science.org/data/ plantgrowth/plantgrowth.php.

The Center’s Web site lists the photosynthetic and dry weight responses of plants growing in CO2-enriched air, arranged by scientific or common plant name. It also provides the full peer-reviewed journal references and experimental conditions in which each study was conducted for each record. We have summarized those results in two tables appearing in Appendix 2 and Appendix 3 to the current report. The first table, Table 7.1.1, indicates the mean biomass response of nearly 1,000 plants to a 300-ppm increase in atmospheric CO2 concentration. The second table, Table 7.1.2, indicates the photosynthetic response to the same CO2 enrichment for a largely similar list of plants.

In the rest of this section of Chapter 7, we provide a review of research on a representative sample of herbaceous and woody plants, chosen with an eye toward their importance to agriculture and the forestry and papermaking industries, followed by several acquatic plants.


Contents

Herbaceous Plants

A 300 ppm increase in the air’s CO2 content typically raises the productivity of most herbaceous plants by about one-third (Cure and Acock, 1986; Mortensen, 1987). This positive response occurs in plants that utilize all three of the major biochemical pathways (C3, C4, and crassulacean acid metabolism (CAM)) of photosynthesis (Poorter, 1993). Thus, with more CO2 in the air, the productivity of nearly all crops rises, as they produce more branches and tillers, more and thicker leaves, more extensive root systems, and more flowers and fruit (Idso, 1989).

On average, a 300 ppm increase in atmospheric CO2 enrichment leads to yield increases of 15 percent for CAM crops, 49 percent for C3 cereals, 20 percent for C4 cereals, 24 percent for fruits and melons, 44 percent for legumes, 48 percent for roots and tubers, and 37 percent for vegetables (Idso and Idso, 2000). It should come as no surprise, therefore, that the father of modern research in this area—Sylvan H. Wittwer—has said “it should be considered good fortune that we are living in a world of gradually increasing levels of atmospheric CO2,” and “the rising level of atmospheric CO2 is a universally free premium, gaining in magnitude with time, on which we can all reckon for the future.”

Chavas et al. (2009) examined potential climate change impacts on the productivity of canola, corn, potato, rice, and winter wheat in eastern China via full-domain simulations of the EPIC agro-ecosystem model for the baseline period AD 1961-1990 and the future period AD 2071-2100 under the IPCC's A2 scenario for projected air CO2 concentrations and accompanying climate change. Their analysis shows that "without the enhanced CO2-fertilization effect, potential productivity declines in all cases ranging from 2.5 to 12%.” However, they report that when the CO2-fertilization effect is included, "aggregate potential productivity increases 6.5% for rice, 8.3% for canola, 18.6% for corn, 22.9% for potato, and 24.9% for winter wheat." If the Chinese slightly adjust the areas where different crops are preferentially grown, the benefits can be expected to be even larger.

Additional information on this topic, including reviews of herbaceous plants not discussed here, can be found at http://www.co2science.org/subject/a/ subject_a.php under the heading Agriculture.


Alfalfa


Morgan et al. (2001) grew the C3 legume alfalfa (Medicago sativa L.) for 20 days post-defoliation in growth chambers maintained at atmospheric CO2 concentrations of 355 and 700 ppm and low or high levels of soil nitrogen to see how these factors affected plant regrowth. They determined that the plants in the elevated CO2 treatment attained total dry weights over the 20-day regrowth period that were 62 percent greater than those reached by the plants grown in ambient air, irrespective of soil nitrogen concentration.

De Luis et al. (1999) grew alfalfa plants in controlled environment chambers in air of 400 and 700 ppm CO2 for two weeks before imposing a two-week water treatment on them, wherein the soil in which half of the plants grew was maintained at a moisture content approaching field capacity while the soil in which the other half grew was maintained at a moisture content that was only 30 percent of field capacity. Under these conditions, the CO2-enriched water-stressed plants displayed an average water-use efficiency that was 2.6 and 4.1 times greater than that of the water-stressed and well-watered plants, respectively, growing in ambient 400-ppm-CO2 air. In addition, under ambient CO2 conditions, the water stress treatment increased the mean plant root:shoot ratio by 108 percent, while in the elevated CO2 treatment it increased it by 269 percent. As a result, the nodule biomass on the roots of the CO2-enriched water-stressed plants was 40 and 100 percent greater than the nodule biomass on the roots of the well-watered and water-stressed plants, respectively, growing in ambient air. Hence, the CO2-enriched water-stressed plants acquired 31 and 97 percent more total plant nitrogen than the well-watered and water-stressed plants, respectively, growing in ambient air. The CO2-enriched water-stressed plants attained 2.6 and 2.3 times more total biomass than the water-stressed and well-watered plants, respectively, grown at 400 ppm CO2.

Luscher et al. (2000) grew effectively and ineffectively nodulating (good nitrogen-fixing vs. poor nitrogen-fixing) alfalfa plants in large free-air CO2 enrichment (FACE) plots for multiple growing seasons at atmospheric CO2 concentrations of 350 and 600 ppm, while half of the plants in each treatment received a high supply of soil nitrogen and the other half received only minimal amounts of this essential nutrient. The extra CO2 increased the yield of effectively nodulating plants by about 50 percent, regardless of soil nitrogen supply; caused a 25 percent yield reduction in ineffectively nodulating plants subjected to low soil nitrogen; and produced an intermediate yield stimulation of 11 percent for the same plants under conditions of high soil nitrogen, which suggests that the ability to symbiotically fix nitrogen is an important factor in eliciting strong positive growth responses to elevated CO2 under conditions of low soil nitrogen supply.

Sgherri et al. (1998) grew alfalfa in open-top chambers at ambient (340 ppm) and enriched (600 ppm) CO2 concentrations for 25 five days, after which water was withheld for five additional days so they could investigate the interactive effects of elevated CO2 and water stress on plant water status, leaf soluble protein and carbohydrate content, and chloroplast thylakoid membrane composition. They found that the plants grown in elevated CO2 exhibited the best water status during the moisture deficit part of the study, as indicated by leaf water potentials that were approximately 30 percent higher (less negative) than those observed in plants grown in ambient CO2. This beneficial adjustment was achieved by partial closure of leaf stomata and by greater production of nonstructural carbohydrates (a CO2-induced enhancement of 50 percent was observed), both of which phenomena can lead to decreases in transpirational water loss, the former by guard cells physically regulating stomatal apertures to directly control the exodus of water from leaves, and the latter by nonstructural carbohydrates influencing the amount of water available for transpiration. This latter phenomenon occurs because many nonstructural carbohydrates are osmotically active solutes that chemically associate with water through the formation of hydrogen bonds, thereby effectively reducing the amount of unbound water available for bulk flow during transpiration. Under water-stressed conditions, however, the CO2-induced difference in total leaf nonstructural carbohydrates disappeared. This may have resulted from an increased mobilization of nonstructural carbohydrates to roots in the elevated CO2 treatment, which would decrease the osmotic potential in that part of the plant, thereby causing an increased influx of soil moisture into the roots. If this did indeed occur, it would also contribute to a better overall water status of CO2-enriched plants during drought conditions.

The plants grown at elevated CO2 also maintained greater leaf chlorophyll contents and lipid to protein ratios, especially under conditions of water stress. Leaf chlorophyll content, for example, decreased by a mere 6 percent at 600 ppm CO2, while it plummeted by approximately 30 percent at 340 ppm, when water was withheld. Moreover, leaf lipid contents in plants grown with atmospheric CO2 enrichment were about 22 and 83 percent higher than those measured in plants grown at ambient CO2 during periods of ample and insufficient soil moisture supply, respectively. Furthermore, at elevated CO2 the average amounts of unsaturation for two of the most important lipids involved in thylakoid membrane composition were approximately 20 and 37 percent greater than what was measured in plants grown at 340 ppm during times of adequate and inadequate soil moisture, respectively. The greater lipid contents observed at elevated CO2, and their increased amounts of unsaturation, may allow thylakoid membranes to maintain a more fluid and stable environment, which is critical during periods of water stress in enabling plants to continue photosynthetic carbon uptake.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturealfalfa.php.


Cotton


As the CO2 content of the air increases, cotton (Gossypium hirsutum L.) plants typically display enhanced rates of photosynthetic carbon uptake, as noted by Reddy et al. (1999), who reported that twice-ambient atmospheric CO2 concentrations boosted photosynthetic rates of cotton by 137 to 190 percent at growth temperatures ranging from 2°C below ambient to 7°C above ambient.

Elevated CO2 also enhances total plant biomass and harvestable yield. Reddy et al. (1998), for example, reported that plant biomass at 700 ppm CO2 was enhanced by 31 to 78 percent at growth temperatures ranging from 20 to 40°C, while boll production was increased by 40 percent. Similarly, Tischler et al. (2000) found that a doubling of the atmospheric CO2 concentration increased seedling biomass by at least 56 percent.

These results indicate that elevated CO2 concentrations tend to ameliorate the negative effects of heat stress on productivity and growth in cotton. In addition, Booker (2000) discovered that elevated CO2 reduced the deleterious effects of elevated ozone on leaf biomass and starch production.

Atmospheric CO2 enrichment also can induce changes in cotton leaf chemistry that tend to increase carbon sequestration in plant biolitter and soils. Booker et al. (2000), for example, observed that biolitter produced from cotton plants grown at 720 ppm CO2 decomposed at rates that were 10 to 14 percent slower than those displayed by ambiently grown plants; a after three years of exposure to air containing 550 ppm CO2, Leavitt et al. (1994) reported that 10 percent of the organic carbon present in soils beneath CO2-enriched FACE plots resulted from the extra CO2 supplied to them.

In summary, as the CO2 content of the air increases, cotton plants will display greater rates of photosynthesis and biomass production, which should lead to greater boll production in this important fiber crop, even under conditions of elevated air temperature and ozone concentration. In addition, carbon sequestration in fields planted to cotton should also increase with future increases in the air’s CO2 content.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturecotton.php.


Maize


Maroco et al. (1999) grew corn (Zea mays L.), or maize as it is often called, for 30 days in plexiglass chambers maintained at either ambient or triple-ambient concentrations of atmospheric CO2 to determine the effects of elevated CO2 on the growth of this important agricultural C4 species. This exercise revealed that elevated CO2 (1,100 ppm) increased maize photosynthetic rates by about 15 percent relative to those measured in plants grown at 350 ppm CO2, in spite of the fact that both rubisco and PEP-carboxylase were down-regulated. This increase in carbon fixation likely contributed to the 20 percent greater biomass accumulation observed in the CO2-enriched plants. In addition, leaves of CO2-enriched plants contained approximately 10 percent fewer stomates per unit leaf area than leaves of control plants, and atmospheric CO2 enrichment reduced stomatal conductance by as much as 71 percent in elevated-CO2-grown plants. As a result of these several different phenomena, the higher atmospheric CO2 concentration greatly increased the intrinsic water-use efficiency of the CO2-enriched plants.

In a study designed to examine the effects of elevated CO2 under real-world field conditions, Leakey et al. (2004) grew maize out-of-doors at the SoyFACE facility in the heart of the United States Corn Belt, while exposing different sections of the field to atmospheric CO2 concentrations of either 354 or 549 ppm. The crop was grown, in the words of the researchers, using cultural practices deemed “typical for this region of Illinois,” during a year that turned out to have experienced summer rainfall that was “very close to the 50-year average for this site, indicating that the year was not atypical or a drought year.” Then, on five different days during the growing season (11 and 22 July, 9 and 21 August, and 5 September), they measured diurnal patterns of photosynthesis, stomatal conductance, and microclimatic conditions.

Contrary to what many people had long assumed would be the case for a C4 crop such as corn growing under even the best of natural conditions, Leakey et al. found that “growth at elevated CO2 significantly increased leaf photosynthetic CO2 uptake rate by up to 41 percent.” The greatest whole-day increase was 21 percent (11 July) followed by 11 percent (22 July), during a period of low rainfall. Thereafter, however, during a period of greater rainfall, there were no significant differences between the photosynthetic rates of the plants in the two CO2 treatments, so that over the entire growing season, the CO2-induced increase in leaf photosynthetic rate averaged 10 percent.

Additionally, on all but the first day of measurements, stomatal conductance was significantly lower (-23 percent on average) under elevated CO2 compared to ambient CO2, which led to reduced transpiration rates in the CO2-enriched plants on those days as well. Since “low soil water availability and high evaporative demand can both generate water stress and inhibit leaf net CO2 assimilation in C4 plants,” they state that the lower stomatal conductance and transpiration rate they observed under elevated CO2 “may have counteracted the development of water stress under elevated CO2 and prevented the inhibition of leaf net CO2 assimilation observed under ambient CO2.”

The implication of their research, in the words of Leakey et al., was that “contrary to expectations, this US Corn Belt summer climate appeared to cause sufficient water stress under ambient CO2 to allow the ameliorating effects of elevated CO2 to significantly enhance leaf net CO2 assimilation.” They concluded that “this response of Z. mays to elevated CO2 indicates the potential for greater future crop biomass and harvestable yield across the US Corn Belt.”

Also germane to this subject and supportive of the above conclusion are the effects of elevated CO2 on weeds associated with corn. Conway and Toenniessen (2003), for example, speak of maize in Africa being attacked by the parasitic weed Striga hermonthica, which sucks vital nutrients from its roots, as well as from the roots of many other C4 crops of the semi-arid tropics, including sorghum, sugar cane, and millet, plus the C3 crop rice, particularly throughout much of Africa, where Striga is one of the region’s most economically important parasitic weeds. Research shows how atmospheric CO2 enrichment greatly reduces the damage done by this devastating weed (Watling and Press, 1997; Watling and Press, 2000).

Baczek-Kwinta and Koscielniak (2003) studied another phenomenon that is impacted by atmospheric CO2 enrichment and that can affect the productivity of maize. Noting the tropical origin of maize and that the crop “is extremely sensitive to chill (temperatures 0-15°C),” they report that it is nevertheless often grown in cooler temperate zones. In such circumstances, however, maize can experience a variety of maladies associated with exposure to periods of low air temperature. To see if elevated CO2 either exacerbates or ameliorates this problem, they grew two hybrid genotypes—KOC 9431 (chill-resistant) and K103xK85 (chill-sensitive)—from seed in air of either ambient (350 ppm) or elevated (700 ppm) CO2 concentration (AC or EC, respectively), after which they exposed the plants to air of 7°C for eleven days, whereupon they let them recover for one day in ambient air of 20°C, all the while measuring several physiological and biochemical parameters pertaining to the plants’ third fully expanded leaves.

The two researchers’ protocol revealed that “EC inhibited chill-induced depression of net photosynthetic rate (PN), especially in leaves of chill-resistant genotype KOC 9431,” which phenomenon “was distinct not only during chilling, but also during the recovery of plants at 20°C.” In fact, they found that “seedlings subjected to EC showed 4-fold higher PN when compared to AC plants.” They also determined that “EC diminished the rate of superoxide radical formation in leaves in comparison to the AC control.” In addition, they found that leaf membrane injury “was significantly lower in samples of plants subjected to EC than AC.” Last, they report that enrichment of the air with CO2 successfully inhibited the decrease in the maximal quantum efficiency of photosystem 2, both after chilling and during the one-day recovery period. And in light of all of these positive effects of elevated CO2, they concluded that “the increase in atmospheric CO2 concentration seems to be one of the protective factors for maize grown in cold temperate regions.”

But what about the effects of climate change, both past and possibly future, on corn production? For nine areas of contrasting environment within the Pampas region of Argentina, Magrin et al. (2005) evaluated changes in climate over the twentieth century along with changes in the yields of the region’s chief crops. Then, after determining upward low-frequency trends in yield due to technological improvements in crop genetics and management techniques, plus the aerial fertilization effect of the historical increase in the air’s CO2 concentration, annual yield anomalies and concomitant climatic anomalies were calculated and used to develop relations describing the effects of changes in precipitation, temperature and solar radiation on crop yields, so that the effects of long-term changes in these climatic parameters on Argentina agriculture could be determined.

Noting that “technological improvements account for most of the observed changes in crop yields during the second part of the twentieth century,” which totaled 110 percent for maize, Magrin et al. report that due to changes in climate between the periods 1950-70 and 1970-99, maize yields increased by 18 percent.

Much the same has been found to be true in Alberta, Canada, where Shen et al. (2005) derived and analyzed long-term (1901-2002) temporal trends in the agroclimate of the region. They report, for example, that “an earlier last spring frost, a later first fall frost, and a longer frost-free period are obvious all over the province.” They also found that May-August precipitation in Alberta increased 14 percent from 1901 to 2002, and that annual precipitation exhibited a similar increasing trend, with most of the increase coming in the form of low-intensity events. In addition, the researchers note that “the area with sufficient corn heat units for corn production, calculated according to the 1973-2002 normal, has extended to the north by about 200-300 km, when compared with the 1913-32 normal, and by about 50-100 km, when compared with the 1943-72 normal.”

In light of these findings, Shen et al. conclude that “the changes of the agroclimatic parameters imply that Alberta agriculture has benefited from the last century’s climate change,” emphasizing that “the potential exists to grow crops and raise livestock in more regions of Alberta than was possible in the past.” They also note that the increase in the length of the frost-free period “can greatly reduce the frost risks to crops and bring economic benefits to Alberta agricultural producers,” and that the northward extension of the corn heat unit boundary that is sufficient for corn production “implies that Alberta farmers now have a larger variety of crops to choose from than were available previously.” Hence, they say “there is no hesitation for us to conclude that the warming climate and increased precipitation benefit agriculture in Alberta.”

With respect to the future, Bootsma et al. (2005) derived relationships between agroclimatic indices and average yields of major grain crops, including corn, from field trials conducted in eastern Canada, after which they used them to estimate potential impacts of projected climate change scenarios on anticipated average yields for the period 2040 to 2069. Based on a range of available heat units projected by multiple General Circulation Model (GCM) experiments, they determined that average yields achievable in field trials could increase by 40 to 115 percent for corn, “not including the direct effect of increased atmospheric CO2 concentrations.” Adding expected CO2 increases to the mix, along with gains in yield anticipated to be achieved through breeding and improved technology, these numbers rose to 114 to 186 percent.

In light of their findings, Bootsma et al. predict there will be a “switch to high-energy and high-protein-content crops (corn and soybeans) that are better adapted to the warmer climate.”

Costa et al. (2009) estimated the impacts of increased temperature and atmospheric CO2 concentration on the yields of maize (Zea mays) and common beans (Phaseolus vulgaris) in Brazil, using the MadCM3 climate model projects specified in the A2 scenario of the IPCC's Special Report on Emissions Scenarios. The authors report that the warming conditions associated with increased greenhouse gases "lead to reductions in the potential productivity of maize and beans for the years 2050 and 2080 by up to 30%." However, they say that the CO2 fertilization effect is expected to overcome the negative response to warming and lead to a net increase in the productivity of common beans. In the case of maize, on the other hand, they find that "the CO2 fertilization feedback is much weaker and cannot cancel out the thermal effect." But when they factor in evolving technology, these effects may be nullified.

In summary, as the air’s CO2 content continues to rise, and even if the climate of the world changes in the ways suggested by GCM and IPCC calculations, maize plants will likely display greater rates of photosynthesis and biomass production, as well as reduced transpirational water losses and increased water-use efficiencies, and more areas of the world will likely become suitable for growing this important crop.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturemaize.php.


Peanut


Stanciel et al. (2000) grew peanuts (Arachis hypogaea L.) hydroponically for 110 days in controlled environment chambers maintained at atmospheric CO2 concentrations of 400, 800 and 1200 ppm, finding that the net photosynthetic rates of plants grown at 800 ppm CO2 were 29 percent greater than those of plants grown at 400 ppm CO2, but that plants grown at 1200 ppm CO2 displayed photosynthetic rates that were 24 percent lower than those exhibited by plants grown in 400-ppm CO2 air. Nevertheless, the number of pods, pod weight and seed dry weight per unit area were all greater at 1200 ppm than at 400 ppm CO2. Also, harvest index, which is the ratio of seed dry weight to pod dry weight, was 19 and 31 percent greater at 800 and 1200 ppm CO2, respectively, than it was at 400 ppm CO2. In addition, as the atmospheric CO2 concentration increased, stomatal conductance decreased, becoming 44 and 50 percent lower at 800 and 1200 ppm than it was at 400 ppm CO2. Thus, atmospheric CO2 enrichment also reduced transpirational water loss, leading to a significant increase in plant water use efficiency.

Prasad et al. (2003) grew Virginia Runner (Georgia Green) peanuts from seed to maturity in sunlit growth chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm and daytime-maximum/nighttime-minimum air temperatures of 32/22, 36/26, 40/30 and 44/34°C, while they assessed various aspects of vegetative and reproductive growth. In doing so, they found that leaf photosynthetic rates were unaffected by air temperature over the range investigated, but they rose by 27 percent in response to the experimental doubling of the air’s CO2 content. Vegetative biomass, on the other hand, increased by 51 percent and 54 percent in the ambient and CO2-enriched air, respectively, as air temperature rose from 32/22 to 40/30°C. A further air temperature increase to 44/34°C, however, caused moderate to slight decreases in vegetative biomass in both the ambient and CO2-enriched air, so that the final biomass increase over the entire temperature range investigated was 27 percent in ambient air and 53 percent in CO2-enriched air. When going from the lowest temperature ambient CO2 treatment to the highest temperature elevated CO2 treatment, however, there was a whopping 106 percent increase in vegetative biomass.

By contrast, seed yields in both the ambient and CO2-enriched air dropped dramatically with each of the three temperature increases studied, declining at the highest temperature regime to but a small percentage of what they were at the lowest temperature regime. Nevertheless, Prasad et al. report that “seed yields at 36.4/26.4°C under elevated CO2 were similar to those obtained at 32/22°C under ambient CO2,” the latter pair of which temperatures they describe as “present-day seasonal temperatures.”

It would appear that a warming of 4.4°C above present-day seasonal temperatures for peanut production would have essentially no effect on peanut seed yields, as long as the atmosphere’s CO2 concentration rose concurrently, by something on the order of 350 ppm. It is also important to note, according to Prasad et al., that “maximum/minimum air temperatures of 32/22°C and higher are common in many peanut-producing countries across the globe.” In fact, they note that “the Anantapur district in Andhra Pradesh, which is one of the largest peanut-producing regions in India, experiences season-long temperatures considerably greater than 32/22°C from planting to maturity.”

In light of these real-world observations, i.e., that some of the best peanut-producing regions in the world currently experience air temperatures considerably greater than what Prasad et al. suggest is optimum for peanuts (something less than 32/22°C), it would appear that real-world declines in peanut seed yields in response to a degree or two of warming, even in air of ambient CO2 concentration, must be very slight or even non-existent, for how else could the places that commonly experience these considerably higher temperatures remain some of the best peanut-producing areas in the world? This in turn suggests that for more realistic values of CO2-induced global warming, i.e., temperature increases on the order of 0.4°C for a doubling of the air’s CO2 content (Idso, 1998), there would likely be a significant increase in real-world peanut production.

In another pertinent study, Vu (2005) grew peanut plants from seed to maturity in greenhouses maintained at atmospheric CO2 concentrations of 360 and 720 ppm and at air temperatures that were 1.5 and 6.0°C above outdoor air temperatures, while he measured a number of parameters related to the plants’ photosynthetic performance. His work revealed that although Rubisco protein content and activity were down-regulated by elevated CO2, the Rubisco photosynthetic efficiency (the ratio of midday light-saturated carbon exchange rate to Rubisco initial or total activity) of the elevated-CO2 plants “was 1.3- to 1.9-fold greater than that of the ambient-CO2 plants at both growth temperatures.” He also determined that “leaf soluble sugars and starch of plants grown at elevated CO2 were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient CO2.” In addition, he discovered that the leaf transpiration of the elevated-CO2 plants relative to that of the ambient-CO2 plants was 12 percent less at near-ambient temperatures and 17 percent less in the higher temperature regime, while the water use efficiency of the elevated-CO2 plants relative to the ambient-CO2 plants was 56 percent greater at near-ambient temperatures and 41 percent greater in the higher temperature environment.

In commenting on his findings, Vu notes that because less Rubisco protein was required by the elevated-CO2 plants, the subsequent redistribution of excess leaf nitrogen “would increase the efficiency of nitrogen use for peanut under elevated CO2,” just as the optimization of inorganic carbon acquisition and greater accumulation of the primary photosynthetic products in the CO2-enriched plants “would be beneficial for peanut growth at elevated CO2.” Indeed, in the absence of other stresses, Vu’s conclusion was that “peanut photosynthesis would perform well under rising atmospheric CO2 and temperature predicted for this century.”

In a somewhat different type of study, Alexandrov and Hoogenboom (2000) studied how year-to-year changes in temperature, precipitation and solar radiation had influenced the yields of peanuts over a 30-year period in the southeastern United States, after which they used the results to predict future crop yields based on climate-change output from various global circulation models (GCMs) of the atmosphere. At ambient CO2 concentrations, the GCM scenarios suggested a decrease in peanut yields by the year 2020, due in part to predicted changes in temperature and precipitation. However, when the yield-enhancing effects of a doubling of the atmospheric CO2 concentration were included, a totally different result was obtained: a yield increase.

Although we have little faith in GCM scenarios, it is interesting to note that their climate change predictions often result in positive outcomes for agricultural productivity when the direct effects of elevated CO2 on plant growth and development are included in the analyses. These results support the research reported later in this chapter describing the stress-ameliorating effects of atmospheric CO2 enrichment on plant growth and development under unfavorable growing conditions characterized by high air temperatures and inadequate soil moisture.

In conclusion, it would appear that even if the climate changes that are typically predicted to result from anticipated increases in the air’s CO2 content were to materialize, the concurrent rise in the air’s CO2 concentration should more than compensate for any deleterious effects those changes in climate might otherwise have had on the growth and yield of peanuts.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/peanut.php.


Potato


In the study of Sicher and Bunce (1999), exposure to twice-ambient atmospheric CO2 concentrations enhanced rates of net photosynthesis in potato plants (Solanum tuberosum L.) by 49 percent; while in the study of Schapendonk et al. (2000), a doubling of the air’s CO2 content led to an 80 percent increase in net photosynthesis. In a study that additionally considered the role of the air’s vapor pressure deficit (VPD), Bunce (2003) found that exposure to twice-ambient atmospheric CO2 concentrations boosted net photosynthesis by 36 percent at a VPD of 0.5 kPa (moist air) but by 70 percent at a VPD of 3.5 kPa (dry air). Yet another complexity was investigated by Olivo et al. (2002), who assessed the effect of a doubling of the air’s CO2 content on the net photosynthetic rates of high-altitude (Solanum curtilobum) and low-altitude (S. tuberosum) and found the rate of the former to be enhanced by 56 percent and that of the latter by 53 percent. In addition, although they did not directly report photosynthetic rates, Louche-Tessandier et al. (1999) noted that photosynthetic acclimation was reduced in CO2-enriched plants that were inoculated with a fungal symbiont, which consequently allowed them to produce greater amounts of biomass than non-inoculated control plants grown in ambient air.

Because elevated CO2 concentrations stimulate photosynthesis in potatoes, it is to be expected they would also increase potato biomass production. Miglietta et al. (1998), for example, reported that potatoes grown at 660 ppm CO2 produced 40 percent more tuber biomass than control plants grown in ambient air. Such reports are common, in fact, with twice-ambient atmospheric CO2 concentrations having been reported to produce yield increases of 25 percent (Lawson et al., 2001), 36 percent (Chen and Setter, 2003), 37 percent (Schapendonk et al., 2000), 40 percent (Olivo et al., 2002), 44 percent (Sicher and Bunce, 1999), 85 percent (Olivo et al., 2002) and 100 percent (Ludewig et al., 1998).

A few studies have been conducted at even higher atmospheric CO2 concentrations. Kauder et al. (2000), for example, grew plants for up to seven weeks in controlled environments receiving an extra 600 ppm CO2, obtaining final tuber yields that were 30 percent greater than those of ambiently grown plants. Also, in a study of potato microcuttings grown for four weeks in environmental chambers maintained at ambient air and air enriched with an extra 1200 ppm CO2, Pruski et al. (2002) found that the average number of nodes per stem was increased by 64 percent, the average stem dry weight by 92 percent, and the average shoot length by 131 percent.

Atmospheric CO2 enrichment also leads to reductions in transpirational water loss by potato plants. Magliulo et al. (2003), for example, grew potatoes in the field within FACE rings maintained at either ambient (370 ppm) or enriched (550 ppm) atmospheric CO2 concentrations for two consecutive years, finding that the CO2-enriched plants used 12 percent less water than the ambient-treatment plants, while they produced 47 percent more tuber biomass. Hence, the CO2-enriched plants experienced a 68 percent increase in water use efficiency, or the amount of biomass produced per unit of water used in producing it. Likewise, Olivo et al. (2002) found that a doubling of the air’s CO2 content increased the instantaneous water-use efficiencies of high-altitude and low-altitude potato species by 90 percent and 80 percent, respectively.

In the final phenomenon considered here, we review the findings of three studies that evaluated the ability of atmospheric CO2 enrichment to mitigate the deleterious effects of ozone pollution on potato growth. Fangmeier and Bender (2002) determined the mean tuber yield of potato as a function of atmospheric CO2 concentration for conditions of ambient and high atmospheric O3 concentrations, as derived from a trans-European set of experiments. At the mean ambient CO2 concentration of 380 ppm, the high O3 stress reduced mean tuber yield by approximately 9 percent. At CO2 concentrations of 540 and 680 ppm, however, the high O3 stress had no significant effect on tuber yield.

Much the same results were obtained by Wolf and van Oijen (2002, 2003), who used the validated potato model LPOTCO to project future European tuber yields. Under two climate change scenarios that incorporated the effects of increased greenhouse gases on climate (i.e., increased air temperature and reduced precipitation), the model generated increases in irrigated tuber production ranging from 2,000 to 4,000 kg of dry matter per hectare across Europe, with significant reductions in the negative effects of O3 pollution.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturepotato.php.


Rice


DeCosta et al. (2003a) grew two crops of rice (Oryza sativa L.) at the Rice Research and Development Institute of Sri Lanka from January to March (the maha season) and from May to August (the yala season) in open-top chambers in air of either ambient or ambient plus 200 pppm CO2, determining that leaf net photosynthetic rates were significantly higher in the CO2-enriched chambers than in the ambient-air chambers: 51-75 percent greater in the maha season and 22-33 percent greater in the yala season. Likewise, in the study of Gesch et al. (2002), where one-month-old plants were maintained at either 350 ppm CO2 or switched to a concentration of 700 ppm for 10 additional days, the plants switched to CO2-enriched air immediately displayed large increases in their photosynthetic rates that at the end of the experiment were still 31 percent greater than those exhibited by unswitched control plants.

With respect to the opposite of photosynthesis, Baker et al. (2000) reported that rates of carbon loss via dark respiration in rice plants decreased with increasing nocturnal CO2 concentrations. As a result, it is not surprising that in the study of Weerakoon et al. (2000), rice plants exposed to an extra 300 ppm of atmospheric CO2 exhibited a 35 percent increase in mean season-long radiation-use efficiency, defined as the amount of biomass produced per unit of solar radiation intercepted. In light of these several observations, therefore, one would logically expect rice plants to routinely produce more biomass at elevated levels of atmospheric CO2.

In conjunction with the study of DeCosta et al. (2003a), DeCosta et al. (2003b) found that CO2-enriched rice plants produced more leaves per hill, more tillers per hill, more total plant biomass, greater root dry weight, and more panicles per plant and had harvest indices that were increased by 4 percent and 2 percent, respectively, in the maha and yala seasons, which suite of benefits led to grain yield increases of 24 percent and 39 percent in those two periods. In another study, Kim et al. (2003) grew rice crops from the seedling stage to maturity at atmospheric CO2 concentrations of ambient and ambient plus 200 ppm using FACE technology and three levels of applied nitrogen—low (LN, 4 g N m-2), medium (MN, 8 and 9 g N m-2), and high (HN, 15 g N m-2)—for three cropping seasons (1998-2000). They found that “the yield response to elevated CO2 in crops supplied with MN (+14.6 percent) or HN (+15.2 percent) was about twice that of crops supplied with LN (+7.4 percent),” confirming the importance of N availability to the response of rice to atmospheric CO2 enrichment that had previously been determined by Kim et al. (2001) and Kobaysahi et al. (2001).

Various environmental stresses can significantly alter the effect of elevated CO2 on rice. In the study of Tako et al. (2001), rice plants grown at twice-ambient CO2 concentrations and ambient temperatures displayed no significant increases in biomass production; but when air temperatures were raised by 2°C, the CO2-enriched plants produced 22 percent more biomass than the plants grown in non-CO2-enriched air. By contrast, Ziska et al. (1997) reported that CO2-enriched rice plants grown at elevated air temperatures displayed no significant increases in biomass; but when the plants were grown at ambient air temperatures, the additional 300 ppm of CO2 boosted their rate of biomass production by 40 percent. In light of these observations, rice growers should select cultivars that are most responsive to elevated CO2 concentrations at the air temperatures likely to prevail in their locality in order to maximize their yield production in a future high-CO2 world.

Water stress can also severely reduce rice production. As an example, Widodo et al. (2003) grew rice plants in eight outdoor, sunlit, controlled-environment chambers at daytime atmospheric CO2 concentrations of 350 and 700 ppm for an entire season. In one set of chambers, the plants were continuously flooded. In another set, drought stress was imposed during panicle initiation. In another, it was imposed during anthesis; and in the last set, drought stress was imposed at both stages. The resultant drought-induced effects, according to the scientists, “were more severe for plants grown at ambient than at elevated CO2.” They report, for example, that “plants grown under elevated CO2 were able to maintain midday leaf photosynthesis, and to some extent other photosynthetic-related parameters, longer into the drought period than plants grown at ambient CO2.”

Recovery from the drought-induced water stress was also more rapid in the elevated CO2 treatment. At panicle initiation, for example, Widodo et al. observed that “as water was added back following a drought induction, it took more than 24 days for the ambient CO2 [water]-stressed plants to recuperate in midday leaf CER, compared with only 6-8 days for the elevated CO2 [water]-stressed plants.” Similarly, they report that “for the drought imposed during anthesis, midday leaf CER of the elevated CO2 [water]-stressed plants were fully recovered after 16 days of re-watering, whereas those of the ambient CO2 [water]-stressed plants were still 21 percent lagging behind their unstressed controls at that date.” Hence, they concluded that “rice grown under future rising atmospheric CO2 should be better able to tolerate drought situations.”

In a somewhat different type of study, Watling and Press (2000) found that rice plants growing in ambient air and infected with a root hemiparasitic angiosperm obtained final biomass values that were only 35 percent of those obtained by uninfected plants. In air of 700 ppm CO2, however, the infected plants obtained biomass values that were 73 percent of those obtained by uninfected plants. Thus, atmospheric CO2 enrichment significantly reduced the negative impact of this parasite on biomass production in rice.

In summary, as the CO2 concentration of the air continues to rise, rice plants will likely experience greater photosynthetic rates, produce more biomass, be less affected by root parasites, and better deal with environmental stresses, all of which effects should lead to greater grain yields.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturerice.php.


Sorghum


Many laboratory and field experiments have demonstrated a significant positive impact of elevated levels of atmospheric CO2 on total biomass and grain production in the C4 crop sorghum (Sorghum bicolor (L.) Moench).

Ottman et al. (2001) grew sorghum plants in a FACE experiment conducted near Phoenix, Arizona, USA, where plants were fumigated with air containing either 360 or 560 ppm CO2 and where they were further subjected to irrigation regimes resulting in both adequate and inadequate levels of soil moisture. Averaged over the two years of their study, the extra CO2 increased grain yield by only 4 percent in the plots receiving adequate levels of soil moisture but by 16 percent in the dry soil moisture plots.

Prior et al. (2005) grew sorghum in two different years in 7-meter-wide x 76-meter-long x 2-m-deep bins filled with a silt loam soil, upon which they constructed a number of clear-plastic-wall open-top chambers they maintained at ambient CO2 concentrations and ambient concentrations plus 300 ppm. In the first of the two years, the extra CO2 increased sorghum residue production by 14 percent, while in the second year it increased crop residue production by 24 percent and grain production by 22 percent. For a CO2 increase of 200 ppm comparable to that employed in the study of Ottman et al., these figures translate to crop residue increases of 9 percent and 16 percent and a grain increase of 15 percent.

In a review of primary research papers describing results obtained from large-scale FACE experiments conducted over the prior 15 years, Ainsworth and Long (2005) determined that, in the mean, sorghum grain yield was increased by approximately 7 percent in response to a 200-ppm increase in the atmosphere’s CO2 concentration.

An experiment with a bit more complexity was carried out several years earlier by Watling and Press (1997), who grew sorghum with and without infection by the parasitic C3 weeds Striga hermonthica and S. asiatica. The study lasted for about two months and was conducted in controlled environment cabinets fumigated with air of either 350 or 700 ppm CO2. In the absence of parasite infection, the extra 350 ppm of CO2 boosted plant biomass production by 35 percent, which adjusted downward to make it compatible with the 200-ppm increase employed in most FACE studies corresponds to an increase of just under 21 percent. When infected with S. asiatica, the biomass stimulation provided by the extra CO2 was about the same; but when infected with S. hermonthica, it was almost 80 percent, which corresponds to a similarly downward adjusted biomass increase of 45 percent.

In light of these several observations, it would appear that although the CO2-induced increase in total biomass and grain yield of sorghum is rather modest, ranging from 4 to 16 percent under well-watered conditions, it can be on the high end of this range when the plants are stressed by a shortage of water (16 percent has been observed) and by parasitic infection (45 percent has been observed). Consequently, elevated levels of atmospheric CO2 seem to help sorghum most when help is most needed.

Additional information on this topic, including reviews on sorghum not discussed here, can be found at http://www.co2science.org/subject/a/subject_a.php under the main heading Agriculture, sub heading Sorghum.


Soybean


Wittwer (1995) reports that the common soybean (Glycine max L.) “provides about two-thirds of the world’s protein concentrate for livestock feeding, and is a valuable ingredient in formulated feeds for poultry and fish.” Bernacchi et al. (2005) characterize the soybean as “the world’s most important seed legume.” Consequently, it is important to determine how soybeans will likely respond to rising atmospheric CO2 concentrations with and without concomitant increases in air temperature and under both well-watered and water-stressed conditions.

Rogers et al. (2004) grew soybeans from emergence to grain maturity in ambient and CO2-enriched air (372 and 552 ppm CO2, respectively) at the SoyFACE facility of the University of Illinois at Urbana-Champaign, Illinois, USA, while CO2 uptake and transpiration measurements were made from pre-dawn to post-sunset on seven days representative of different developmental stages of the crop. Across the growing season, they found that the mean daily integral of leaf net photosynthesis rose by 24.6 percent in the elevated CO2 treatment, while mid-day stomatal conductance dropped by 21.9 percent, in response to the 48 percent increase in atmospheric CO2 employed in their study. With respect to photosynthesis, they additionally report “there was no evidence of any loss of stimulation toward the end of the growing season,” noting that the largest stimulation actually occurred during late seed filling. Nevertheless, they say that the photosynthetic stimulation they observed was only “about half the 44.5 percent theoretical maximum increase calculated from Rubisco kinetics.” Thus, there is an opportunity for soybeans to perhaps become even more responsive to atmospheric CO2 enrichment than they are currently, which potential could well be realized via future developments in the field of genetic engineering.

Bunce (2005) grew soybeans in the field in open-top chambers maintained at atmospheric CO2 concentrations of ambient and ambient +350 ppm at the Beltsville Agricultural Research Center in Maryland, USA, where net CO2 exchange rate measurements were performed on a total of 16 days between 18 July and 11 September of 2000 and 2003, during flowering to early pod-filling. Over the course of this study, daytime net photosynthesis per unit leaf area was 48 percent greater in the plants growing in the CO2-enriched air, while nighttime respiration per unit leaf area was not affected by elevated CO2. However, because the elevated CO2 increased leaf dry mass per unit area by an average of 23 percent, respiration per unit mass was significantly lower for the leaves of the soybeans growing in the CO2-enriched air, producing a sure recipe for accelerated growth and higher soybean seed yields.

Working in Australia, Japan, and the United States, Ziska et al. (2001b) observed a recurrent diurnal pattern of atmospheric CO2 concentration, whereby maximum values of 440-540 ppm occurred during a three-hour pre-dawn period that was followed by a decrease to values of 350-400 ppm by mid-morning, after which there was a slow but steady increase in the late afternoon and early evening that brought the air’s CO2 concentration back to its pre-dawn maximum value. In an attempt to see if the pre-dawn CO2 spikes they observed affected plant growth, they grew soybeans for one month in controlled-environment chambers under three different sets of conditions: a constant 24-hour exposure to 370 ppm CO2, a constant 370 ppm CO2 exposure during the day followed by a constant 500 ppm CO2 exposure at night, and a CO2 exposure of 500 ppm from 2200 to 0900 followed by a decrease to 370 ppm by 1000, which was maintained until 2200, somewhat mimicking the CO2 cycle they observed in nature. This program revealed that the 24-hour exposure to 370 ppm CO2 and the 370-ppm-day/500-ppm-night treatments produced essentially the same results in terms of biomass production after 29 days. However, the CO2 treatment that mimicked the observed atmospheric CO2 pattern resulted in a plant biomass increase of 20 percent.

In a study that evaluated a whole range of atmospheric CO2 concentrations, from far below ambient levels to high above them, Allen et al. (1998) grew soybeans for an entire season in growth chambers maintained at atmospheric CO2 concentrations of 160, 220, 280, 330, 660, and 990 ppm. In doing so, they observed a consistent increase in total nonstructural carbohydrates in all vegetative components including roots, stems, petioles, and especially the leaves, as CO2 concentrations rose. There was, however, no overall significant effect of treatment CO2 concentration on nonstructural carbohydrate accumulation in soybean reproductive components, including podwalls and seeds, which observations indicate that the higher yields reported in the literature for soybeans exposed to elevated CO2 most likely result from increases in the number of pods produced per plant, and not from the production of larger individual pods or seeds.

The increasing amounts of total nonstructural carbohydrates that were produced with each additional increment of CO2 provided the raw materials to support greater biomass production at each CO2 level. Although final biomass and yield data were not reported in this paper, the authors did present biomass data obtained at 66 days into the experiment. Relative to above-ground biomass measured at 330 ppm CO2, the plants that were grown in sub-ambient CO2 concentrations of 280, 220, and 160 ppm exhibited 12, 33, and 60 percent less biomass, respectively, while plants grown in atmospheric CO2 concentrations of 660 and 990 ppm displayed 46 and 66 percent more biomass.

In a study of two contrasting soybean cultivars, Ziska and Bunce (2000) grew Ripley, which is semi-dwarf and determinate in growth, and Spencer, which is standard-size and indeterminate in growth, for two growing seasons in open-top chambers maintained at atmospheric CO2 concentrations of ambient and ambient plus 300 ppm. Averaged over both years, the elevated CO2 treatment increased photosynthetic rates in the Ripley and Spencer varieties by 76 and 60 percent, respectively. However, Spencer showed a greater CO2-induced increase in vegetative biomass than Ripley (132 vs. 65 percent). Likewise, elevated CO2 enhanced seed yield in Spencer by 60 percent but by only 35 percent in Ripley, suggesting that cultivar selection for favorable yield responses to atmospheric CO2 enrichment could have a big impact on future farm productivity.

In another study of contrasting types of soybeans, Nakamura et al. (1999) grew nodulated and non-nodulated plants in pots within controlled-environmental cabinets maintained at atmospheric CO2 concentrations of 360 and 700 ppm in combination with low and high soil nitrogen supply for three weeks. They found that at low nitrogen, elevated CO2 increased total plant dry mass by approximately 40 and 80 percent in nodulated soybeans grown at low and high nitrogen supply, respectively, while non-nodulated plants exhibited no CO2-induced growth response at low nitrogen but an approximate 60 percent growth enhancement at high nitrogen supply. Hence, it would appear that as the air’s CO2 content continues to rise, non-nodulated soybeans will display increases in biomass only if they are grown in nitrogen-rich soils. Nodulated soybeans, however, should display increased growth in both nitrogen-rich and nitrogen-poor soils, with their responses being about twice as large in high as in low soil nitrogen conditions.

In yet another study of soybeans with different genetic characteristics, Ziska et al. (2001a) grew one modern and eight ancestral soybean genotypes in glasshouses maintained at atmospheric CO2 concentrations of 400 and 710 ppm, finding that the elevated CO2 increased photosynthetic rates in all cultivars by an average of 75 percent. This photosynthetic enhancement led to CO2-induced increases in seed yield that averaged 40 percent, except for one of the ancestral varieties that exhibited an 80 percent increase in seed yield.

To get a glimpse of what might happen if future temperatures also continue to rise, Ziska (1998) grew soybeans for 21 days in controlled environments having atmospheric CO2 concentrations of approximately 360 (ambient) or 720 ppm and soil temperatures of 25° (ambient) or 30°C. He found that elevated CO2 significantly increased whole plant net photosynthesis at both temperatures, with the greatest effect occurring at 30°C. As time progressed, however, this photosynthetic stimulation dropped from 50 percent at 13 days into the experiment to 30 percent at its conclusion eight days later; in spite of this partial acclimation, which was far from complete, atmospheric CO2 enrichment significantly enhanced total plant dry weight at final harvest by 36 and 42 percent at 25° and 30°C, respectively.

Studying the complicating effects of water stress were Serraj et al. (1999), who grew soybeans from seed in pots within a glasshouse until they were four weeks old, after which half of the plants were subjected to an atmospheric CO2 concentration of 360 ppm, while the other half were exposed to an elevated concentration of 700 ppm. In addition, half of the plants at each CO2 concentration were well-watered and half of them were allowed to experience water stress for a period of 18 days. This protocol revealed that short-term (18-day) exposure of soybeans to elevated CO2 significantly decreased daily and cumulative transpirational water losses compared to plants grown at 360 ppm CO2, regardless of water treatment. In fact, elevated CO2 reduced total water loss by 25 and 10 percent in well-watered and water-stressed plants, respectively. Also, drought stress significantly reduced rates of net photosynthesis among plants of both CO2 treatments. However, plants grown in elevated CO2 consistently exhibited higher photosynthetic rates than plants grown at ambient CO2, regardless of soil water status.

At final harvest, the elevated CO2 treatment had little effect on the total dry weight of plants grown at optimal soil moisture, but it increased the total dry weight of water-stressed plants by about 33 percent. Also, while root dry weight declined for plants grown under conditions of water stress and ambient CO2 concentration, no such decline was exhibited by plants subjected to atmospheric CO2 enrichment and water stress.

Studying both water and high-temperature stress were Ferris et al. (1999), who grew soybeans in glasshouses maintained at atmospheric CO2 concentrations of 360 and 700 ppm for 52 days, before having various environmental stresses imposed on them for eight days during early seed filling. For the eight-day stress period, some plants were subjected to air temperatures that were 15°C higher than those to which the control plants were exposed, while some were subjected to a water stress treatment in which their soil moisture contents were maintained at 40 percent of that experienced by the control plants. Averaged across all stress treatments and harvests, this protocol revealed that the high CO2 treatment increased total plant biomass by 41 percent. Both high-temperature and water-deficit treatments, singly or in combination, reduced overall biomass by approximately the same degree, regardless of CO2 treatment. Thus, even when the greatest biomass reductions of 17 percent occurred in the CO2-enriched and ambiently grown plants, in response to the combined stresses of high temperature and low soil moisture, plants grown in elevated CO2 still exhibited an average biomass that was 24 percent greater than that displayed by plants grown in ambient CO2.

Averaged across all stress treatments and harvests, elevated CO2 increased seed yield by 32 percent. In addition, it tended to ameliorate the negative effects of environmental stresses. CO2-enriched plants that were water stressed, for example, had an average seed yield that was 34 percent greater than that displayed by water-stressed controls grown at ambient CO2, while CO2-enriched plants exposed to high temperatures produced 38 percent more seed than their respectively stressed counterparts. In fact, the greatest relative impact of elevated CO2 on seed yield occurred in response to the combined stresses of high temperature and low soil moisture, with CO2-enriched plants exhibiting a seed yield that was 50 percent larger than that of similarly stressed plants grown in ambient CO2.

In a predictive application of this type of knowledge, but based on a different means of obtaining it, Alexandrov and Hoogenboom (2000) studied how temperature, precipitation, and solar radiation influenced soybean yields over a 30-year period in the southeastern United States, after which they used the results they obtained to predict future crop yields based on climate output from various global circulation models of the atmosphere. At ambient CO2 concentrations, the model-derived scenarios pointed to a decrease in soybean yields by the year 2020, due in part to predicted changes in temperature and precipitation. However, when the yield-enhancing effects of a doubling of the air’s CO2 concentration were included in the simulations, a completely different projection was obtained: a yield increase.

Shifting to the subject of soybean seed quality, Caldwell et al. (2005) write that “the beneficial effects of isoflavone-rich foods have been the subject of numerous studies (Birt et al., 2001; Messina, 1999),” and that “foods derived from soybeans are generally considered to provide both specific and general health benefits,” presumably via these substances. Hence, it is only natural they would wonder how the isoflavone content of soybean seeds may be affected by the ongoing rise in the air’s CO2 content, and that they would conduct a set of experiments to find the answer.

The scientists grew well-watered and fertilized soybean plants from seed to maturity in pots within two controlled-environment chambers, one maintained at an atmospheric CO2 concentration of 400 ppm and one at 700 ppm. The chambers were initially kept at a constant air temperature of 25°C. At the onset of seed fill, however, air temperature was reduced to 18°C until seed development was complete, in order to simulate average outdoor temperatures at this stage of plant development. In a second experiment, this protocol was repeated, except the temperature during seed fill was maintained at 23°C, with and without drought (a third treatment), while in a third experiment, seed-fill temperature was maintained at 28°C, with or without drought.

In the first experiment, where air temperature during seed fill was 18°C, the elevated CO2 treatment increased the total isoflavone content of the soybean seeds by 8 percent. In the second experiment, where air temperature during seed fill was 23°C, the extra CO2 increased total seed isoflavone content by 104 percent, while in the third experiment, where air temperature during seed fill was 28°C, the CO2-induced isoflavone increase was 101 percent. Finally, when drought-stress was added as a third environmental variable, the extra CO2 boosted total seed isoflavone content by 186 percent when seed-fill air temperature was 23°C, while at a seed-fill temperature of 28°C, it increased isoflavone content by 38 percent.

Under all environmental circumstances studied, enriching the air with an extra 300 ppm of CO2 increased the total isoflavone content of soybean seeds. In addition, the percent increases measured under the stress situations investigated were always greater than the percent increase measured under optimal growing conditions.

Also writing on the subject of soybean seed quality, Thomas et al. (2003) say “the unique chemical composition of soybean has made it one of the most valuable agronomic crops worldwide,” noting that “oil and protein comprise ~20 and 40%, respectively, of the dry weight of soybean seed.” Consequently, they explored the effects of elevated CO2 plus temperature on soybeans that were grown to maturity in sunlit controlled-environment chambers with sinusoidally varying day/night max/min temperatures of 28/18°, 32/22°, 36/26°, 40/30°, and 44/34°C and atmospheric CO2 concentrations of 350 and 700 ppm. This work revealed that the effect of temperature on seed composition and gene expression was “pronounced,” but that “there was no effect of CO2.” In this regard, however, they note that “Heagle et al. (1998) observed a positive significant effect of CO2 enrichment on soybean seed oil and oleic acid concentration,” the latter of which parameters their own study found to rise with increasing temperature all the way from 28/18° to 44/34°C. In addition, they determined that “32/22°C is optimum for producing the highest oil concentration in soybean seed,” that “the degree of fatty acid saturation in soybean oil was significantly increased by increasing temperature,” and that crude protein concentration increased with temperature to 40/30°C.

In commenting on these observations, Thomas et al. note that “the intrinsic value of soybean seed is in its supply of essential fatty acids and amino acids in the oil and protein, respectively.” Hence, we conclude that the temperature-driven changes they identified in these parameters, as well as the CO2 effect observed by Heagle et al., bode well for the future production of this important crop and its value to society in a CO2-enriched and warming world. Thomas et al. note, however, that “temperatures during the soybean-growing season in the southern USA are at, or slightly higher than, 32/22°C,” and that warming could negatively impact the soybean oil industry in this region. For the world as a whole, however, warming would be a positive development for soybean production; while in the southern United States, shifts in planting zones could readily accommodate changing weather patterns associated with this phenomenon.

In conclusion, as the air’s CO2 content continues to rise, soybeans will likely respond by displaying significant increases in growth and yield, with possible improvements in seed quality; these beneficial effects will likely persist even if temperatures rise or soil moisture levels decline, regardless of their cause.

Additional information on this topic, including reviews on sorghum not discussed here, can be found at http://www.co2science.org/subject/a/subject_a.php under the main heading Agriculture, sub heading Soybean.


Strawberry


In the open-top chamber study of Bunce (2001), strawberry plants (Fragaria x ananassa) exposed to air containing an extra 300 and 600 ppm CO2 displayed photosynthetic rates that were 77 and 106 percent greater, respectively, than rates displayed by plants grown in ambient air containing 350 ppm CO2. Similarly, Bushway and Pritts (2002) reported that strawberry plants grown at atmospheric CO2 concentrations between 700 and 1,000 ppm exhibited photosynthetic rates that were consistently more than 50 percent greater than rates displayed by control plants.

Because elevated CO2 stimulates rates of photosynthesis in strawberry plants, it is expected that it would also increase biomass production in this important agricultural species. After growing plants in air containing an additional 170 ppm CO2 above ambient concentrations, Deng and Woodward (1998) reported that total fresh fruit weights were 42 and 17 percent greater than weights displayed by control plants receiving high and low soil nitrogen inputs, respectively. In addition, Bushway and Pritts (2002) reported that a two- to three-fold increase in the air’s CO2 content boosted strawberry fruit yield by 62 percent.

As the air’s CO2 content continues to rise, strawberry plants will likely exhibit enhanced rates of photosynthesis and biomass production, which should lead to greater fruit yields.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturestraw.php.


Sunflower


As the CO2 content of the air increases, sunflower plants (Helianthus annus L.) will likely display enhanced rates of photosynthetic carbon uptake. In the study of Sims et al. (1999), exposure to twice-ambient atmospheric CO2 concentrations enhanced rates of net photosynthesis in individual upper-canopy sunflower leaves by approximately 50 percent. Similarly, Luo et al. (2000) reported that sunflowers grown at 750 ppm CO2 displayed canopy carbon uptake rates that were fully 53 percent greater than those exhibited by plants grown at 400 ppm CO2.

The study of Zerihun et al. (2000) reported that twice-ambient CO2 concentrations increased whole plant biomass in sunflowers by 44, 13, and 115 percent when the plants were simultaneously exposed to low, medium, and high levels of soil nitrogen, respectively.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturesun.php.


Tomato


In the study of Ziska et al. (2001), tomato plants (Lycopersicon esculentum Mill.) grown at a nocturnal atmospheric CO2 concentration of 500 ppm displayed total plant biomass values that were 10 percent greater than those exhibited by control plants growing in air containing 370 ppm CO2. This result was likely the consequence of the elevated CO2 reducing the rate of nocturnal respiration in the plants, which would have allowed them to utilize the retained carbon to produce more biomass.

This CO2-induced benefit, as well as a host of other positive effects of atmospheric CO2 enrichment, are also manifest under unfavorable growing conditions. Jwa and Walling (2001), for example, reported that fungal infection reduced plant biomass in tomatoes growing in normal air by about 30 percent. However, in fungal-infected plants grown at twice-ambient atmospheric CO2 concentrations, the elevated CO2 completely ameliorated the growth-reducing effects of the pathogen.

In another stressful situation, Maggio et al. (2002) reported that a 500-ppm increase in the air’s CO2 concentration increased the average value of the root-zone salinity threshold in tomato plants by about 60 percent. In addition, they reported that the water-use efficiency of the CO2-enriched plants was about twice that of the ambiently grown plants.

As the CO2 content of the air increases, tomato plants will likely display greater rates of photosynthesis and biomass production, which should consequently lead to greater fruit yields, even under stressful conditions of fungal infection and high soil salinity.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/agriculturetomato.php.


Wheat


In one study, Dijkstra et al. (1999) grew winter wheat (Triticum aestivum L.) in open-top chambers and field-tracking sun-lit climatized enclosures maintained at atmospheric CO2 concentrations of ambient and ambient plus 350 ppm CO2 for two years, determining that the elevated CO2 increased both final grain yield and total above-ground biomass by 19 percent. In another study, Masle (2000) grew two varieties of wheat for close to a month in greenhouses maintained at atmospheric CO2 concentrations of 350 and 900 ppm, finding that the CO2-enriched plants exhibited biomass increases of 52 to 93 percent, depending upon variety and vernalization treatment.

Based on a plethora of experimental observations of this nature, many scientists have developed yield prediction models for wheat. Using the output of several such models, Alexandrov and Hoogenboom (2000) estimated the impact of typically predicted climate changes on wheat production in Bulgaria in the twenty-first century, finding that a doubling of the air’s CO2 concentration would likely enhance wheat yields there between 12 and 49 percent in spite of a predicted 2.9° to 4.1°C increase in air temperature. Likewise, Eitzinger et al. (2001) employed the WOFOST crop model to estimate wheat production in northeastern Austria in the year 2080. For a doubled atmospheric CO2 concentration with concomitant climate changes derived from five different general circulation models of the atmosphere, they obtained simulated yield increases of 30 to 55 percent, even in the face of predicted changes in both temperature and precipitation.

Southworth et al. (2002) used the CERES-Wheat growth model to calculate winter wheat production during the period 2050-2059 for 10 representative farm locations in Indiana, Illinois, Ohio, Michigan, and Wisconsin, USA, for six future climate scenarios. They report that some of the southern portions of this group of states would have exhibited climate-induced yield decreases had the aerial fertilization effect of the CO2 increase that drove the predicted changes in climate not been included in the model. When they did include the increase in the air’s CO2 concentration (to a value of 555 ppm), however, they note that “wheat yields increased 60 to 100% above current yields across the central and northern areas of the study region,” while in the southern areas “small increases and small decreases were found.” The few minor decreases, however, were associated with the more extreme Hadley Center greenhouse run that presumed a 1 percent increase in greenhouse gases per year and a doubled climate variability; hence, they would have to be considered highly unlikely.

In discussing their findings, Southworth et al. note that other modeling studies have obtained similar results for other areas. They report, for example, that Brown and Rosenberg (1999) found winter wheat yields across other parts of the United States to increase “under all climate change scenarios modeled (1, 2.5, and 5°C temperature increases),” and that Cuculeanu et al. (1999) found modeled yields of winter wheat in southern Romania to increase by 15 to 21 percent across five sites. Also, they note that Harrison and Butterfield (1996) “found increased yields of winter wheat across Europe under all the climate change scenarios they modeled.”

Van Ittersum et al. (2003) performed a number of simulation experiments with the Agricultural Production Systems Simulator (APSIM)-Nwheat model in which they explored the implications of possible increases in atmospheric CO2 concentration and near-surface air temperature for wheat production and deep drainage at three sites in Western Australia differing in precipitation, soil characteristics, nitrogenous fertilizer application rates, and wheat cultivars. They first assessed the impact of the ongoing rise in the air’s CO2 content, finding that wheat grain yield increased linearly at a rate of 10-16 percent for each 100-ppm increase in atmospheric CO2 concentration, with only a slight concomitant increase in deep drainage (a big win, small loss outcome). For a likely future CO2 increase of 200 ppm, increases in grain yield varied between 3 and 17 percent for low nitrogen fertilizer application rates and between 21 and 34 percent for high rates of nitrogen application, with the greatest relative yield response being found for the driest site studied.

When potential warming was factored into the picture, the results proved even better. The positive effects of the CO2 increase on wheat grain yield were enhanced an extra 3-8 percent when temperatures were increased by 3°C in the model simulations. These yield increases were determined to result in an increased financial return to the typical Western Australian wheat farmer of 15-35 percent. In addition, the imposition of the simultaneous temperature increase led to a significant decline in deep drainage, producing a truly win-win situation that enhanced the average farmer’s net income by an additional 10-20 percent. Consequently, it was determined that the CO2-induced increase in temperature predicted by the IPCC could well increase the net profitability of Western Australian wheat farmers by anywhere from 25-55 percent, while at the same time mitigating what van Ittersum et al. refer to as “one of Australia’s most severe land degradation problems.”

In a wide variety of circumstances, atmospheric CO2 enrichment significantly increases the biomass production and yield of wheat plants, thereby benefiting both wheat producers and consumers alike.

Additional information on this topic, including reviews on sorghum not discussed here, can be found at http://www.co2science.org/subject/a/subject_a.php under the main heading Agriculture, sub heading Wheat.


Grapes


Although climate model projections are generally thought to be unreliable, there are potential positive outcomes that are being posited from projected climate changes – as is the case with grapes. In a 2011 study, a team of six scientists from Portugal responded to the IPCC prediction that “more frequent extreme weather will increase summer air temperature and water stress, namely for regions with a Mediterranean-type environment. Expected changes in the climate of viticultural regions may alter significantly both the spectrum and the distribution of grape varieties currently used.” Red wine produced in Demarcated Region of Douro (DRD) is one of the most important products for the Portuguese economy.

The team grew grapes in open-top chambers in 2004, 2005 and 2006 with ambient (365 ppm) and elevated (500 ppm) atmospheric CO2 concentrations, and found that the elevated CO2 concentration increased net photosynthetic rate, intrinsic water use efficiency and leaf thickness. In 2004, 2005 and 2006 the elevated CO2 increased the yield by 50%, 27% and 50%, respectively.

A second team of scientists from Portugal and Germany approached the future of grapes in the Douro Region using an entirely different methodology. Santos et al. remind us that “atmospheric factors, such as temperature, precipitation and radiation strongly control grapevine growth and development, primarily by affecting photosynthetic rate.” They collected grape yield, monthly temperature and precipitation data from the DRD over the period 1986 to 2008. Based on IPCC predictions for the DRD over the next 100 years, the authors state that “grapevine yield in the DRD is expected to undergo an upward trend until the end of this century, which might be further enhanced by projected upward trends in CO2 concentration.”


Other Plants


At current levels of crop productivity, global food requirements may outpace crop production by the middle of this century. Thus, according to Rosenthal et al. (2011), "direct improvements in photosynthetic efficiency will be needed if we are to meet global food needs in the future." Plants "would have a greater increase in productivity in elevated CO2 when compared to wild type plants" if crops could be engineered with an increased capacity for the regeneration of RuBP (Ribulose-1,5-bisphosphate, an organic substance that is involved in photosynthesis). Therefore, wild type tobacco (Nicotiana tabacum) and "transformed" tobacco that overexpresses the C3 cycle enzyme sedoheptulose-1,7 bisphosphatase (SBPase) were grown under ambient (385 ppm) and enriched (585 ppm) atmospheric CO2 concentrations, during which time a number of plant properties and processes were assessed, in order to determine the ultimate impact of the aerial fertilization effect of atmospheric CO2 enrichment on the two types of plants.

The scientists determined that growth under elevated CO2 stimulated instantaneous net photosynthesis more in the transformed plants than in the wild type plants; and although there was some evidence of photosynthetic acclimation in both sets of plants, there was still a greater CO2-induced stimulation of final biomass in the transformed plants than in the wild-type plants (22% vs. 13%). These results "provide proof of concept that increasing [the] content and activity of a single photosynthesis enzyme can enhance carbon assimilation and yield of C3 crops grown at CO2 concentrations expected by the middle of the 21st century," which productivity boost they feel will be needed in order to adequately feed the world's projected population at that time.

Woody Plants

The growth response of woody plants to atmospheric CO2 enrichment has also been extensively studied. Ceulemans and Mousseau (1994), for example, tabulated the results of 95 separate experimental investigations related to this topic. The review of Poorter (1993) includes 41 additional sets of pertinent results, and the two reviews of Wullschleger et al. (1995, 1997) contain 40 other sets of applicable data. When averaged together, these 176 individual woody plant experiments reveal a mean growth enhancement on the order of 50 percent for an approximate doubling of the air’s CO2 content, which is about one-and-a-half times as much as the response of non-woody herbaceous plants.

It is possible, however, that this larger result is still an underestimate of the capacity of trees and shrubs to respond to atmospheric CO2 enrichment; for the mean duration of the 176 woody plant experiments described above was only five months, which may not have been sufficient for the long-term equilibrium effects of the CO2 enrichment of the air to be manifest. In the world’s longest such experiment, for example, Kimball et al. (2007) observed a 70 percent sustained increase in biomass production over the entire last decade of a 17-year study in response to a 75 percent increase in the air’s CO2 content employed throughout the experiment. Likewise, studies of Eldarica pine trees conducted at the same location have revealed a similarly increasing growth response over the same length of time (Idso and Kimball, 1994).

In the subsections that follow, we highlight the results of studies that have examined the growth response of several woody plants to atmospheric CO2 enrichment. We end the section with discussions of the effect of CO2 enhancement on wood density and forest productivity and carbon sequestration. For more information on this topic, see http://www.co2 science.org/data/plant_growth/plantgrowth.php.


Aspen-Poplar


Several studies have documented the effects of elevated levels of atmospheric CO2 on photosynthesis in various aspen clones (Populus tremuloides). In the short-term study of Kruger et al. (1998), aspen seedlings grown for 70 days at atmospheric CO2 concentrations of 650 ppm exhibited photosynthetic rates that were approximately 10 percent greater than those displayed by seedlings maintained at ambient CO2 concentrations. In the longer five-month study of Kubiske et al. (1998), atmospheric CO2 enrichment significantly increased photosynthetic rates in four aspen genotypes, regardless of soil nitrogen status.

In an even longer 2.5-year study, Wang and Curtis (2001) also observed significant CO2-induced photosynthetic increases in two male and two female aspen clones; when six aspen genotypes were grown in open-top chambers for 2.5 years at atmospheric CO2 concentrations of 350 and 700 ppm, Curtis et al. (2000) reported that the elevated CO2 concentrations increased rates of net photosynthesis by 128 and 31 percent at high and low soil nitrogen contents, respectively. In addition, in a study that looked only at air temperature effects that was conducted at ambient CO2 concentrations, King et al. (1999) determined that increasing the air temperature from 13° to 29°C enhanced photosynthetic rates in four different aspen clones by an average of 35 percent.

In a FACE study, where O3-sensitive and O3-tolerant clones were grown for six months in field plots receiving 360 and 560 ppm CO2 in combination with ambient and enriched (1.5 times ambient) O3 levels, Noormets et al. (2001) reported that CO2-induced increases in photosynthetic rates were at least maintained, and sometimes even increased, when clones were simultaneously exposed to elevated O3. After an entire year of treatment exposure, in fact, Karnosky et al. (1999) noted that the powerful ameliorating effect of elevated CO2 on ozone-induced damage was still operating strongly in this system. O3-induced foliar damages in O3-sensitive and O3-tolerant clones were reduced from 55 and 17 percent, respectively, at ambient CO2, to 38 and 3 percent, respectively, at elevated CO2.

With respect to biomass production, Pregitzer et al. (2000) reported that 2.5 years of exposure to twice-ambient concentrations of atmospheric CO2 increased fine-root biomass in six aspen genotypes by an average of 65 and 17 percent on nitrogen-rich and nitrogen-poor soils, respectively. Using this same experimental system, Zak et al. (2000) determined that elevated CO2 enhanced total seedling biomass by 38 percent at high soil nitrogen and by 16 percent at low soil nitrogen. Similar results were reported in the two-year open-top chamber study of Mikan et al. (2000), who observed 50 and 25 percent CO2-induced increases in total seedling biomass at high and low soil nitrogen levels, respectively.

As the air’s CO2 content continues to increase, aspen seedlings will likely display enhanced rates of photosynthesis and biomass production, regardless of genotype, gender, O3-sensitvity, and soil nitrogen status. Consequently, greater amounts of carbon will likely be sequestered in the tissues of this most abundant of North American tree species and in the soils in which they are rooted in the years and decades ahead.

Working to explore a plant’s capacity to allocate assimilated CO2 to greater biomass production, Cseke et al. (2009) grew two quaking aspen (Populus tremuloides Michx.) clones (216 and 271) from the seedling stage in replicate plots maintained at either 372 or 560 ppm CO2, respectively, throughout each year's growing season (May-September), assessing their stem volume (a surrogate for biomass) over an eight-year period, during which time they measured the trees' maximum light-saturated rates of leaf net photosynthesis, the transcriptional activity of leaf elevated-CO2-responsive genes, and numerous leaf primary and secondary carbon-based compounds.

Although the CO2-induced increase in the maximum light-saturated rate of leaf net photosynthesis in clone 216 was more than twice as great as that of clone 271, just the opposite relationship was manifest in the CO2-induced increases in the trees' stem volumes. The researchers suggest that this is because "the CO2-responsive clone (271) partitions carbon into pathways associated with active defense/response to stress, carbohydrate/starch biosynthesis and subsequent growth," while "the CO2-unresponsive clone (216) partitions carbon into pathways associated with passive defense and cell wall thickening."

This study indicates that there is significant variation in genotypic expression patterns among trees in response to long-term exposure to elevated CO2. Therefore, "future efforts to improve productivity or other advantageous traits for carbon sequestration should include an examination of genetic variability in CO2 responsiveness."

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesaspen.php.


Beech


Egli and Korner (1997) rooted eight beech saplings (genus Fagus) directly into calcareous or acidic soils in open-top chambers and exposed them to atmospheric CO2 concentrations of either 370 or 570 ppm. Over the first year of their study, the saplings growing on calcareous soil in CO2-enriched air exhibited a 9 percent increase in stem diameter; they speculated that this initial small difference may “cumulate to higher ‘final’ tree biomass through compounding interest.” At the end of three years of differential CO2 exposure, the trees in the CO2-enriched chambers were experiencing net ecosystem carbon exchange rates that were 58 percent greater than the rates of the trees in the ambient CO2 chambers, regardless of soil type; the stem dry mass of the CO2-enriched trees was increased by about 13 percent over that observed in the ambient-air chambers (Maurer et al., 1999).

In a similar but much shorter experiment, Dyckmans et al. (2000) grew three-year-old seedlings of beech for six weeks in controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm, finding that the doubling of the air’s CO2 content increased seedling carbon uptake by 63 percent. They also noted that the majority of the assimilated carbon was allocated to the early development of leaves, which would be expected to subsequently lead to greater absolute amounts of photosynthetic carbon fixation.

In the two-year study of Grams et al. (1999), beech seedlings grown at ambient CO2 concentrations displayed large reductions in photosynthetic rates when simultaneously exposed to twice-ambient levels of ozone. However, at twice-ambient CO2 concentrations, twice-ambient ozone concentrations had no negative effects on the trees’ photosynthetic rates. Thus, atmospheric CO2 enrichment completely ameliorated the negative effects of ozone on photosynthesis in this species.

Similarly, Polle et al. (1997) reported that beech seedlings grown at 700 ppm CO2 for two years displayed significantly reduced activities of catalase and superoxide dismutase, which are antioxidative enzymes responsible for detoxifying highly reactive oxygenated compounds within cells. Their data imply that CO2-enriched atmospheres are conducive to less oxidative stress and, therefore, less production of harmful oxygenated compounds than typically occurs in ambient air. Consequently, the seedlings growing in the CO2-enriched air were likely able to remobilize a portion of some of their valuable raw materials away from the production of detoxifying enzymes and reinvest them into other processes required for facilitating optimal plant development and growth.

With respect to this concept of resource optimization, Duquesnay et al. (1998) studied the relative amounts of 12C and 13C in tree rings of beech growing for the past century in northeastern France and determined that the intrinsic water-use efficiency of the trees had increased by approximately 33 percent over that time period, no doubt in response to the concomitant rise in the air’s CO2 concentration over the past 100 years.

In conclusion, as the CO2 content of the air increases, beech trees will likely display enhanced rates of photosynthesis and decreased damage resulting from oxidative stress. Together, these phenomena should allow greater optimization of raw materials within beech, allowing them to produce greater amounts of biomass ever more efficiently as the atmospheric CO2 concentration increases.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesbeech.php.


Birch


In the relatively short-term study of Wayne et al. (1998), yellow birch seedlings (Betula pendula) grown for two months at atmospheric CO2 concentrations of 800 ppm exhibited photosynthetic rates that were about 50 percent greater than those displayed by control seedlings fumigated with air containing 400 ppm CO2. Similarly, in the three-month study of Tjoelker et al. (1998a), paper birch seedlings grown at 580 ppm CO2 displayed photosynthetic rates that were approximately 30 percent greater than those exhibited by seedlings exposed to 370 ppm CO2. Likewise, Kellomaki and Wang (2001) reported that birch seedlings exposed to an atmospheric CO2 concentration of 700 ppm for five months displayed photosynthetic rates that were about 25 percent greater than seedlings grown at 350 ppm CO2. Finally, in the much longer four-year study conducted by Wang et al. (1998), silver birch seedlings grown in open-top chambers receiving twice-ambient concentrations of atmospheric CO2 displayed photosynthetic rates that were fully 110 percent greater than rates displayed by their ambiently grown counterparts. Thus, short-term photosynthetic enhancements resulting from atmospheric CO2 enrichment appear to persist for several years or longer.

Because elevated CO2 enhances photosynthetic rates in birch trees, it likely will also lead to increased biomass production in these important deciduous trees, as it has in several experiments. In the three-month study of Tjoelker et al. (1998b), for example, a 57 percent increase in the air’s CO2 content increased the biomass of paper birch seedlings by 50 percent. When similar seedlings were grown at 700 ppm CO2 for four months, Catovsky and Bazzaz (1999) reported that elevated CO2 increased total seedling biomass by 27 and 130 percent under wet and dry soil moisture regimes, respectively. In the interesting study of Godbold et al. (1997), paper birch seedlings grown at 700 ppm for six months not only increased their total biomass, but also increased the number of root tips per plant by more than 50 percent. In the longer two-year study of Berntson and Bazzaz (1998), twice-ambient levels of CO2 increased the biomass of a mixed yellow and white birch mesocosm by 31 percent; and in another two-year study, Wayne et al. (1998) reported that yellow birch seedlings grown at 800 ppm CO2 produced 60 and 227 percent more biomass than seedlings grown at 400 ppm CO2 at ambient and elevated air temperatures, respectively. Finally, after exposing silver birch seedlings to twice-ambient CO2 concentrations for four years, Wang et al. (1998) noted that CO2-enriched seedlings produced 60 percent more biomass than ambiently grown seedlings. Hence, atmospheric CO2 enrichment clearly enhances birch biomass in both short- and medium-term experiments.

In some studies, elevated CO2 also reduced stomatal conductances in birch trees, thereby boosting their water-use efficiencies. Tjoelker et al. (1998a), for example, reported that paper birch seedlings grown at 580 ppm CO2 for three months experienced 10-25 percent reductions in stomatal conductance, which contributed to 40-80 percent increases in water-use efficiency. Similar CO2-induced reductions in stomatal conductance (21 percent) were reported in silver birch seedlings grown for four years at 700 ppm CO2 by Rey and Jarvis (1998).

The results of these several studies suggest that the ongoing rise in the air’s CO2 content will likely increase rates of photosynthesis and biomass production in birch trees, as well as improve their water use efficiencies, irrespective of any concomitant changes in air temperature and/or soil moisture status that might occur. Consequently, rates of carbon sequestration by this abundant temperate forest species should also increase in the years and decades ahead.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesbirch.php.


Citrus Trees


How does atmospheric CO2 enrichment affect the growth and development of citrus trees and the fruit they produce?

In the study of Keutgen and Chen (2001), cuttings of Citrus madurensis grown for three months at 600 ppm CO2 displayed rates of photosynthesis that were more than 300 percent greater than those measured on control cuttings grown at 300 ppm CO2. In addition, elevated CO2 concentrations have been shown to increase photosynthetic rates in mango (Schaffer et al., 1997), mangosteen (Schaffer et al., 1999), and sweet orange (Jifon et al., 2002). In the study of Jifon et al., it was further reported that twice-ambient CO2 concentrations increased photosynthetic rates in mycorrhizal- and non-mycorrhizal-treated sour orange seedlings by 118 and 18 percent, respectively.

Such CO2-induced increases in photosynthesis should lead to enhanced biomass production; and so they do. Idso and Kimball (2001), for example, have documented how a 75 percent increase in the air’s CO2 content has boosted the long-term production of above-ground wood and fruit biomass in sour orange trees by 80 percent in a study that has been ongoing since November 1987. Furthermore, Idso et al. (2002) have additionally demonstrated that the 300-ppm increase in the air’s CO2 content has increased the fresh weight of individual oranges by an average of 4 percent and the vitamin C content of their juice by an average of 5 percent.

In summary, these peer-reviewed studies suggest that as the air’s CO2 content slowly but steadily rises, citrus trees will respond by increasing their rates of photosynthesis and biomass production. In addition, they may also increase the vitamin C content of their fruit, which may help to prevent an array of human health problems brought about by insufficient intake of vitamin C.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treescitrus.php.


Eucalyptus


In the eight-month study of Roden et al. (1999), Eucalyptus pauciflora seedlings growing at 700 ppm CO2 displayed seasonal rates of net photosynthesis that were approximately 30 percent greater than those exhibited by their ambiently grown counterparts. In another eight-month study, Palanisamy (1999) reported that well-watered Eucalyptus cladocalyx seedlings exposed to 800 ppm CO2 exhibited photosynthetic rates that were 120 percent higher than those observed in control plants growing at 380 ppm CO2. Moreover, after a one-month period of water stress, photosynthetic rates of CO2-enriched seedlings were still 12 percent greater than rates displayed by ambiently grown water-stressed seedlings.

Because elevated CO2 enhances photosynthetic rates in eucalyptus species, this phenomenon should lead to increased biomass production in these rapidly growing trees. And so it does. In the eight-month experiment of Gleadow et al. (1998), for example, Eucalyptus cladocalyx seedlings growing at 800 ppm CO2 displayed 134 and 98 percent more biomass than seedlings growing at 400 ppm CO2 at low and high soil nitrogen concentrations, respectively. Similarly, Eucalyptus pauciflora seedlings growing at twice-ambient CO2 concentrations for eight months produced 53 percent more biomass than control seedlings (Roden et al., 1999).

In summary, as the CO2 content of the air increases, eucalyptus seedlings will likely display enhanced rates of photosynthesis and biomass production, regardless of soil moisture and nutrient status. Consequently, greater amounts of carbon will likely be sequestered by this rapidly growing tree species.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treeseuc.php.


Fruit-Bearing


Several studies have recently documented the effects of elevated atmospheric CO2 concentrations on photosynthesis in various fruiting trees. In an eight-day experiment, Pan et al. (1998) found that twice-ambient CO2 concentrations increased rates of net photosynthesis in one-year-old apple seedlings by 90 percent. In a longer three-month study, Keutgen and Chen (2001) noted that cuttings of Citrus madurensis exposed to 600 ppm CO2 displayed rates of photosynthesis that were more than 300 percent greater than rates observed in control cuttings exposed to 300 ppm CO2. Likewise, in the review paper of Schaffer et al. (1997), it was noted that atmospheric CO2 enrichment had previously been shown to enhance rates of net photosynthesis in various tropical and sub-tropical fruit trees, including avocado, banana, citrus, mango, and mangosteen. Finally in the two-year study of Centritto et al. (1999a), cherry seedlings grown at 700 ppm CO2 exhibited photosynthetic rates that were 44 percent greater than those displayed by seedlings grown in ambient air, independent of a concomitant soil moisture treatment.

Because elevated CO2 enhances the photosynthetic rates of fruiting trees, it should also lead to increased biomass production in them. In the two-year study of Centritto et al. (1999b), for example, well-watered and water-stressed seedlings growing at twice-ambient CO2 concentrations displayed basal trunk areas that were 47 and 51 percent larger than their respective ambient controls. Similarly, in a study spanning more than 13 years, Idso and Kimball (2001) demonstrated that the above-ground wood biomass of mature sour orange trees growing in air enriched with an additional 300 ppm of CO2 was 80 percent greater than that attained by control trees growing in ambient air.

As the CO2 content of the air increases, fruit trees will likely display enhanced rates of photosynthesis and biomass production, regardless of soil moisture conditions. Consequently, greater amounts of carbon will likely be sequestered in the woody trunks and branches of such species. Moreover, fruit yields may increase as well. In the study of Idso and Kimball, for example, fruit yields were stimulated to essentially the same degree as above-ground wood biomass; i.e., by 80 percent in response to a 75 percent increase in the air’s CO2 content.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesfruit.php.


Nitrogen-Fixing Trees


In the six-week study of Schortemeyer et al. (1999), seedlings of Australian blackwood (Acacia melanoxylon) grown at twice-ambient atmospheric CO2 concentrations displayed photosynthetic rates that were 22 percent greater than those of ambiently grown seedlings. In addition, the CO2-enriched seedlings exhibited biomass values that were twice as large as those displayed by control seedlings grown in air of 350 ppm CO2. Likewise, Polley et al. (1999) reported that a doubling of the atmospheric CO2 concentration for three months increased honey mesquite (Prosopis glandulosa) seedling root and shoot biomass by 37 and 46 percent, respectively.

Several studies have investigated the effects of elevated CO2 on black locust (Robinia pseudoacacia) seedlings. Uselman et al. (2000), grew seedlings for three months at 700 ppm CO2 and reported that this treatment increased the root exudation of organic carbon compounds by 20 percent, while Uselman et al. (1999) reported no CO2-induced increases in the root exudation of organic nitrogen compounds. Nonetheless, elevated CO2 enhanced total seedling biomass by 14 percent (Uselman et al., 2000).

In the study of Olesniewicz and Thomas (1999), black locust seedlings grown at twice-ambient CO2 concentrations for two months exhibited a 69 percent increase in their average rate of nitrogen-fixation when they were not inoculated with an arbuscular mycorrhizal fungal species. It was further determined that the amount of seedling nitrogen derived from nitrogen-fixation increased in CO2-enriched plants by 212 and 90 percent in non-inoculated and inoculated seedlings, respectively. Elevated CO2 enhanced total plant biomass by 180 and 51 percent in non-inoculated and inoculated seedlings, respectively.

As the CO2 content of the air increases, nitrogen-fixing trees respond by exhibiting enhanced rates of photosynthesis and biomass production, as well as enhanced rates of nitrogen fixation.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesnitrofix.php.


Oak


How do oak (genus Quercus) trees respond to atmospheric CO2 enrichment? In the two-month study of Anderson and Tomlinson (1998), northern red oak seedlings exposed to 700 ppm CO2 displayed photosynthetic rates that were 34 and 69 percent greater than those displayed by control plants growing under well-watered and water-stressed conditions, respectively. Similarly, in the four-month study of Li et al. (2000), Quercus myrtifolia seedlings growing at twice-ambient CO2 concentrations exhibited rates of photosynthesis at the onset of senescence that were 97 percent greater than those displayed by ambiently growing seedlings.

In the year-long study of Staudt et al. (2001), Quercus ilex seedlings grown at 700 ppm CO2 displayed trunk and branch biomasses that were 90 percent greater than those measured on seedlings growing at 350 ppm CO2. Also, in the eight-month inter-generational study performed by Polle et al. (2001), seedlings produced from acorns collected from ambient and CO2-enriched mother trees and germinated in air of either ambient or twice-ambient atmospheric CO2 concentration displayed whole-plant biomass values that were 158 and 246 percent greater, respectively, than those exhibited by their respective control seedlings growing in ambient air.

In another study, Schulte et al. (1998) grew oak seedlings for 15 weeks at twice-ambient CO2 concentrations, finding that elevated CO2 enhanced seedling biomass by 92 and 128 percent under well-watered and water-stressed conditions, respectively. In a similar study conducted by Tomlinson and Anderson (1998), water-stressed seedlings growing at 700 ppm CO2 displayed biomass values that were similar to those exhibited by well-watered plants growing in ambient air. Thus, atmospheric CO2 enrichment continues to benefit oak trees even under water-stressed conditions.

Additional studies have demonstrated that oak seedlings also respond positively to atmospheric CO2 enrichment when they are faced with other environmental stresses and resource limitations. When pedunculate oak seedlings were subjected to two different soil nutrient regimes, for example, Maillard et al. (2001) reported that a doubling of the atmospheric CO2 concentration enhanced seedling biomass by 140 and 30 percent under high and low soil nitrogen conditions, respectively. And in the study of Usami et al. (2001), saplings of Quercus myrsinaefolia that were grown at 700 ppm CO2 displayed biomass increases that were 110 and 140 percent greater than their ambiently grown counterparts when they were simultaneously subjected to air temperatures that were 3° and 5°C greater than ambient temperature, respectively. Thus, elevated CO2 concentrations tend to ameliorate some of the negative effects caused by growth-reducing stresses in oaks. In fact, when Schwanz and Polle (1998) reported that elevated CO2 exposure caused reductions in the amounts of several foliar antioxidative enzymes in mature oak trees, they suggested that this phenomenon was the result of atmospheric CO2 enrichment causing the trees to experience less oxidative stress and, therefore, they had less need for antioxidative enzymes.

In some studies, elevated CO2 has been shown to reduce stomatal conductances in oak trees, thus contributing to greater tree water-use efficiencies. Tognetti et al. (1998a), for example, reported that oak seedlings growing near a natural CO2-emitting spring exhibited less water loss and more favorable turgor pressures than trees growing further away from the spring. The resulting improvement in water-use efficiency was so significant that Tognetti et al. (1998b) stated, “such marked increases in water-use efficiency under elevated CO2 might be of great importance in Mediterranean environments in the perspective of global climate change.”

In summary, it is clear that as the CO2 content of the air increases, oak seedlings will likely display enhanced rates of photosynthesis and biomass production, regardless of air temperature, soil moisture, and soil nutrient status. Consequently, greater amounts of carbon will likely be removed from the atmosphere by the trees of this abundant genus and stored in their tissues and the soils in which they are rooted.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesoak.php.


Pine



Loblolly


Tissue et al. (1997) grew seedlings of loblolly pine trees (Pinus taeda L.) for a period of four years in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 650 ppm in a study of the long-term effects of elevated CO2 on the growth of this abundant pine species. This experiment indicated there was a mean biomass accumulation in the seedlings grown in CO2-enriched air that was 90 percent greater than that attained by the seedlings grown in ambient air.

Johnson et al. (1998) reviewed 11 of their previously published papers, describing the results of a series of greenhouse and open-top chamber studies of the growth responses of loblolly pine seedlings to a range of atmospheric CO2 and soil nitrogen concentrations. This work indicated that when soil nitrogen levels were so low as to be extremely deficient, or so high as to be toxic, growth responses to atmospheric CO2 enrichment were negligible. For moderate soil nitrogen deficiencies, however, a doubling of the air’s CO2 content sometimes boosted growth by as much as 1,000 percent. Consequently, since the nitrogen status of most of earth’s ecosystems falls somewhere between extreme deficiency and toxicity, these results suggest that loblolly pine trees may experience large increases in growth as the air’s CO2 content continues to climb.

Naidu and DeLucia (1999) described the results of working one full year in 30-meter-diameter circular FACE plots maintained at atmospheric CO2 concentrations of either 350 or 560 ppm in an originally 13-year-old loblolly pine plantation in North Carolina, USA, where they determined the effects of the elevated CO2 treatment on the productivity of the trees, which were growing in soil that was characteristically low in nitrogen and phosphorus. After the first year of atmospheric CO2 enrichment in this Duke Forest Face Study, the growth rate of the CO2-enriched trees was about 24 percent greater than that of the trees exposed to ambient CO2, in spite of the likelihood of soil nutrient limitations and a severe summer drought (rainfall in August 1997 was about 90 percent below the 50-year average).

After four years of work at the Duke Forest Face Site, Finzi et al. (2002) reported that the extra 200 or so ppm of CO2 had increased the average yearly dry matter production of the CO2-enriched trees by 32 percent, while at the eight-year point of the experiment Moore et al. (2006) reported there had been a sustained increase in trunk basal area increment that varied between 13 and 27 percent with variations in weather and the timing of growth. What is more, they say “there was no evidence of a decline in the relative enhancement of tree growth by elevated CO2 as might be expected if soil nutrients were becoming progressively more limiting,” which many people had expected would occur in light of the site’s low soil nitrogen and phosphorus content. In addition, at the six-year point of the study Pritchard et al. (2008) determined that the extra CO2 had increased the average standing crop of fine roots by 23 percent.

Gavazzi et al. (2000) grew one-year-old loblolly pine seedlings for about four months in pots placed within growth chambers maintained at atmospheric CO2 concentrations of either 360 or 660 ppm and adequate or inadequate levels of soil moisture, while the pots were seeded with a variety of C3 and C4 weeds. In the course of this experiment, they found that total seedling biomass was always greater under well-watered as opposed to water-stressed conditions, and that elevated CO2 increased total seedling biomass by 22 percent in both water treatments. In the elevated CO2 and water-stressed treatment, however, they also found that seedling root-to-shoot ratios were about 80 percent greater than they were in the elevated CO2 and well-watered treatment, due to a 63 percent increase in root biomass. In the case of the weeds, total biomass was also always greater under well-watered compared to water-stressed conditions. However, the elevated CO2 did not increase weed biomass; in fact, it reduced it by approximately 22 percent. Consequently, in assessing the effects of elevated CO2 on competition between loblolly pine seedlings and weeds, the seedlings were definitely the winners, with the researchers concluding that the CO2-induced increase in root-to-shoot ratio under water-stressed conditions may “contribute to an improved ability of loblolly pine to compete against weeds on dry sites.”

Working with data obtained from stands of loblolly pine plantations at 94 locations scattered throughout the southeastern United States, Westfall and Amateis (2003) employed mean height measurements made at three-year intervals over a period of 15 years to calculate a site index related to mean growth rate for each of the five three-year periods, which index would be expected to increase monotonically if growth rates were being enhanced above “normal” by some monotonically increasing growth-promoting factor. This protocol indicated, in their words, that “mean site index over the 94 plots consistently increased at each remeasurement period,” which would suggest, as they phrase it, that “loblolly pine plantations are realizing greater than expected growth rates,” and, we would add, that the growth rate increases are growing larger and larger with each succeeding three-year period.

As for what might be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern region of the United States, the two researchers say that in addition to rising atmospheric CO2 concentrations, “two other likely factors that could affect growth are temperature and precipitation.” However, they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggest that if increased nitrogen deposition were the cause, “such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots.” Hence, they are partial to the aerial fertilization effect of atmospheric CO2 enrichment as the explanation. What is more, they note that “similar results were reported by Boyer (2001) for natural stands of longleaf pine, where increases in dominant stand height are occurring over generations on the same site.”

The studies reported here indicate that as the CO2 content of the air continues to rise, loblolly pine trees will likely experience significant increases in biomass production, even on nutrient-poor soils, during times of drought, and in competition with weeds.

Additional information on this topic, including reviews loblolly pine trees not discussed here, can be found at http://www.co2science.org/subject/t/subject _t.php, under the heading Trees, Types, Pine, Loblolly.


Ponderosa


Walker et al. (1998b) grew seedlings of Ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws) for an entire year in controlled environment chambers with atmospheric CO2 concentrations of either 350 (ambient), 525, or 700 ppm. In addition, low or high levels of nitrogen and phosphorus were supplied to determine the main and interactive effects of atmospheric CO2 enrichment and soil nutrition on seedling growth and fungal colonization of the seedlings’ roots. After 12 months, they found that phosphorus supply had little impact on overall seedling growth, while high nitrogen increased nearly every parameter measured, including root, shoot, and total biomass, as did atmospheric CO2 enrichment. Averaged over all nitrogen and phosphate treatments, total root dry weights at 525 and 700 ppm CO2 were 92 and 49 percent greater, respectively, than those observed at ambient CO2, while shoot dry weights were 83 and 26 percent greater. Consequently, seedlings grown at 525 and 700 ppm CO2 had total dry weights that were 86 and 35 percent greater, respectively, than those measured at ambient CO2. In addition, elevated CO2 increased the total number of ectomycorrhizal fungi on roots by 170 percent at 525 ppm CO2 and 85 percent at 700 ppm CO2 relative to the number observed at ambient CO2.

Walker et al. (1998a) grew Ponderosa pine seedlings for two growing seasons out-of-doors in open-top chambers having atmospheric CO2 concentrations of 350, 525, and 700 ppm on soils of low, medium, and high nitrogen content to determine the interactive effects of these variables on juvenile tree growth. The elevated CO2 concentrations had little effect on most growth parameters after the first growing season, with the one exception of below-ground biomass, which increased with both CO2 and soil nitrogen. After two growing seasons, however, elevated CO2 significantly increased all growth parameters, including tree height, stem diameter, shoot weight, stem volume, and root volume, with the greatest responses typically occurring at the highest CO2 concentration in the highest soil nitrogen treatment. Root volume at 700 ppm CO2 and high soil nitrogen, for example, exceeded that of all other treatments by at least 45 percent, as did shoot volume by 42 percent. Similarly, at high CO2 and soil nitrogen coarse root and shoot weights exceeded those at ambient CO2 and high nitrogen by 80 and 88 percent, respectively.

Johnson et al. (1998) reviewed 11 of their previously published papers (including the two discussed above) in which they describe the results of a series of greenhouse and open-top chamber studies of the growth responses of Ponderosa pine seedlings to a range of atmospheric CO2 and soil nitrogen concentrations. These studies indicated that when soil nitrogen levels were so low as to be extremely deficient, or so high as to be toxic, growth responses to atmospheric CO2 enrichment were negligible. For moderate soil nitrogen deficiencies, however, a doubling of the air’s CO2 content sometimes boosted growth by as much as 1,000 percent. In addition, atmospheric CO2 enrichment mitigated the negative growth response of ponderosa pine to extremely high soil nitrogen in two separate studies.

Maherali and DeLucia (2000) grew Ponderosa pine seedlings for six months in controlled environment chambers maintained at atmospheric CO2 concentrations ranging from 350 to 1,100 ppm, while they were subjected to either low (15/25°C night/day) or high (20/30°C night/day) temperatures. This study revealed that although elevated CO2 had no significant effect on stomatal conductance, seedlings grown in the high temperature treatment exhibited a 15 percent increase in this parameter relative to seedlings grown in the low temperature treatment. Similarly, specific hydraulic conductivity, which is a measure of the amount of water moving through a plant relative to its leaf or needle area, also increased in the seedlings exposed to the high temperature treatment. In addition, biomass production rose by 42 percent in the low temperature treatment and 62 percent in the high temperature treatment when the atmospheric CO2 concentration was raised from 350 to 1,100 ppm.

Tingey et al. (2005) studied the effects of atmospheric CO2 enrichment (to approximately 350 ppm above ambient) on the fine-root architecture of Ponderosa pine seedlings growing in open-top chambers via minirhizotron tubes over a period of four years. This experiment showed that “elevated CO2 increased both fine root extensity (degree of soil exploration) and intensity (extent that roots use explored areas) but had no effect on mycorrhizae,” the latter of which observations was presumed to be due to the fact that soil nitrogen was not limiting to growth in this study. More specifically, they report that “extensity increased 1.5- to 2-fold in elevated CO2 while intensity increased only 20 percent or less,” noting that similar extensity results had been obtained over shorter periods of four months to two years by Arnone (1997), Berntson and Bazzaz (1998), DeLucia et al. (1997) and Runion et al. (1997), while similar intensity results had been obtained by Berntson (1994).

Last, Soule and Knapp (2006) studied Ponderosa pine trees growing naturally at eight sites within the Pacific Northwest of the United States, in order to see how they may have responded to the increase in the atmosphere’s CO2 concentration that occurred after 1950. In selecting these sites, they chose locations that “fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance.” They also say they selected locations with “a variety of climatic and topoedaphic conditions, ranging from extremely water-limiting environments … to areas where soil moisture should be a limiting factor for growth only during extreme drought years,” additionally noting that all sites were located in areas “where ozone concentrations and nitrogen deposition are typically low.”

At each of the eight sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included “the potentially oldest trees on each site,” so that their results would indicate, as they put it, “the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations.” Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates during matched wet and dry years pre- and post-1950.

So what did they find? Overall, the two researchers report finding a post-1950 radial growth enhancement that was “more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site.” As for the magnitude of the response, they determined that “the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%.”

With respect to the meaning and significance of their observations, Soule and Knapp say their results “showing that radial growth has increased in the post-1950s period … while climatic conditions have generally been unchanged, suggest that nonclimatic driving forces are operative.” In addition, they say that “these radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth and Long, 2005).” Hence, they say their findings suggest that “elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions.”

Summarizing their findings Soule and Knapp recount how they had “hypothesized that ponderosa pine … would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally harsh sites,” and they state in their very next sentence that their results “support these hypotheses.” Hence, they conclude their paper by stating it is likely that “an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural conditions within the [USA’s] interior Pacific Northwest.”

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesponderosa.php.


Scots


Rouhier and Read (1998) grew seedlings of Scots pine (Pinus sylvestris L.) for four months in growth cabinets maintained at atmospheric CO2 concentrations of either 350 or 700 ppm. In addition, one-third of the seedlings were inoculated with one species of mycorrhizal fungi, one-third were inoculated with another species, and one-third were not inoculated at all, in order to determine the effects of elevated CO2 on mycorrhizal fungi and their interactive effects on seedling growth. These procedures resulted in the doubled atmospheric CO2 content increasing seedling dry mass by an average of 45 percent regardless of fungal inoculation. In addition, the extra CO2 increased the number of hyphal tips associated with seedling roots by about 62 percent for both fungal species. Hyphal growth was also accelerated by elevated CO2; after 55 days of treatment, the mycorrhizal network produced by one of the fungal symbionts occupied 444 percent more area than its counterpart exposed to ambient CO2.

These results suggest that as the air’s CO2 content continues to rise, fungal symbionts of Scots pine will likely receive greater allocations of carbon from their host. This carbon can be used to increase their mycorrhizal networks, which would enable the fungi to explore greater volumes of soil in search of minerals and nutrients to benefit the growth of its host. In addition, by receiving greater allocations of carbon, fungal symbionts may keep photosynthetic down regulation from occurring, as they provide an additional sink for leaf-produced carbohydrates.

Janssens et al. (1998) grew three-year-old Scots pine seedlings in open-top chambers kept at ambient and 700 ppm atmospheric CO2 concentrations for six months while they studied the effects of elevated CO2 on root growth and respiration. In doing so, they learned that the elevated CO2 treatment significantly increased total root length by 122 percent and dry mass by 135 percent relative to the roots of seedlings grown in ambient-CO2 air. In addition, although starch accumulation in the CO2-enriched roots was nearly 90 percent greater than that observed in the roots produced in the ambient-CO2 treatment, the carbon-to-nitrogen ratio of the CO2-enriched roots was significantly lower than that of the control-plant roots, indicative of the fact that they contained an even greater relative abundance of nitrogen. The most important implication of this study, therefore, was that Scots pine seedlings will likely be able to find the nitrogen they need to sustain large growth responses to atmospheric CO2 enrichment with the huge root systems they typically produce in CO2-enriched air.

Kainulainen et al. (1998) constructed open-top chambers around Scots pine trees that were about 20 years old and fumigated them with combinations of ambient or CO2-enriched air (645 ppm) and ambient or twice-ambient (20 to 40 ppb) ozone-enriched air for three growing seasons to study the interactive effects of these gases on starch and secondary metabolite production. In doing so, they determined that elevated CO2 and O3 (ozone) had no significant impact on starch production in Scots pine, even after two years of treatment exposure. However, near the end of the third year, the elevated CO2 alone significantly enhanced starch production in current-year needles, although neither extra CO2, extra O3, nor combinations thereof had any significant effects on the concentrations of secondary metabolites they investigated.

Kellomaki and Wang (1998) constructed closed-top chambers around 30-year-old Scots pine trees, which they fumigated with air containing either 350 or 700 ppm CO2 at ambient and elevated (ambient plus 4°C) air temperatures for one full year, after which they assessed tree water-use by measuring cumulative sap flow for 32 additional days. This protocol revealed that the CO2-enriched air reduced cumulative sap flow by 14 percent at ambient air temperatures, but that sap flow was unaffected by atmospheric CO2 concentration in the trees growing at the elevated air temperatures. These findings suggest that cumulative water-use by Scotts pine trees in a CO2-enriched world of the future will likely be less than or equal to—but no more than—what it is today.

Seven years later, Wang et al. (2005) published a report of a study in which they measured sap flow, crown structure, and microclimatic parameters in order to calculate the transpiration rates of individual 30-year-old Scots pine trees that were maintained for a period of three years in ambient air and air enriched with an extra 350 ppm of CO2 and/or warmed by 2° to 6°C in closed-top chambers constructed within a naturally seeded stand of the trees. As they describe it, the results of this experiment indicated that “(i) elevated CO2 significantly enhanced whole-tree transpiration rate during the first measuring year [by 14%] due to a large increase in whole-tree foliage area, 1998, but reduced it in the subsequent years of 1999 and 2000 [by 13% and 16%, respectively] as a consequence of a greater decrease in crown conductance which off-set the increase in foliage area per tree; (ii) trees growing in elevated temperature always had higher sap flow rates throughout three measuring years [by 54%, 45% and 57%, respectively]; and (iii) the response of sap flow to the combination of elevated temperature and CO2 was similar to that of elevated temperature alone, indicating a dominant role for temperature and a lack of interaction between elevated CO2 and temperature.” These observations suggest that as the air’s CO2 content continues to rise, we probably can expect to see a decrease in evaporative water loss rates from naturally occurring stands of Scots pine trees … unless there is a large concurrent increase in air temperature.

Also working with closed-top chambers that were constructed around 20-year-old Scots pines and fumigated with air containing 350 and 700 ppm CO2 at ambient and elevated (ambient plus 4°C) air temperatures for a period of three years were Peltola et al. (2002), who studied the effects of elevated CO2 and air temperature on stem growth in this coniferous species when it was growing on a soil low in nitrogen. After three years of treatment, they found that cumulative stem diameter growth in the CO2-enriched trees growing at ambient air temperatures was 57 percent greater than that displayed by control trees growing at ambient CO2 and ambient air temperatures, while the trees exposed to elevated CO2 and elevated air temperature exhibited cumulative stem-diameter growth that was 67 percent greater than that displayed by trees exposed to ambient-CO2 air and ambient air temperatures. Consequently, as the air’s CO2 content continues to rise, Scots pine trees will likely respond by increasing stem-diameter growth, even if growing on soils low in nitrogen, and even if air temperatures rise by as much as 4°C.

In a somewhat different type of study, Kainulainen et al. (2003) collected needle litter beneath 22-year-old Scots pines that had been growing for the prior three years in open-top chambers that had been maintained at atmospheric CO2 concentrations of 350 and 600 ppm in combination with ambient and elevated (approximately 1.4 x ambient) ozone concentrations to determine the impacts of these variables on the subsequent decomposition of senesced needles. This they did by enclosing the needles in litterbags and placing the bags within a native litter layer in a Scots pine forest, where decomposition rates were assessed by measuring accumulated litterbag mass loss over a period of 19 months. Interestingly, the three researchers found that exposure to elevated CO2 during growth did not affect subsequent rates of needle decomposition, nor did elevated O3 exposure affect decomposition, nor did exposure to elevated concentrations of the two gases together affect it.

Finally, Bergh et al. (2003) used a boreal version of the process-based BIOMASS simulation model to quantify the individual and combined effects of elevated air temperature (2° and 4°C above ambient) and CO2 concentration (350 ppm above ambient) on the net primary production (NPP) of Scots pine forests growing in Denmark, Finland, Iceland, Norway, and Sweden. This work revealed that air temperature increases of 2° and 4°C led to mean NPP increases of 11 and 20 percent, respectively. However, when the air’s CO2 concentration was simultaneously increased from 350 to 700 ppm, the corresponding mean NPP increases rose to 41 and 55 percent. Last, when the air’s CO2 content was doubled at the prevailing ambient temperature, the mean value of the NPP rose by 27 percent. Consequently, as the air’s CO2 content continues to rise, Ponderosa pines of Denmark, Finland, Iceland, Norway, and Sweden should grow ever more productively; and if air temperature also rises, they will likely grow better still.

Given the above results, as the air’s CO2 content continues to rise, we can expect to see the root systems of Scots pines significantly enhanced, together with the mycorrhizal fungal networks that live in close association with them and help secure the nutrients the trees need to sustain large CO2-induced increases in biomass production. Concurrently, we can expect to see much smaller changes in total evaporative water loss, which means that whole-tree water use efficiency should also be significantly enhanced.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesscots.php.


Spruce


Several studies have recently documented the effects of elevated CO2 on photosynthesis in various varieties of spruce (genus Picea). In the relatively short-term study of Tjoelker et al. (1998a), black spruce (Picea mariana) seedlings grown for three months at atmospheric CO2 concentrations of 580 ppm exhibited photosynthetic rates that were about 28 percent greater than those displayed by control seedlings fumigated with air containing 370 ppm CO2. Similarly, Egli et al. (1998) reported that Norway spruce (Picea abies) seedlings grown at 570 ppm CO2 displayed photosynthetic rates that were 35 percent greater than those exhibited by seedlings grown at 370 ppm. In two branch bag studies conducted on mature trees, it was demonstrated that twice-ambient levels of atmospheric CO2 enhanced rates of photosynthesis in current-year needles by 50 percent in Norway spruce (Roberntz and Stockfors, 1998) and 100 percent in Sitka spruce (Picea sitchensis) (Barton and Jarvis, 1999). Finally, in the four-year open-top chamber study of Murray et al. (2000), the authors reported that Sitka spruce seedlings growing at 700 ppm CO2 exhibited photosynthetic rates that were 19 and 33 percent greater than those observed in control trees growing in ambient air and receiving low and high amounts of nitrogen fertilization, respectively.

Because elevated CO2 enhances photosynthetic rates in spruce species, this phenomenon should lead to increased biomass production in these important coniferous trees. In the short-term three-month study of Tjoelker et al. (1998b), for example, black spruce seedlings receiving an extra 210 ppm CO2 displayed final dry weights that were about 20 percent greater than those of seedlings growing at ambient CO2. Similarly, after growing Sitka spruce for three years in open-top chambers, Centritto et al. (1999) reported that a doubling of the atmospheric CO2 concentration enhanced sapling dry mass by 42 percent.

In summary, it is clear that as the CO2 content of the air increases, spruce trees will likely display enhanced rates of photosynthesis and biomass production, regardless of soil nutrient status. Consequently, rates of carbon sequestration by this abundant coniferous forest species will likely be enhanced.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treesspruce.php.


Tropical


Several studies have recently documented the effects of elevated atmospheric CO2 concentrations on photosynthesis in various tropical and sub-tropical trees. In the relatively short-term study of Lovelock et al. (1999a), for example, seedlings of the tropical tree Copaifera aromatica that were grown for two months at an atmospheric CO2 concentration of 860 ppm exhibited photosynthetic rates that were consistently 50-100 percent greater than those displayed by control seedlings fumigated with air containing 390 ppm CO2. Similarly, Lovelock et al. (1999b) reported that a 10-month 390-ppm increase in the air’s CO2 content boosted rates of net photosynthesis in 30-m tall Luehea seemannii trees by 30 percent. Likewise, in the review paper of Schaffer et al. (1999), it was noted that atmospheric CO2 enrichment had previously been shown to enhance rates of net photosynthesis in a number of tropical and sub-tropical fruit trees, including avocado, banana, citrus, mango, and mangosteen. Even at the ecosystem level, Lin et al. (1998) found that a 1,700-m2 synthetic rainforest mesocosm displayed a 79 percent enhancement in net ecosystem carbon exchange rate in response to a 72 percent increase in the air’s CO2 content.

Because elevated CO2 enhances photosynthetic rates in tropical and sub-tropical trees, it should also lead to increased carbohydrate and biomass production in these species. At a tropical forest research site in Panama, twice-ambient CO2 concentrations enhanced foliar sugar concentrations by up to 30 percent (Wurth et al., 1998), while doubling the foliar concentrations of starch (Lovelock et al., 1998) in a number of tree species. Also, in the study of Hoffmann et al. (2000), elevated CO2 (700 ppm) enhanced dry weights of an “uncut” Brazilian savannah tree species (Keilmeyera coriacea) by about 50 percent, while it enhanced the dry weight of the same “cut” species by nearly 300 percent. Although not specifically quantified, Schaffer et al. (1997) noted that twice-ambient CO2 exposure for one year obviously enhanced dry mass production in two mango ecotypes. Finally, in the six-month study of Sheu et al. (1999), a doubling of the atmospheric CO2 concentration increased seedling dry weight in Schima superba by 14 and 49 percent when grown at ambient and elevated (5°C above ambient) air temperatures, respectively.

It is clear that as the air’s CO2 content rises, tropical and sub-tropical trees will likely display enhanced rates of photosynthesis and biomass production, even under conditions of herbivory and elevated air temperature. Consequently, greater carbon sequestration will also likely occur within earth’s tropical and sub-tropical forests as ever more CO2 accumulates in the atmosphere.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/t/treestropical.php.


Wood Density


Numerous experiments have demonstrated that trees grown in air enriched with CO2 nearly always sequester more biomass in their trunks and branches than do trees grown in ambient air. Several studies also have looked at the effects of elevated CO2 on the density of that sequestered biomass.

Rogers et al. (1983) observed no difference in the wood density of loblolly pine (Pinus taeda) trees grown at 340 and 718 ppm CO2 for 10 weeks; but they found a 33 percent CO2-induced increase in the wood density of sweetgum (Liquidambar styraciflua) trees that were grown at these concentrations for only eight weeks. Doyle (1987) and Telewski and Strain (1987) studied the same two tree species over three growing seasons in air of 350 and 650 ppm CO2, finding no effect of atmospheric CO2 enrichment on the stem density of sweetgum, but a mean increase of 9 percent in the stem density of loblolly pine.

Conroy et al. (1990) grew seedlings of two Pinus radiata families at 340 and 660 ppm CO2 for 114 weeks, finding CO2-induced trunk density increases for the two families of 5.4 and 5.6 percent when soil phosphorus was less than adequate and increases of 5.6 and 1.2 percent when it was non-limiting. In a similar study, Hattenschwiler et al. (1996) grew six genotypes of clonally propagated four-year-old Norway spruce (Picea abies) for three years at CO2 concentrations of 280, 420, and 560 ppm at three different rates of wet nitrogen deposition. On average, they found that wood density was 12 percent greater in the trees grown at the two higher CO2 concentrations than it was in the trees grown at 280 ppm.

Norby et al. (1996) grew yellow poplar or “tulip” trees (Liriodendron tulipifera) at ambient and ambient plus 300 ppm CO2 for three years, during which time the wood density of the trees increased by approximately 7 percent. Tognetti et al. (1998) studied two species of oak tree—one deciduous (Quercus pubescens) and one evergreen (Quercus ilex)—growing in the vicinity of CO2 springs in central Italy that raised the CO2 concentration of the surrounding air by approximately 385 ppm. This increase in the air’s CO2 content increased the wood density of the deciduous oaks by 4.2 percent and that of the evergreen oaks by 6.4 percent.

Telewski et al. (1999) grew loblolly pine trees for four years at ambient and ambient plus 300 ppm CO2. In their study, wood density determined directly from mass and volume measurements was increased by 15 percent by the extra CO2; average ring density determined by X-ray densitometry was increased by 4.5 percent.

Beismann et al. (2002) grew different genotypes of spruce and beech (Fagus sylvatica) seedlings for four years in open-top chambers maintained at atmospheric CO2 concentrations of 370 and 590 ppm in combination with low and high levels of wet nitrogen application on both rich calcareous and poor acidic soils to study the effects of these factors on seedling toughness (fracture characteristics) and rigidity (bending characteristics such as modulus of elasticity). They found that some genotypes of each species were sensitive to elevated CO2, while others were not. Similarly, some were responsive to elevated nitrogen deposition, while others were not. Moreover, such responses were often dependent upon soil type. Averaged across all tested genotypes, however, atmospheric CO2 enrichment increased wood toughness in spruce seedlings grown on acidic soils by 12 and 18 percent at low and high levels of nitrogen deposition, respectively. In addition, atmospheric CO2 enrichment increased this same wood property in spruce seedlings grown on calcareous soils by about 17 and 14 percent with low and high levels of nitrogen deposition, respectively. By contrast, elevated CO2 had no significant effects on the mechanical wood properties of beech seedlings, regardless of soil type.

Finally, Kilpelainen et al. (2003) erected 16 open-top chambers within a 15-year-old stand of Scots pines growing on a nutrient-poor sandy soil of low nitrogen content near the Mekrijarvi Research Station of the University of Joensuu, Finland. Over the next three years they maintained the trees within these chambers in a well-watered condition, while they enriched the air in half of the chambers to a mean daytime CO2 concentration of approximately 580 ppm and maintained the air in half of each of the two CO2 treatments at 2°C above ambient. In the ambient temperature treatment the 60 percent increase in the air’s CO2 concentration increased latewood density by 27 percent and maximum wood density by 11 percent, while in the elevated-temperature treatment it increased latewood density by 25 percent and maximum wood density by 15 percent. These changes led to mean overall CO2-induced wood density increases of 2.8 percent in the ambient-temperature treatment and 5.6 percent in the elevated-temperature treatment.

In light of these several observations, it is clear that different species of trees respond differently to atmospheric CO2 enrichment, and that they respond with still greater variety under different sets of environmental conditions. In general, however, atmospheric CO2 enrichment tends to increase wood density in both seedlings and mature trees more often than not, thereby also increasing a number of strength properties of their branches and trunks.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/w/wooddensity.php.


Forests


Forests contain perennial trees that remove CO2 from the atmosphere during photosynthesis and store its carbon within their woody tissues for decades to periods sometimes in excess of a thousand years. Thus, it is important to understand how increases in the air’s CO2 content affect forest productivity and carbon sequestration, which has a great effect on the rate of rise of the air’s CO2 concentration. In this summary, we review several recent scientific publications pertaining to these subjects.

By examining various properties of tree rings, researchers can deduce how historical increases in the air’s CO2 concentration have already affected tree productivity and water use efficiency. Duquesnay et al. (1998), for example, analyzed the relative amounts of 12C and 13C present in yearly growth rings of beech trees raised in silviculture regimes in northeastern France, determining that their intrinsic water use efficiencies rose by approximately 33 percent during the past century, as the air’s CO2 concentration rose from approximately 280 to 360 ppm. In another case, Rathgeber et al. (2000) used tree-ring density data to create a historical productivity baseline for forest stands of Pinus halepensis in southeastern France, from which they determined that the net productivity of such forests would increase by 8 to 55 percent with a doubling of the air’s CO2 content. Finally, when running a forest growth model based on empirical observations reported in the literature, Lloyd (1999) determined that the rise in the atmospheric CO2 concentration since the onset of the Industrial Revolution likely increased the net primary productivity of mature temperate deciduous forests by about 7 percent. In addition, he determined that a proportional increase in anthropogenic nitrogen deposition likely increased forest net primary productivity by 25 percent. And when he combined the two effects, the net primary productivity stimulation rose to 40 percent, which is more than the sum of the individual growth enhancements resulting from the increases in CO2 and nitrogen.

The results of these studies demonstrate that historic increases in the air’s CO2 content have already conferred great benefits upon earth’s forests. But will future increases in the air’s CO2 concentration continue to do so? Several research teams have embarked on long-term studies of various forest communities in an attempt to address this important question. What follows are some important observations that have been made from their mostly ongoing CO2-enrichment studies.

In 1996, circular FACE plots (30-m diameter) receiving atmospheric CO2 concentrations of 360 and 560 ppm were established in a 15-year-old loblolly pine (Pinus taeda) plantation in North Carolina, USA, to study the effects of elevated CO2 on the growth and productivity of this particular forest community, which also had several hardwood species present in the understory beneath the primary coniferous canopy. Using this experimental set-up as a platform for several experiments, Hymus et al. (1999) reported that net photosynthetic rates of CO2-enriched loblolly pines trees were 65 percent greater than rates observed in control trees exposed to ambient air. These greater rates of carbon fixation contributed to the 24 percent greater growth rates observed in the CO2-enriched pine trees in the first year of this long-term study (Naidu and DeLucia 1999). In addition, DeLucia and Thomas (2000) reported that the elevated CO2 increased rates of net photosynthesis by 50 to 160 percent in four subdominant hardwood species present in the forest understory. Moreover, for one species—sweetgum (Liquidambar styraciflua)—the extra CO2 enhanced rates of net photosynthesis in sun and shade leaves by 166 and 68 percent, respectively, even when the trees were naturally subjected to summer seasonal stresses imposed by high temperature and low soil water availability. Consequently, after two years of atmospheric CO2 enrichment, total ecosystem net primary productivity in the CO2-enriched plots was 25 percent greater than that measured in control plots fumigated with ambient air.

In a similar large-scale study, circular (25-m diameter) FACE plots receiving atmospheric CO2 concentrations of 400 and 530 ppm were constructed within a 10-year-old sweetgum plantation in Tennessee, USA, to study the effects of elevated CO2 on the growth and productivity of this forest community. After two years of treatment, Norby et al. (2001) reported that the modest 35 percent increase in the air’s CO2 content boosted tree biomass production by an average of 24 percent. In addition, Wullschleger and Norby (2001) noted that CO2-enriched trees displayed rates of transpirational water loss that were approximately 10 percent lower than those exhibited by control trees grown in ambient air. Consequently, elevated CO2 enhanced seasonal water use efficiencies of these mature sweetgum trees by 28 to 35 percent.

On a smaller scale, Pritchard et al. (2001) constructed idealized ecosystems (containing five different species) representative of regenerating longleaf pine (Pinus palustris Mill.) communities of the southeastern USA, fumigating them for 18 months with air containing 365 and 720 ppm CO2 to study the effects of elevated CO2 on this forest community. They reported that elevated CO2 increased the above- and below-ground biomass of the dominant longleaf pine individuals by 20 and 62 percent, respectively. At the ecosystem level, elevated CO2 stimulated total above-ground biomass production by an average of 35 percent. Similar results for regenerating temperate forest communities have been reported by Berntson and Bazzaz (1998), who documented a 31 percent increase in Transition Hardwood-White Pine-Hemlock forest mesocosm biomass in response to two years of fumigation with twice-ambient concentrations of atmospheric CO2.

While previously assumed that forests reached their maximum productivity at an intermediate age, changing from carbon sinks to carbon sources in mature and old-growth stands, a study by Zhou et al. (2011) follows along the line of the above research by proving otherwise. The researchers tested the change in carbon sequestration from 1981 to 2010 of mixed, coniferous and birch forests on Changbai Mountain in China. Their results show that these forests experienced increases in above-ground, below-ground and total carbon densities by 84, 29 and 55 tons of carbon per hectare, respectively. Contrary to what had long been believed by many, Zhou et al. say their findings are "consistent with the suggestion of Luyssaert et al. (2008) and Zhou et al. (2006) that old-growth forests can continue to accumulate carbon” as they age.

Authors Peng et al. (2009) set out to validate the process-based TRIPLEX model of forest growth and carbon and nitrogen cycling by comparing it against observed data. Based on their anaylsis, the authors reported that "the model validation results show that the simulated tree total volume, net primary productivity (NPP), total biomass and soil carbon are consistent with observed data across the Northeast of China, demonstrating that the improved TRIPLEX model is able to simulate forest growth and carbon dynamics.” Peng et al. proceeded to use the calibrated model to investigate the potential impacts of projected increases in atmospheric CO2 content on the climate of northeast China. The model indicates that climate change would increase forest NPP and biomass carbon but would decrease overall soil carbon under all three of the climate change scenarios they studied, but that "the combined effects of climate change and CO2 fertilization on the increase of NPP were estimated to be 10-12% for [the] 2030s and 28-37% in [the] 2090s," because "the simulated effects of CO2 fertilization significantly offset the soil carbon loss due to climate change alone."These findings led the scientists to conclude that future climate change and increasing atmospheric CO2 would have a significant beneficial impact on the forest ecosystems of Northeastern China. These results are consistent with recent experiments in temperate forests in North America and Europe.

It is clear that as the air’s CO2 concentration continues to rise, forests will likely respond by exhibiting significant increases in total primary productivity and biomass production. Consequently, forests will likely grow much more robustly and significantly expand their ranges, as has already been documented in many parts of the world, including gallery forest in Kansas, USA (Knight et al., 1994) and the Budal and Sjodal valleys in Norway (Olsson et al., 2000). Such CO2-induced increases in growth and range expansion should result in large increases in global carbon sequestration within forests.

Additional information on this topic, including reviews on forests not discussed here, can be found at http://www.co2science.org/subject/f/subject_f.php under the heading Forests.

Aquatic Plants

We have shown how atmospheric CO2 enrichment typically enhances the growth and productivity of nearly all terrestrial plants. But what about aquatic plants? In this section we seek to answer that question.


Freshwater Algae


How do freshwater algae respond to increases in the air’s CO2 content? The subject has not been thoroughly researched, but the results of the studies discussed below provide a glimpse of what the future may hold as the atmosphere’s CO2 concentration continues its upward course.

Working with cells of the freshwater alga Chlorella pyrenoidosa, Xia and Gao (2003) cultured them in Bristol’s solution within controlled environment chambers maintained at low and high light levels (50 and 200 µmol/m²/s) during 12-hour light periods that were followed by 12-hour dark periods for a total of 13 days, while the solutions in which the cells grew were continuously aerated with air of either 350 or 700 ppm CO2. When the cells were harvested (in the exponential growth phase) at the conclusion of this period, the biomass (cell density) of the twice-ambient CO2 treatment was found to be 10.9 percent and 8.3 percent greater than that of the ambient-air treatment in the low- and high-light regimes, respectively, although only the high-light result was statistically significant. The two scientists concluded from these observations that a “doubled atmospheric CO2 concentration would affect the growth of C. pyrenoidosa when it grows under bright solar radiation, and such an effect would increase by a great extent when the cell density becomes high.” Their data also suggest the same may well be true when the alga grows under not-so-bright conditions.

Working on a much larger scale “in the field” with six 1.5-m-diameter flexible plastic cylinders placed in the littoral zone of Lake Hampen in central Jutland, Denmark (three maintained at the ambient CO2 concentration of the air and three enriched to 10 times the ambient CO2 concentration), Andersen and Andersen (2006) measured the CO2-induced growth response of a mixture of several species of filamentous freshwater algae dominated by Zygnema species, but containing some Mougeotia and Spirogyra. After one full growing season (May to November), they determined that the biomass of the microalgal mixture in the CO2-enriched cylinders was increased by 220 percent in early July, by 90 percent in mid-August, and by a whopping 3,750 percent in mid-November.

In another study of the subject, Schippers et al. (2004a) say “it is usually thought that unlike terrestrial plants, phytoplankton will not show a significant response to an increase of atmospheric CO2,” but they note, in this regard, that “most analyses have not examined the full dynamic interaction between phytoplankton production and assimilation, carbon-chemistry and the air-water flux of CO2,” and that “the effect of photosynthesis on pH and the dissociation of carbon (C) species have been neglected in most studies.”

In an attempt to rectify this situation, Schippers et al. developed “an integrated model of phytoplankton growth, air-water exchange and C chemistry to analyze the potential increase of phytoplankton productivity due to an atmospheric CO2 elevation,” and as a test of their model, they let the freshwater alga Chlamydomonas reinhardtii grow in 300-ml bottles filled with 150 ml of a nutrient-rich medium at enclosed atmospheric CO2 concentrations of 350 and 700 ppm that they maintained at two air-water exchange rates characterized by CO2 exchange coefficients of 2.1 and 5.1 m day-1, as described by Shippers et al. (2004b), while periodically measuring the biovolume of the solutions by means of an electronic particle counter. The results of this effort, as they describe it, “confirm the theoretical prediction that if algal effects on C chemistry are strong, increased phytoplankton productivity because of atmospheric CO2 elevation should become proportional to the increased atmospheric CO2,” which suggests that algal productivity “would double at the predicted increase of atmospheric CO2 to 700 ppm.” Although they note that “strong algal effects (resulting in high pH levels) at which this occurs are rare under natural conditions,” they still predict that effects on algal production in freshwater systems could be such that a “doubling of atmospheric CO2 may result in an increase of the productivity of more than 50%.”

In the last of the few papers we have reviewed in this area, Logothetis et al. (2004) note that “the function and structure of the photosynthetic apparatus of many algal species resembles that of higher plants (Plumley and Smidt, 1984; Brown, 1988; Plumley et al., 1993),” and that “unicellular green algae demonstrate responses to increased CO2 similar to those of higher plants in terms of biomass increases (Muller et al., 1993).” However, they also note that “little is known about the changes to their photosynthetic apparatus during exposure to high CO2,” which deficiency they began to correct via a new experiment, wherein batches of the unicellular green alga Scenedesmus obliquus (wild type strain D3) were grown autotrophically in liquid culture medium for several days in a temperature-controlled water bath of 30°C at low (55 µmol m-2 s-1) and high (235 µmol m-2 s-1) light intensity while they were continuously aerated with air of either 300 or 100,000 ppm CO2. This protocol revealed that exposure to the latter high CO2 concentration produces, in their words, a “reorganization of the photosynthetic apparatus” that “leads to enhanced photosynthetic rates, which … leads to an immense increase of biomass.” After five days under low light conditions, for example, the CO2-induced increase in biomass was approximately 300 percent, while under high light conditions it was approximately 600 percent.

Based on these few observations, it is not possible to draw any sweeping conclusions about the subject. However, they do indicate there may be a real potential for the ongoing rise in the air’s CO2 content to significantly stimulate the productivity of this freshwater contingent of earth’s plants.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/aquaticplants.php.


Freshwater Macrophytes


In this section we discuss the findings of papers that investigate the influence of CO2 concentrations as it applies to submersed, floating, and emergent freshwater macrophytes, beginning with studies of aquatic plants that live their lives totally submersed in freshwater environments.

For several multi-week periods, Idso (1997) grew specimens of corkscrew vallisneria (Vallisneria tortifolia) in several 10- and 29-gallon glass tanks (containing 10-cm bottom-layers of common aquarium gravel) that were filled with tap water maintained within 0.5°C of either 18.2°C or 24.5°C, while the semi-sealed air spaces above these “Poor Man’s Biospheres,” as he christened them, were maintained at a number of different CO2 concentrations. With the harvesting of plants at the end of the study, this protocol revealed that the CO2-induced growth enhancement of the plants was linear (in contrast to the gradually declining CO2-induced growth enhancements typically exhibited by most terrestrial plants as the air’s CO2 content climbs ever higher), and that the linear relationship extended to the highest atmospheric CO2 concentration studied: 2,100 ppm. In addition, he found that the CO2-induced growth increase experienced by the plants in the higher of the two water temperature treatments (a 128 percent increase in going from an atmospheric CO2 concentration of 365 ppm to one of 2,100 ppm) was 3.5 times greater than that of the plants in the lower water temperature treatment. Although this response may seem rather dramatic, it is not unique; Idso reports that Titus et al. (1990), who studied the closely related Vallisneria americana, “observed that the biomass of their experimental plants also rose linearly with the CO2 content of the air above the water within which they grew, and that [it] did so from the value of the [then] current global mean (365 ppm) to a concentration fully ten times larger.”

In another study of a closely allied species, Yan et al. (2006) collected turions of Vallisneria spinulosa from Liangzi Lake, Hubei Province, China, and planted them in tanks containing 15-cm-deep layers of fertile lake sediments, topped with 40 cm of lake water, that were placed in two glasshouses—one maintained at the ambient atmospheric CO2 concentration of 390 ppm and the other maintained at an elevated concentration of 1,000 ppm—where the plants grew for a period of 120 days, after which they were harvested and the dry weights of their various organs determined. As they describe it, this work indicated that the “total biomass accumulation of plants grown in the elevated CO2 was 2.3 times that of plants grown in ambient CO2, with biomass of leaves, roots and rhizomes increasing by 106%, 183% and 67%, respectively.” Most spectacularly of all, they report that “turion biomass increased 4.5-fold,” because “the mean turion numbers per ramet and mean biomass per turion in elevated CO2 were 1.7-4.3 and 1.9-3.4 times those in ambient CO2.”

In Denmark, in a study of small slow-growing evergreen perennials called isoetids that live submersed along the shores of numerous freshwater lakes, Andersen et al. (2006) grew specimens of Littorella uniflora in sediment cores removed from Lake Hampen in 75-liter tanks with 10-cm overburdens of filtered lake water for a period of 53 days, while measuring various plant, water, and sediment properties, after which they destructively harvested the plants and measured their biomass. Throughout this period, half of the tanks had ambient air bubbled through their waters, while the other half were similarly exposed to a mixture of ambient air and pure CO2 that produced a 10-fold increase in the air’s CO2 concentration. This ultra-CO2-enrichment led to a 30 percent increase in plant biomass, as well as “higher O2 release to the sediment which is important for the cycling and retention of nutrients in sediments of oligotrophic softwater lakes.” And when the ultra-CO2-enrichment was maintained for an entire growing season (May-November), Andersen and Andersen (2006) report that the 10-fold increase in aquatic CO2 concentration enhanced the biomass production of Littorella uniflora by a much larger 78 percent.

In a study of an “in-between” type of plant that has submersed roots and rhizomes that are anchored in water-body sediments, but which has floating leaves on the surface of the water and emergent flowers that protrude above the water surface, Idso et al. (1990) grew water lilies (Nymphaea marliac) for two consecutive years in sunken metal stock tanks located out-of-doors at Phoenix, Arizona (USA) and enclosed within clear-plastic-wall open-top chambers through which air of either 350 or 650 ppm CO2 was continuously circulated. This work revealed that in addition to the leaves of the plants being larger in the CO2-enriched treatment, there were 75 percent more of them than there were in the ambient-air tanks at the conclusion of the initial five-month-long growing season. Each of the plants in the high-CO2 tanks also produced twice as many flowers as the plants growing in ambient air; and the flowers that blossomed in the CO2-enriched air were more substantial than those that bloomed in the air of ambient CO2 concentration: they had more petals, the petals were longer, and they had a greater percent dry matter content, such that each flower consequently weighed about 50 percent more than each flower in the ambient-air treatment. In addition, the stems that supported the flowers were slightly longer in the CO2-enriched tanks, and the percent dry matter contents of both the flower and leaf stems were greater, so that the total dry matter in the flower and leaf stems in the CO2-enriched tanks exceeded that of the flower and leaf stems in the ambient-air tanks by approximately 60 percent.

Just above the surface of the soil that covered the bottoms of the tanks, there were also noticeable differences. Plants in the CO2-enriched tanks had more and bigger basal rosette leaves, which were attached to longer stems of greater percent dry matter content, which led to the total biomass of these portions of the plants being 2.9 times greater than the total biomass of the corresponding portions of the plants in the ambient-air tanks. In addition, plants in the CO2-enriched tanks had more than twice as many unopened basal rosette leaves.

The greatest differences of all, however, were hidden within the soil that covered the bottoms of the stock tanks. When half of the plants were harvested at the conclusion of the first growing season, for example, the number of new rhizomes produced over that period was discovered to be 2.4 times greater in the CO2-enriched tanks than it was in the ambient-air tanks; the number of major roots produced there was found to be 3.2 times greater. As with all other plant parts, the percent dry matter contents of the new roots and rhizomes were also greater in the CO2-enriched tanks. Overall, therefore, the total dry matter production within the submerged soils of the water lily ecosystems was 4.3 times greater in the CO2-enriched tanks than it was in the ambient-air tanks; the total dry matter production of all plant parts—those in the submerged soil, those in the free water, and those in the air above—was 3.7 times greater in the high-CO2 enclosures.

Over the second growing season, the growth enhancement in the high-CO2 tanks was somewhat less; but the plants in those tanks were so far ahead of the plants in the ambient-air tanks that in their first five months of growth, they produced what it took the plants in the ambient-air tanks fully 21 months to produce.

Moving on to plants that are exclusively floating freshwater macrophytes, Idso (1997) grew many batches of the common water fern (Azolla pinnata) over a wide range of atmospheric CO2 concentrations at two different water temperatures (18.2°C and 24.5°C) in Poor Man’s Biospheres for periods of several weeks. This work revealed that a 900-ppm increase in the CO2 concentration of the air above the tanks led to only a 19 percent increase in the biomass production of the plants floating in the cooler water, but that it led to a 66 percent biomass increase in the plants floating in the warmer water.

In an earlier study of Azolla pinnata, Idso et al. (1989) conducted three separate two- to three-month experiments wherein they grew batches of the floating fern out-of-doors in adequately fertilized water contained in sunken metal stock tanks located within clear-plastic-wall open-top chambers that were continuously maintained at atmospheric CO2 concentrations of either 340 or 640 ppm, during which time the plants were briefly removed from the water and weighed at weekly intervals, while their photosynthetic rates were measured at hourly intervals from dawn to dusk on selected cloudless days. As a result of this protocol, they found the photosynthetic and growth rates of the plants growing in ambient air “first decreased, then stagnated, and finally became negative when mean air temperature rose above 30°C.” In the high CO2 treatment, on the other hand, they found that “the debilitating effects of high temperatures were reduced: in one case to a much less severe negative growth rate, in another case to merely a short period of zero growth rate, and in a third case to no discernible ill effects whatsoever—in spite of the fact that the ambient treatment plants in this instance all died.”

Last, in a study of an emergent freshwater macrophyte, Ojala et al. (2002) grew water horsetail (Equisetum fluviatile) plants at ambient and double-ambient atmospheric CO2 concentrations and ambient and ambient + 3°C air temperatures for three years, although the plants were subjected to the double-ambient CO2 condition for only approximately five months of each year. This work revealed that the increase in air temperature boosted maximum shoot biomass by 60 percent, but the elevated CO2 had no effect on this aspect of plant growth. However, elevated CO2 and temperature—both singly and in combination—positively affected root growth, which was enhanced by 10, 15, and 25 percent by elevated air temperature, CO2, and the two factors together, respectively.

In light of the several experimental findings discussed above, we conclude that the ongoing rise in the air’s CO2 content will likely have significant positive impacts on most freshwater macrophytes, including submersed, floating, and emergent species.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/aquaticmacrophytes.php.


Marine Macroaglae


How do marine macroalgae respond to increases in the air’s CO2 content? The results of the studies discussed below provide a glimpse of what the future may hold in this regard, as the atmosphere’s CO2 concentration continues its upward climb.

Gao et al. (1993) grew cultures of the red macroalgae Gracilaria sp. and G. chilensis in vessels enriched with nitrogen and phosphorus that were continuously aerated with normal air containing 350 ppm CO2, air enriched with an extra 650 ppm CO2, and air enriched with an extra 1,250 ppm CO2 for a period of 19 days. Compared to the control treatments, the relative growth enhancements in the + 650-ppm and +1250-ppm CO2 treatments were 20 percent and 60 percent, respectively, for G. chilensis, and 130 percent and 190 percent, respectively, for the Gracilaria sp.

With respect to these findings, the researchers comment that “in their natural habitats, photosynthesis and growth of Gracilaria species are likely to be CO2-limited, especially when the population density is high and water movement is slow.” Hence, as the air’s CO2 content continues to rise, these marine macroalgae should grow ever better in the years ahead. Such should also be the case with many other macroalgae, for Gao et al. note that “photosynthesis by most macroalgae is probably limited by inorganic carbon sources in natural seawater,” citing the studies of Surif and Raven (1989), Maberly (1990), Gao et al. (1991), and Levavasseur et al. (1991) as evidence for this statement.

In a subsequent study, Kubler et al. (1999) grew Lomentaria articulata, a red seaweed common to the Northeast Atlantic intertidal zone, for three weeks in hydroponic cultures subjected to various atmospheric CO2 and O2 concentrations. In doing so, they found that oxygen concentrations ranging from 10 to 200 percent of ambient had no significant effect on either the seaweed’s daily net carbon gain or its total wet biomass production rate. By contrast, CO2 concentrations ranging from 67 to 500 percent of ambient had highly significant effects on these parameters. At twice the ambient CO2 concentration, for example, daily net carbon gain and total wet biomass production rates were 52 and 314 percent greater than they were at ambient CO2.

More recently, Zou (2005) collected specimens of the brown seaweed Hizikia fusiforme from intertidal rocks along the coast of Nanao Island, Shantou, China, and maintained them in glass aquariums that contained filtered seawater enriched with 60 µM NaNO3 and 6.0 µM NaH2PO4, while continuously aerating the aquariums with air of either 360 or 700 ppm CO2 and periodically measuring seaweed growth and nitrogen assimilation rates, as well as nitrate reductase activities. By these means they determined that the slightly less than a doubling of the air’s CO2 concentration increased the seaweed’s mean relative growth rate by about 50 percent, its mean rate of nitrate uptake during the study’s 12-hour light periods by some 200 percent, and its nitrate reductase activity by approximately 20 percent over a wide range of substrate nitrate concentrations.

As a subsidiary aspect of the study, Zou notes that “the extract of H. fusiforme has an immunomodulating activity on humans and this ability might be used for clinical application to treat several diseases such as tumors (Suetsuna, 1998; Shan et al., 1999).” He also reports that the alga “has been used as a food delicacy and an herbal ingredient in China, Japan and Korea.” In fact, he says that it “is now becoming one of the most important species for seaweed mariculture in China, owing to its high commercial value and increasing market demand.” The ongoing rise in the air’s CO2 content bodes well for all of these applications. In addition, Zou notes that “the intensive cultivation of H. fusiforme would remove nutrients more efficiently with the future elevation of CO2 levels in seawater, which could be a possible solution to the problem of ongoing coastal eutrophication,” suggesting that rising CO2 levels may also assist in the amelioration of this environmental problem.

In light of these several observations, the ongoing rise in the air’s CO2 content should help marine macroalgae to become more productive with the passage of time.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/aquaticmacroalgae.php.


Marine Microalgae


How do marine microalgae respond to increases in the air’s CO2 content? Based on the late twentieth century work of Riebesell et al. (1993), Hein and Sand-Jensen (1997), and Wolf-Gladrow et al., (1999), it would appear that the productivity of earth’s marine microalgae may be significantly enhanced by elevated concentrations of atmospheric CO2. More recent work by other researchers suggests the same.

In a study of the unicellular marine diatom Skeletonema costatus, which is widely distributed in coastal waters throughout the world and is a major component of most natural assemblages of marine phytoplankton, Chen and Gao (2004) grew cell cultures of the species in filtered nutrient-enriched seawater maintained at 20°C under a light/dark cycle of 12/12 hours at a light intensity of 200 µmol m-2 s-1, while continuously aerating the culture solutions with air of either 350 or 1,000 ppm CO2 and measuring a number of physiological parameters related to the diatom’s photosynthetic activity. They report that cell numbers of the alga “increased steadily throughout the light period and they were 1.6 and 2.1 times higher after the 12 h light period for the alga grown at 350 and 1000 ppm CO2, respectively.” They also say that chlorophyll a concentrations “increased 4.4- and 5.4-fold during the middle 8 h of the light period for the alga grown at 350 and 1000 ppm CO2, respectively,” and that “the contents of cellular chlorophyll a were higher for the alga grown at 1000 ppm CO2 than that at 350 ppm CO2.” In addition, they note that the initial slope of the light saturation curve of photosynthesis and the photochemical efficiency of photosystem II “increased with increasing CO2, indicating that the efficiency of light-harvesting and energy conversion in photosynthesis were increased.” The end result of these several responses, in the words of Chen and Gao, was that “S. costatum benefited from CO2 enrichment.”

In another report of a study of marine microalgae that would appear to have enormous implications, Gordillo et al. (2003) begin by noting that “one of the main queries for depicting future scenarios of evolution of atmospheric composition and temperature is whether an atmospheric CO2 increase stimulates primary production, especially in aquatic plants.” Why do they say that? They say it because, as they put it, “aquatic primary producers account for about 50 percent of the total carbon fixation in the biosphere (Falkowski and Raven, 1997).”

Although the question addressed by Gordillo et al. sounds simple enough, its answer is not straightforward. In many phytoplankton, both freshwater and marine, photosynthesis appears to be saturated under current environmental conditions. Raven (1991), however, has suggested that those very same species, many of which employ carbon-concentrating mechanisms, could well decrease the amount of energy they expend in this latter activity in a CO2-enriched world, which metabolic readjustment would leave a larger proportion of their captured energy available for fueling enhanced growth.

To explore this possibility, the four researchers studied various aspects of the growth response of the microalgal chlorophyte Dunaliella viridis (which possesses a carbon concentrating mechanism and has been used as a model species for the study of inorganic carbon uptake) to atmospheric CO2 enrichment. Specifically, they batch-cultured the chlorophyte, which is one of the most ubiquitous eukaryotic organisms in hypersaline environments, in 250-ml Perspex cylinders under controlled laboratory conditions at high (5 mM) and low (0.5 mM) nitrate concentrations, while continuously aerating half of the cultures with ambient air of approximately 350 ppm CO2 and the other half with air of approximately 10,000 ppm CO2. In doing so, they discovered that atmospheric CO2 enrichment had little effect on dark respiration in both N treatments. Likewise, it had little effect on photosynthesis in the low-N treatment. In the high-N treatment, the extra CO2 increased photosynthesis by 114 percent. In the case of biomass production, the results were even more extreme: in the low-N treatment elevated CO2 had no effect at all, while in the high-N treatment it nearly tripled the cell density of the culture solution.

In discussing their findings, Gordillo et al. note that “it has long been debated whether phytoplankton species are growth-limited by current levels of CO2 in aquatic systems, i.e. whether an increase in atmospheric CO2 could stimulate growth (Riebesell et al., 1993).” Their results clearly indicate that it can, as long as sufficient nitrogen is available. But that was not all that Gordillo et al. learned. In the high-N treatment, where elevated CO2 greatly stimulated photosynthesis and biomass production, once the logarithmic growth phase had run its course and equilibrium growth was attained, approximately 70 percent of the carbon assimilated by the chlorophyte was released to the water, while in the low-CO2 treatment only 35 percent was released.

With respect to this suite of observations, Gordillo et al. say “the release of organic carbon to the external medium has been proposed as a mechanism for maintaining the metabolic integrity of the cell (Ormerod, 1983),” and that “according to Wood and Van Valen (1990), organic carbon release would be a sink mechanism protecting the photosynthetic apparatus from an overload of products that cannot be invested in growth or stored.” They additionally state that stores of photosynthetic products “are reduced to avoid overload and produce a high demand for photosynthates.” Under these conditions, they conclude that “the process would then divert assimilated C to either the production of new biomass, or the release to the external medium once the culture conditions do not allow further exponential growth.”

A second consequence of enhanced organic carbon release in the face of atmospheric CO2 enrichment and sufficient N availability is that the internal C:N balance of the phytoplankton is maintained within a rather tight range. This phenomenon has also been observed in the green seaweed Ulva rigida (Gordillo et al., 2001) and the cyanobacterium Spirulina platensis (Gordillo et al., 1999). Hence, what the study of Gordillo et al. implies about the response of Dunaliella viridis to atmospheric CO2 enrichment may well be widely applicable to many, if not most, aquatic plants, not the least of which may be the zooxanthellae that by this means (enhanced organic carbon release) could provide their coral hosts with the source of extra energy they need to continue building their skeletons at a non-reduced rate in the face of the negative calcification pressure produced by the changes in seawater chemistry that have been predicted to result from the ongoing rise in the air’s CO2 concentration.

In light of these several observations, there would appear to be ample reason to be optimistic about the response of earth’s marine macroalgae to the ongoing rise in the air’s CO2 content.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/a/aquaticmicroalgae.php.


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