Other Plant Responses to Atmospheric CO2 Enrichment
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Contents |
Transpirations and Water Use Efficiency
In a study of marsh plants, Li et al. (2010) write, “wetlands evapotranspire more water than other ecosystems, including agricultural, forest and grassland ecosystems,” but the “effects of elevated atmospheric carbon dioxide concentration on wetland evapotranspiration (ET) are largely unknown.” In an effort to help fill this knowledge void, they present the results of “twelve years of measurements of ET, net ecosystem CO2 exchange (NEE), and ecosystem water use efficiency (EWUE, i.e., NEE/ET) at 13:00–15:00 hours in July and August for a Scirpus olneyi (C3 sedge) community and a Spartina patens (C4 grass) community exposed to ambient and elevated (ambient + 340 ppm) CO2 in a Chesapeake Bay wetland.”
The results of their study indicate “although a decrease in stomatal conductance at elevated CO2 in the S. olneyi community was counteracted by an increase in leaf area index (LAI) to some extent, ET was still reduced by 19% on average over 12 years,” while “in the S. patens community, LAI was not affected by elevated CO2 and the reduction of ET was 34%.” They found NEE “was stimulated about 36% at elevated CO2 in the S. olneyi community but was not significantly affected by elevated CO2 in the S. patens community.” Merging the ET and NEE responses of the two species, they determined “EWUE was increased about 83% by elevated CO2 in both the S. olneyi and S. patens communities.”
Li et al. conclude rising concentrations of atmospheric CO2 “could have significant impacts on the hydrologic cycles of coastal wetlands,” noting “reduced ET could increase carbon uptake by mitigating the effects of drought on carbon uptake (Rasse et al., 2005),” and it “could also facilitate ground water recharge to counteract salinity intrusion in coastal areas caused by rising sea levels from global warming.” They state salinity intrusion has been identified as “a serious problem in the United States” based on the work of Stevenson et al. (1988) and Day et al. (2000).
Mateos-Naranjo et al. (2010) also worked with a marsh plant—S. maritima, which they obtained from a low-marsh site along the southwest coast of Spain. They watered clumps of the plant with Hoagland’s solution of three different salinities (0, 170, or 510 mM NaCl), conducting an experiment with the plants in controlled environment chambers maintained at atmospheric CO2 concentrations of either 380 or 700 ppm for periods of 30 days, during which time they measured several plant properties and processes. They found the 84 percent increase in the atmosphere’s CO2 concentration stimulated the growth of S. maritima by about 65 percent in all three salinity treatments, while the halophyte’s water use efficiency was increased by about 10 percent, 100 percent, and 160 percent in the 0, 170, and 510 mM salinity treatments, respectively. They conclude, “increasing CO2 concentration has a positive effect on the photochemical apparatus, helping to counteract salt stress experienced by plants at current CO2 concentrations.”
In another recent study, Shimono et al. (2010) write, “by 2050, the world’s population will have increased by about 37%, from the current level of 6.7 billion to an estimated 9.2 billion (UN, 2009), with a corresponding increase in global food demand.” They also state “about 0.6 billion Mg of rice is produced annually from an area of 1.5 million km2, making rice one of the most important crops for supporting human life.” As noted by Pritchard and Amthor (2005), rice supplies the planet’s human population with an estimated 20 percent of its energy needs (on a caloric basis) and 14 percent of its protein requirements (on a weight basis).
The six scientists further note “rice production depends heavily on water availability,” as “irrigated lowlands account for 55% of the total area of harvested rice and typically produce two to three times the crop yield of rice grown under non-irrigated conditions (IRRI, 2002).” And because the demand for ever-greater quantities of water will continue to rise due to the need to feed growing numbers of people, “efficient use of water will thus be essential for future rice production.”
In an attempt to determine how agriculture may be affected by the ongoing rise in the air’s CO2 content, the Japanese researchers conducted a two-year free-air CO2 enrichment (FACE) study in fields at Shizukuishi, Iwate (Japan) to learn how elevated CO2 may reduce crop water use via its impact on the leaf stomatal conductance (gs) of three varieties of rice (Oryza sativa L.): early-maturing Kirara397, intermediate-maturing Akitakomachi, and latest-maturing Hitomebore.
In response to the 53 percent increase in daytime atmospheric CO2 concentration employed in their experiments, Shimono et al. report, “the reduction in gs due to elevated CO2 was similar across measurements, averaging around 20% in the morning, 24% around noon and 23% in the afternoon across all growth stages.” They add “there was no significant CO2 x cultivar interaction.” Consequently, with the concomitant increase in grain yield that also results from atmospheric CO2 enrichment, it should be apparent that the ongoing rise in the air’s CO2 content will play a major role in enabling a growing world population to meet its food needs.
Moving on from crops to grasslands, we consider Barbosa et al. (2010), who studied an alpine grassland in Switzerland. Citing the principle expressed by DeNiro and Epstein (1978), who coined the phrase “you are what you eat isotopically,” they decided to use the horns of numerous deceased alpine ibex (Capra ibex) because they are composed of yearly growth layers that possess a temporal archive of the δ13C values of the alpine grassland plants the animals ate while they were alive. The researchers were given access by the Natural History Museum of Bern to the horns of 24 males that had lived in the grassland they were studying. These horns covered the period from 1938 to 2006 and provided a total of 233 yearly δ13C data points.
From information obtained from the ibex horns, Barbosa et al. determined the intrinsic water-use efficiency (iWUE) of the alpine grassland plants had increased by approximately 18 percent over the 69-year period from 1938 to 2006, when the atmosphere’s CO2 concentration rose by about 23 percent. Between 1955 and 2006, however, meteorological data indicate the vapor pressure deficit (or evaporative demand) of the air in their study area had risen by about 0.1 kPa, just enough to offset the iWUE benefit provided by the rise in the air’s CO2 content. Thus, although the net effect of the increase in the air’s CO2 content (which tended to reduce plant water loss) and the increase in the air’s dryness (which tended to enhance plant water loss) resulted in no net change in plant iWUE, it can be appreciated that had the air’s CO2 content not risen over the period in question, the alpine plants would have fared far worse than they did in reality.
Moving on to trees, we begin with Silva et al. (2009), who studied Araucaria angustifolia, which they describe as “an indigenous conifer tree restricted to the southern region of South America that plays a key role in the dynamics of regional ecosystems where forest expansion over grasslands has been observed.” Working with various types of tree-ring data obtained from trees growing in both forest and grassland sites in southern Brazil, they compared changes in intrinsic water use efficiency (iWUE)—which they defined as the ratio of the rate of CO2 assimilation by the trees’ needles to their stomatal conductance—with historical changes in temperature, precipitation, and atmospheric CO2 concentration that occurred in the region over the past century.
The four researchers report that during the past several decades, “iWUE increased over 30% in both habitats,” and “this increase was highly correlated with increasing levels of CO2 in the atmosphere.” Over this latter period, however, tree growth remained rather stable, due to lower-than-normal precipitation and higher-than-normal temperatures, which would normally tend to depress the growth of this species: Katinas and Crisci (2008) describe A. angustifolia as being “intolerant of dry seasons and requiring cool temperatures.” Therefore, Silva et al. conclude the “climatic fluctuations during the past few decades,” which would normally be expected to have been deleterious to the growth of A. angustifolia, appear to have had their growth-retarding effects “compensated by increases in atmospheric CO2 and changes [i.e., increases] in iWUE.”
Regarding global water scarcity, Kummu et al. (2010) write, “due to the rapidly increasing population and water use per capita in many areas of the world, around one third of the world’s population currently lives under physical water scarcity (e.g. Vorosmarty et al., 2000; Alcamo et al., 2003; Oki and Kanae, 2006).” They note that despite the large number of water scarcity studies conducted over the years, “no global assessment is available of how this trend has evolved over the past several centuries to millennia.” To fill this void, Kummu et al. conducted a study of AD 0 to 2005. This analysis was carried out for ten different time slices, defined as those times at which the human population of the globe was approximately double the population of the previous time slice. Global population data for these analyses were derived from the 5’ latitude x 5’ longitude-resolution global HYDE dataset of Klein Goldewijk (2005) and Klein Goldewijk et al. (2010), and evaluation of water resources availability over the same period was based on monthly temperature and precipitation output from the climate model ECBilt-CLIO-VECODE, as calculated by Renssen et al. (2005).
These operations indicated “moderate water shortage first appeared around 1800, but it commenced in earnest from about 1900, when 9% of the world population experienced water shortage, of which 2% was under chronic water shortage (<1000 m3/capita/year).” Thereafter, from 1960 onwards, “water shortage increased extremely rapidly, with the proportion of global population living under chronic water shortage increasing from 9% (280 million people) in 1960 to 35% (2,300 million) in 2005.” Currently, they note, “the most widespread water shortage is in South Asia, where 91% of the population experiences some form of water shortage,” while “the most severe shortage is in North Africa and the Middle East, where 77% and 52% of the total population lives under extreme water shortage (<500 m3/capita/year), respectively.”
To alleviate these freshwater shortages, Kummu et al. note measures generally have been taken to increase water availability, such as building dams and extracting groundwater. However, they state, “there are already several regions in which such measures are no longer sufficient, as there is simply not enough water available in some regions.” They also note “this problem is expected to increase in the future due to increasing population pressure (e.g. United Nations, 2009), higher welfare (e.g. Grubler et al., 2007), [and] production of water intensive biofuels (e.g. Varis, 2007; Berndes, 2008).” Hence, they conclude there will be an increasing need for many nonstructural measures to be implemented, the first and foremost of which they indicate to be “increasing the efficiency of water use,” a property of plants almost universally promoted by atmospheric CO2 enrichment.
Flowers, Leaves and Seeds
Focusing on flowers, Johnston and Reekie (2008) state “there have been marked changes in plant phenology over the past century,” and they indicate these changes “have been interpreted as a consequence of the increase in temperature that has been observed over this time.” However, they add a new twist to the phenomenon, speculating that “the concentration of atmospheric CO2 may also directly affect time of flowering, even in the absence of temperature change.”
In exploring this possibility, the two researchers examined the effects of elevated atmospheric CO2 concentration by itself (ambient and ambient + 330 ppm), as well as the combined effect of elevated CO2 and elevated air temperature (ambient + 1.5°C), on the flowering phenology of 22 species of plants in the family Asteraceae, which were grown under natural seasonally varying temperature and daylength in separate compartments of a glasshouse in Wolfville, Nova Scotia, Canada. This work revealed, as they describe it, that “on average, elevated CO2 by itself advanced flowering by four days,” while “increasing temperature as well as CO2 advanced flowering by an additional three days.” They also found “CO2 was more likely to hasten phenology in long- than in short-day species,” and “early- and late-flowering species did not differ in response to elevated CO2, but the combined effect of elevated CO2 and temperature hastened flowering more in early- than late-flowering species.” In light of their several findings, they concluded that with respect to time of flowering in Asteraceae species, “the direct effect of CO2 on phenology may be as important as its indirect effect through climate change.”
Concentrating on leaves, McGrath et al. (2010) note “early spring leaf-out is critical to the growth and survival of competing trees in deciduous forests (Augspurger, 2008),” and “individuals or genotypes that more quickly reach high LAI [leaf area index] will more successfully compete with neighbors for light energy and space.” Therefore, working at the Aspen FACE facility, where aspen clones had been grown since 1997 in conditions simulating CO2 and O3 concentrations predicted for the mid-twenty-first century (560 ppm CO2 and 1.5 times current-ambient O3), the three researchers documented the history of leaf area development and leaf photosynthetic operating efficiency over the first month of spring leaf-out 11 years later in 2008. They found the trees in the elevated CO2 plots showed a 36 percent stimulation of leaf area index, whereas the trees in the elevated O3 plots showed a 20 percent reduction in LAI. In addition, they report the photosynthetic operating efficiency of the CO2-enriched aspen leaves was enhanced by 51 percent.
Studying seeds at the Duke Forest FACE facility in the Piedmont region of North Carolina, which was established in an unmanaged plantation of 13-year-old loblolly pine (Pinus taeda L.) trees, Way et al. (2010) collected in ground traps the seeds released by the trees, counted the number of seeds collected, and analyzed various properties of the seeds over the 12-year period 1997–2008, during which time the trees were growing in either ambient air or air enriched with an extra 200 ppm of CO2. At the conclusion of their experiment, they determined “the number of mature, viable seeds doubled per unit basal area in high-CO2 plots from 1997 to 2008 (P<0.001),” but “there was no CO2 effect on mean seed mass, viability, or nutrient content,” which they state “is consistent with observations from the few other studies examining reproductive responses to elevated CO2 in trees,” citing the earlier work of LaDeau and Clark (2001, 2006), Stiling et al. (2004), and Kimball et al. (2007). In addition, they report, “the mass of male catkins collected in ground traps was more than doubled over two years in elevated CO2 plots compared with ambient plots.” Based on their and others’ findings, the seven scientists concluded, “increased production of high-quality seeds by woody species in response to rising CO2 would give them a reproductive advantage over herbaceous species that produce more seeds but cannot maintain seed quality,” and they suggest this phenomenon “may facilitate woody encroachment into herbaceous communities, a wide-spread phenomenon that has already been linked to rising CO2 (Bond and Midgley, 2000; Davis et al., 2007).”
In another study, De Frenne et al. (2010) collected seeds of Anemone nemorosa L.—a model species for slow-colonizing herbaceous forest plants—found in populations growing along a 2400-km latitudinal gradient stretching from northern France to northern Sweden during three separate growing seasons (2005, 2006, and 2008). They then conducted sowing trials in incubators, a greenhouse, and under field conditions in a forest, where they measured effects of different temperature treatments (growing degree hours, or GDH) on seed and seedling traits. Based on their analyses, they report, “seed mass, germination percentage, germinable seed output and seedling mass all showed a positive response to increased GDH experienced by the parent plant.” Seed and seedling mass increased by 9.7 percent and 10.4 percent, respectively, for every 1,000 °C-hours increase in GDH, which they state is equivalent to a 1°C increase in temperature over a 42-day period. As a result, the 19 researchers—from Belgium, Estonia, France, Germany, and Sweden—concluded, “if climate warms, this will have a pronounced positive impact on the reproduction of A. nemorosa, especially in terms of seed mass, germination percentage and seedling mass,” because “if more seeds germinate and resulting seedlings show higher fitness, more individuals may be recruited to the adult stage.” In addition, they write, since “rhizome growth also is likely to benefit from higher winter temperatures (Philipp and Petersen, 2007), it can be hypothesized that the migration potential of A. nemorosa may increase as the climate in NW-Europe becomes warmer in the coming decades.” And increasing migration potential implies decreasing extinction potential.
In a third seed study, with the help of real-world micrometeorological data measured during the vegetative growth period (May–September) of ten consecutive years (1998–2007) in a boreal forest of southern Finland, Kuparinen et al. (2009) investigated the effects of a warming-induced increase in local convective turbulence (due to a postulated 3°C increase in local temperature) on the long-distance dispersal (LDD) of seeds and pollen based on mechanistic models of wind dispersal (Kuparinen et al., 2007) and population spread (Clark et al., 2001). For light-seeded herbs, they found spread rates increased by 35–42 m/yr (6.--9.2 percent), while for heavy-seeded herbs the increase was 0.01–0.06 m/yr (1.9–6.7 percent). Similarly, light-seeded trees increased their spread rates by 27–39 m/yr (3.5–6.2 percent), while for heavy-seeded trees the increase was 0.2–0.5 m/yr (4.0–8.5 percent). In addition, they discovered “climate change driven advancements of flowering and fruiting phenology can increase spread rates of plant populations because wind conditions in spring tend to produce higher spread rates than wind conditions later in the year.”
The four researchers (from France, Germany, Israel, and the United States), write that, in addition to the obvious benefits of greater LLD (being better able to move towards a more hospitable part of the planet), the increased wind dispersal of seeds and pollen may “promote geneflow between populations, thus increasing their genetic diversity and decreasing the risk of inbreeding depression,” citing Ellstrand (1992) and Aguilar et al. (2008). They further note “increased gene flow between neighboring populations can accelerate adaptation to environmental change,” citing Davis and Shaw (2001) and Savolainen et al. (2007). These phenomena are all very positive developments. In fact, they report the “dispersal and spread of populations are widely viewed as a means by which species can buffer negative effects of climate change.”
Secondary Carbon Compounds
Condensed tannins are one example of naturally occurring secondary carbon compounds produced in the leaves of several different plants that often deter herbivorous insects. In New Zealand, the Legume Lotus is a good source of these substances. Scientists with the country’s AgResearch Grasslands institute have additionally determined that sheep and cattle feeding on forage that contains this plant may see their methane emissions significantly reduced.
In a study designed to further explore this phenomenon, for a period of four years (2000–2003), Kelly et al. (2010) grew twice-weekly-watered six-year-old quaking aspen (Populus tremuloides) clones, two-year-old white willow (Salix alba) clones, and two-year-old sugar maple (Acer saccharum) siblings outdoors at the University of Michigan Biological Station in northern Michigan (USA) in open-bottom root boxes enclosed within clear-plastic-wall open-top chambers continuously supplied throughout the growing season (from May until leaf senescence in November) with either ambient-CO2-level air (360 ppm) or elevated-CO2-air (720 ppm). At the conclusion of the four-year period, the fallen leaves were collected, dried, and analyzed for simple phenolic and condensed tannin concentrations.
From Kelly et al.’s tabular results, it can be calculated that the 360-ppm CO2 increase employed in their study boosted the simple phenolics concentrations of the aspen, maple, and willow leaves by 16, 30, and 22 percent, respectively, while it boosted their condensed tannin concentrations by 60, 85, and 26 percent, respectively. Because both foliar phenolics and condensed tannins often enhance plant resistance to herbivore and pathogen attack, plus the fact that ruminants browsing on foliage containing condensed tannins may have a tendency to expel less methane (an important greenhouse gas) to the atmosphere, the increased concentrations of these substances in the leaves of trees grown in CO2-enriched air bodes well for the health of the trees and for people concerned about CO2- and methane-induced global warming.
In another study dealing with tannins, Huttunen et al. (2009) grew, from seed, well-watered silver birch (Betula pendula) plants in small containers filled with peat that were supplied with nitrogen (N) at low, moderate, and high rates equivalent to 0, 150, or 500 kg N per hectare per year, respectively, and maintained within climate-controlled closed-top chambers located outdoors at the University of Joensuu in Finland at either ambient or elevated air temperature (T or T + 2°C), at either ambient or elevated air CO2 concentrations (360 or 720 ppm), from mid-June 1999 to the end of the 2000 growing season. The researchers then harvested the trees’ leaves and determined their insoluble condensed tannin concentrations. As best as can be estimated from the graphical presentations of their results, the doubling of the atmospheric CO2 concentration they imposed on the tree seedlings led to the following increases in insoluble condensed tannin concentrations: 52 percent (low N), 17 percent (moderate N), and 99 percent (high N) under the ambient air temperature regime, and 61 percent (low N), 67 percent (moderate N), and 20 percent (high N) under the elevated air temperature regime. With all air temperature and soil nitrogen treatments showing CO2-induced increases in insoluble condensed tannin concentrations in silver birch leaves, it can be expected that this phenomenon would help to protect the trees’ foliage from predation by voracious insect herbivores and reduce methane emissions from ruminants that might eat birch-tree foliage produced in CO2-enriched air.
Another important group of secondary carbon compounds is that composed of reactive oxygen species (ROS), which can cause severe oxidative damage in plants. To ascertain whether atmospheric CO2 enrichment could alleviate the harm done by higher plant ROS concentrations caused by the stress of soil salinity, Perez-Lopez et al. (2009) grew two barley (Hordeum vulgare L.) cultivars, Alpha and Iranis, within controlled-environment growth chambers at either ambient (350 ppm) or elevated (700 ppm) atmospheric CO2 concentrations in a 3:1 perlite:vermiculite mixture watered with Hoagland’s solution every two days (until the first leaf was completely expanded at 14 days), after which a salinity treatment was administered by adding 0, 80, 160, or 240 mM NaCl to the Hoagland’s solution every two days for 14 more days. After a total of 28 days, the primary leaf of each barley plant was harvested and assessed for several biochemical properties.
The seven scientists report that in the various ambient-air salinity treatments, the deleterious effects of reactive oxygen species (ROS) on barley leaves were made apparent through ion leakage and increases in thiobarbituric acid reactive substances (TBARS), which rose ever-higher as salt concentrations increased. “On the other hand,” they continue, “when [the] salinity treatment was imposed under elevated CO2 conditions, lower solute leakage and TBARS levels were observed, suggesting that the oxidative stress caused by salinity was lower.” In interpreting their findings, Perez-Lopez et al. conclude, “elevated CO2 protects barley cultivars from oxidative stress,” noting “the relief of oxidative stress damage observed in our barley leaves grown under [a] CO2 enriched atmosphere has also been observed in alfalfa (Sgherri et al., 1998), pine (Vu et al., 1999) and oak (Schwanz and Polle, 2001).” Hence, it would appear the ongoing rise in the air’s CO2 content may help a wide variety of plants cope with the many serious problems caused by high soil salinity, and perhaps additional stresses as well.
Still other secondary carbon compounds comprise what are known as biogenic volatile organic compounds or BVOCs. Plants re-emit a substantial portion of their assimilated CO2 back to the atmosphere as BVOCs, and these substances affect both the chemical and physical properties of the air, where they generate large quantities of organic aerosols that can affect the planet’s climate by forming cloud condensation nuclei that may lead to increased cooling during the day by reflecting a greater portion of the incoming solar radiation back to space. In addition, many BVOCs protect plants from a host of insect pests. But not all BVOCs are so helpful.
Isoprene, for example, is a highly reactive non-methane hydrocarbon (NMHC) emitted in copious quantities by vegetation and responsible for the production of vast amounts of tropospheric ozone, which is a debilitating scourge of plant and animal life alike. It has been calculated by Poisson et al. (2000), for example, that current levels of NMHC emissions may increase surface ozone concentrations by up to 40 percent in the marine boundary-layer and by 50–60 percent over land, and that the current tropospheric ozone content extends the atmospheric lifetime of methane—one of the world’s most powerful greenhouse gases—by approximately 14 percent. Thus, it is readily understood that anything that reduces isoprene emissions from vegetation is something to be desired.
In a recent paper on the subject, Lathiere et al. (2010) (1) describe the development and analysis of a new model based on the Model of Emissions of Gases and Aerosols from Nature (MEGAN) developed by Guenther et al. (2006) for estimating isoprene emissions from terrestrial vegetation, (2) validate the new model with compilations of published field-based canopy-scale observations, and (3) use the new model to calculate changes in isoprene emissions from the terrestrial biosphere in response to climate change, atmospheric CO2 increase, and land use change throughout the twentieth century.
The scientists found that between 1901 and 2002, climate change at the global scale “was responsible for a 7% increase in isoprene emissions,” but “rising atmospheric CO2 caused a 21% reduction,” and “by the end of the 20th century, anthropogenic cropland expansion had the largest impact, reducing isoprene emissions by 15%,” so that “overall, these factors combined to cause a 24% decrease in global isoprene emissions during the 20th century.”
These findings represent good news, as the factors identified should reduce the undesirable consequences of increases in tropospheric ozone and methane concentrations. The three scientists warn, however, that “the possible rapid expansion of biofuel production with high isoprene-emitting plant species (e.g., oil palm, willow and poplar) may reverse the trend by which conversion of land to food crops leads to lower isoprene emissions.” This provides yet another reason not to force use of biofuels as replacements for fossil fuels.
Finally, the reader is referred to the discussion in Chapter 2, Section 2.1, of papers by Kiendler-Scharr et al., Kiemann, and Ziemann that appeared in Nature in 2009. Those authors warned that if vegetative isoprene emissions were to increase, driven directly by rising temperatures and/or indirectly by warming-induced changes in the species composition of boreal forests, the resulting decrease in cloud condensation nuclei “could lead to increased global-warming trends.” However, and as almost an afterthought, Ziemann mentions “the potential suppression of terpene emissions by elevated carbon dioxide concentrations.” In fact, that suppression is more than sufficient to offset any increase in isoprene emissions from plants, as shown by the literature review of Young et al. (2009), also summarized in Chapter 2.
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