Ecosystems: Forests
From ClimateWiki
Moving up from individual species and small groups of plants to the ecosystem scale, we consider the case of natural and plantation-type forests, beginning with studies of the latter type, where the air around groups of trees has been experimentally enriched with CO2, starting with the study of McCarthy et al. (2010). Conducted at the Duke Forest Free-Air CO2-Enrichment (FACE) facility, this study is a long-term experiment designed to investigate the effects of an extra 200 ppm of atmospheric CO2 on the growth and development of a plantation of loblolly pine (Pinus taeda) trees with an understory of various broadleaf species, including Liriodendron tulipifera, Liquidambar styraciflua, Acer rubrum, Ulmus alata, and Cornus florida, plus various other trees, shrubs, and vines. All of these were grown on a soil that Finzi and Schlesinger (2003) describe as being in “a state of acute nutrient deficiency that can only be reversed with fertilization.” Many researchers had long thought such fertility deficiency would stifle the ability of the extra aerial supply of CO2 to significantly stimulate the forest’s growth on a continuing basis.
Working with data for the years 1996–2004, the team of nine researchers writes, “net primary productivity [NPP] for pines, hardwoods and the entire stand was calculated as the sum of the production of coarse wood (stems, branches, coarse roots), leaf litter (lagged for pines), fine roots and reproductive structures.” The results of this protocol indicated “elevated CO2 increased pine biomass production, starting in 1997 and continuing every year thereafter,” and “the CO2-induced enhancement remained fairly consistent as the stand developed.” In addition, they found “elevated CO2 increased stand (pine plus all other species) biomass production every year from 1997 onwards with no trend over time,” while the average yearly increase in NPP caused by the approximate 54 percent increase in the air’s CO2 content was 28 percent. Thus, and in spite of the original belief of many scientists that low levels of soil nitrogen—especially an acute deficiency—would preclude any initial growth stimulation provided by atmospheric CO2 enrichment from long persisting, the suite of trees, bushes, and shrubs that constitute the Duke Forest has continued to maintain the extra CO2-enabled vitality it exhibited right from the start of the study, with no sign of it even beginning to taper off.
Further extending the results of the Duke Forest FACE study were Jackson et al. (2009), who describe new belowground data they obtained there, after which they present a synthesis of these and other results obtained from 1996 through 2008, seeking to determine “which, if any, variables show evidence for a decrease in their response to atmospheric CO2 during that time frame.”
Among many other things, Jackson et al. report “on average, in elevated CO2, fine-root biomass in the top 15 cm of soil increased by 24%,” and in recent years the fine-root biomass increase “grew stronger, averaging ~30% at high CO2.” Regarding coarse roots having diameters greater than 2 mm and extending to a soil depth of 32 cm, they report, “biomass sampled in 2008 was twice as great in elevated CO2.” We calculate from the graphical representation of their results that the coarse-root biomass was fully 130 percent greater, which is astounding, particularly given that the extra 200 ppm of CO2 supplied to the air surrounding the CO2-enriched trees represented only about a 55 percent increase over ambient conditions. In the concluding sentence of their paper’s abstract, Jackson et al. state, “overall, the effect of elevated CO2 belowground shows no sign of diminishing.”
In expanding on this statement, the four researchers note “if progressive nitrogen limitation were occurring in this system, we would expect differences in productivity to diminish for trees in the elevated vs. ambient CO2 plots,” but they state, “in fact there is little evidence from estimates of aboveground or total net primary productivity in the replicated Duke experiment that progressive nitrogen limitation is occurring there or at other forest FACE experiments,” even “after more than a decade of manipulation” of the air’s CO2 content, citing in this regard—with respect to the latter portion of their statement—the report of Finzi et al. (2007). Consequently, there is very good reason to believe the “aerial fertilization effect” of atmospheric CO2 enrichment will continue to benefit Earth’s forests significantly as long as the atmosphere’s CO2 concentration continues to rise.
Also studying this important subject were Darbah et al. (2010), who worked with photosynthesis data they and others collected over 11 years at the Aspen FACE site near Rhinelander, Wisconsin (USA). They too evaluated the progressive nitrogen limitation hypothesis, working with two different quaking aspen (Populus tremuloides Michx.) clones (42E and 271), which were exposed to all combinations of ambient and elevated (560 ppm) CO2 and ambient and elevated (1.5 times ambient) ozone (O3). As an added bonus, they investigated whether the same principle might apply to leaf stomatal conductance.
In a crisp and clear report of what they learned, the eight researchers state their results “suggest no long-term photosynthetic and stomatal acclimation to elevated CO2, O3 or CO2 + O3 in aspen trees exposed to elevated CO2 and/or O3 gases for 11 years,” and that the aspen trees “have sustained their maximum instantaneous photosynthesis stimulation for over a decade.” In commenting on their findings, Darbah et al. state they support the observations of (1) Liberloo et al. (2007), who measured a 49 percent increase in net photosynthetic rate in poplar trees after six years of exposure to elevated CO2, (2) the findings of Sholtis et al. (2004), who reported a 44 percent stimulation of net photosynthesis in sweetgum trees after three years of exposure to elevated CO2, (3) Crous and Ellsworth (2004), who found a photosynthetic enhancement of 51–69 percent in Pinus taeda trees after six years of exposure to elevated CO2, and (4) Davey et al. (2006) and (5) Paoletti et al. (2007), of whose work Darbah et al. state, “there was no photosynthetic acclimation (down-regulation) occurring in Quercus ilex under long-term CO2 enrichment.” In addition, they remark that (6) even in white clover (Trifolium repens), Ainsworth et al. (2003) found photosynthetic stimulation “remained after nine years of exposure to elevated CO2.”
Additional support for this upbeat view of the matter is provided by Bader et al. (2010), who, while working at the Swiss Canopy Crane facility in a species-rich deciduous forest 15 km south of Basel, Switzerland, where the 100-year-old stand reaches canopy heights of 30 to 35 meters, measured light-saturated rates of net photosynthesis between 8:30 am and 12:20 pm at ambient (380 ppm) and elevated (550 ppm) atmospheric CO2 concentrations, the latter of which were maintained throughout all daylight periods over the course of the eighth growing season of their long-term study—just as they had been similarly maintained over the prior seven years—in three Quercus petraea trees, three Carpinus betulus trees, one Tilia platyphyllos tree, and one Acer campestre tree.They then compared the results of their measurements with those obtained in earlier years of the experiment.
Bader et al. report the mean net photosynthetic rate of the upper-canopy foliage was 48 percent greater in the CO2-enriched foliage than in the ambient-treatment foliage in July and 42 percent greater in September, yielding an average increase of 45 percent in response to the 45 percent increase in the air’s CO2 content. They go on to observe, “in the same stand, Zotz et al. (2005) found 36 and 49% photosynthetic enhancement in the mid and late growing season, respectively.” This represents a mean response of 42.5 percent, slightly less than what Bader et al. observed most recently.
As for the significance of their findings, the three Swiss scientists write, “the lack of photosynthetic down-regulation is consistent with the findings for mature and understory sweetgum trees growing at the ORNL- and Duke-FACE sites, respectively, and three poplar species growing at short-rotation coppice at the POP-FACE site, as well as for aspen and birch at the ASPEN-FACE stands (Herrick and Thomas, 2001; Sholtis et al., 2004; Liberloo et al., 2007; Uddling et al., 2009).” And in their final comment about their own study, they state their findings suggest “the enhancement of photosynthesis may persist in these mature deciduous trees under high future atmospheric CO2 concentrations,” while adding in the abstract of their paper that they will likely do so “without reductions in photosynthetic capacity.”
Kets et al. (2010) also explored the phenomenon of acclimation by investigating diurnal changes in the rate of light-saturated net photosynthesis (Pn) under both ambient and elevated CO2 and/or ozone (O3) concentrations over wide ranges of leaf stomatal conductance, leaf water potential, intercellular CO2 concentration, leaf temperature, and vapor pressure difference between leaf and air in two clones (271 and 42E) of quaking aspen (Populus tremuloides Michx.) trees that differed in their sensitivity to ozone and had been growing at the Aspen FACE site for seven to eight years. In describing their findings, they state Pn was typically enhanced by 33–46 percent in the CO2-enriched treatments over the course of their study, and there was a small increase in leaf chlorophyll concentration as well.
Consequently, and noting that “previous Aspen FACE studies have reported 25–36% increases in Pn (Noormets et al., 2001; Takeuchi et al., 2001; Sharma et al., 2003; Ellsworth et al., 2004),” the six scientists emphasize that the aerial fertilization effect of atmospheric CO2 enrichment on Pn observed in their study “has rather been increasing in time than decreasing,” stating this phenomenon may be caused by the “slight but significant increase in leaf chlorophyll content per leaf area, which is rather positive acclimation in photosynthetic apparatus than negative acclimation.” In support of this conclusion they also cite the studies of Centritto and Jarvis (1999) and Eichelmann et al. (2004). Hence, their experiment demonstrated that some of the benefits of elevated atmospheric CO2 concentrations may actually increase with the passage of time.
Taking the study of this important subject a major step forward, via a totally different approach, Phillips et al. (2008) begin by noting there is “a long held view,” as they describe it, that “old trees exhibit little potential for growth.” Hence, they write, “it may seem reasonable to conclude that old trees are not responsive to increased CO2,” as some researchers do indeed claim. They go on, however, to demonstrate this view is far from the truth.
The three researchers begin their analysis of the subject by stating, “hydraulic constraints in tall trees,” such as those of great age, “constitute a fundamental form of water limitation; indeed, one that is indistinguishable from soil water limitations,” citing Koch et al. (2004) and Woodruff et al. (2004). They also report “recent research indicates that tree size and its hydraulic correlates, rather than age per se, controls carbon gain in old trees,” as indicated by the study of Mencuccini et al. (2005). These findings imply, in their words, that “factors that alleviate internal or external resource constraints on old trees could improve physiological function and ultimately growth,” which is something elevated CO2 does quite well by increasing plant water use efficiency. They list several phenomena that suggest “a fundamental potential for old growth trees to show greater photosynthesis and growth under industrial age increases in CO2 than they would under constant, pre-industrial CO2 levels.”
Drawing from their own work, Phillips et al. find “500- and 20-year-old Douglas-fir trees both show high sensitivity of photosynthesis to atmospheric CO2,” presenting data that clearly demonstrate, as they phrase it, “under optimal conditions there exists the potential for an approximately 30% increase in photosynthetic rate with an increase in CO2 from pre-industrial to current levels [i.e., from 280 to 385 ppm] in old trees.” And they go on to note “the phenomenon of twentieth-century ring-width increase,” which could thus be expected to accompany the twentieth-century increase in the air’s CO2 content, has in fact been detected in several other studies, including those of LaMarche et al., (1984), Jacoby (1986), Graybill (1987), Kienast and Luxmoore (1988), Graumlich (1991), Knapp et al. (2001), Bunn et al. (2005), and Soule and Knapp (2006), to which could be added the study of Graybill and Idso (1993).
Further commenting on the significance of the findings of these studies, the three researchers write that the results of LaMarche et al. (1984) “could not be explained by temperature or precipitation variation over this time period, but were consistent with, and attributed to, the rise in atmospheric CO2,” which also was the case with the results of Graybill and Idso (1993). Although these data, in their words, “appear to represent compelling circumstantial evidence for carbon fertilization of old growth trees,” they note “this possibility has been discounted and climate change has instead been implicated for the observed responses in subsequent research.” That invalid discounting is likely based on the erroneous claim that twentieth-century global warming was unprecedented over the past one to two millennia. Instead, it is quite probable that a good portion of the twentieth-century increase in tree growth was a consequence of the growth-promoting and water-use-efficiency-enhancing increase in the air’s CO2 content.
In summation, the analysis of Phillips et al. (2008) provides substantial support for the two-part thesis that (1) old-growth forests can continue to sequester carbon for multiple centuries in the face of ever-increasing atmospheric CO2 concentrations, and (2) the global temperature history employed by the Intergovernmental Panel on Climate Change depicts an unrealistically large temperature increase over the course of the twentieth century.
Additional evidence for this thesis has recently come from Pan et al. (2010). They examined “how changes in atmospheric composition (CO2, O3 and N deposition), climate and land-use affected carbon dynamics and sequestration in Mid-Atlantic temperate forests during the 20th century,” by modifying and applying “a well established process-based ecosystem model with a strong foundation of ecosystem knowledge from experimental studies,” which they validated “using the U.S. Forest Inventory and Analysis data.”
For previously harvested and currently regrowing forests, the calibrated model produced the following percentage changes in net ecosystem productivity (NEP) due to observed changes in N deposition (+32 percent), CO2 (+90 percent), O3 (-40 percent), CO2 + O3 (+60 percent), CO2 + N deposition (+184 percent), and CO2 + N deposition + O3 (+138 percent), while corresponding changes in NEP for undisturbed forests were +18 percent, +180 percent, -75 percent, +78 percent, +290 percent, and +208 percent. In addition, the results of Pan et al. revealed “the ‘fertilization’ effect of N deposition mainly stimulates carbon allocation to short-lived tissues such as foliage and fine roots,” but “the ‘fertilization’ effect by elevated CO2 likely enhances more sustainable carbon storage such as woody biomass (including coarse roots).”
The four USDA Forest Service scientists conclude the evidence indicates “the change in atmospheric composition, particularly elevated CO2, will gradually account for more of the carbon sink of temperate forests in the Mid-Atlantic region,” and they state, “such a significant ‘fertilization effect’ on the forest carbon sequestration could eventually result in a ‘greener world’ after a long period of chronic change in atmospheric composition and cumulative impact.”
Real-world evidence for this phenomenon is provided by Cole et al. (2010). They begin by noting that quaking aspen (Populus tremuloides Michx.) is a dominant forest type in north-temperate, montane, and boreal regions of North America,” stating that it is, in fact, “the most widely distributed tree species on the continent,” while further noting that aspen—and related poplars—are “quintessential foundation species (Ellison et al., 2005), shaping the structure and function of the communities and ecosystems in which they occur (Whitham et al., 2006; Schweitzer et al., 2008; Madritch et al., 2009).” This being the case, they considered it important to determine how this keystone species may have responded to the increase in atmospheric CO2 concentration that has occurred over the past several decades, especially within the context of the climatic changes that occurred concurrently.
The four researchers collected branches from 919 trees after their leaves had dropped in the fall, obtaining samples that represented 189 genets or clones (five trees per clone) at 11 sites distributed throughout three regions of Wisconsin (USA). The sampled trees ranged from five to 76 years of age and came from second-growth unmanaged forests south of the areas defoliated by forest tent caterpillars in 1980–1982, 1989–1990, and 2001–2002. In addition, they recorded trunk diameter at breast height for each sampled tree. This parameter, they write, “is very highly correlated with total biomass in aspen,” citing Bond-Lamberty et al. (2002).
The Minnesota and Wisconsin scientists learned that “age-specific ring width increased over time” and “the greatest increase occurred for relatively young trees, so that young trees grew faster in recent years than did young trees several decades ago.” During the past half-century, for example, they found the growth of trees 11–20 years old rose by 60 percent. In addition, they observed “rising CO2 causes ring width to increase at all moisture levels, apparently resulting from improved water use efficiency,” so that “the overall increase results from historical increases in both CO2 and water availability.” And when they separated out the impacts of the two factors, they found “the effect of rising CO2 had been to increase ring width by about 53%,” as a result of the “19.2% increase in ambient CO2 levels during the growing season, from 315.8 ppm in 1958 (when CO2 records began) to 376.4 ppm in 2003.”
Cole et al. comment, “the magnitude of the growth increase uncovered by this analysis raises the question of how much other major forest species have responded to the joint effects of long-term changes in CO2 and precipitation.” In this regard, we note many additional tree species may indeed have experienced similar growth stimulation, particularly in light of the analysis of Tans (2009), who demonstrated that Earth’s land surfaces were a net source of CO2 to the atmosphere until about 1940—primarily due to the felling of forests and the plowing of grasslands to make way for expanded agricultural activities—but from 1940 onward the terrestrial biosphere had become, in the mean, an increasingly greater sink for CO2, and that it has done so even in the face of massive global deforestation, for which it apparently more than compensated. The combined findings of the studies of Tans and Cole et al. clearly testify to the phenomenal ability of the ongoing rise in the air’s CO2 content to transform the face of the Earth.
Other studies complement these findings. For example, Martinez-Vilalta et al.(2008) used tree-ring data from the Catalan Ecological and Forest Inventory “to study the temporal variability of Scots pine stem radial growth (period 1901–1997) across a relatively large region (Catalonia, NE Spain) situated close to the southern limit of the distribution of the species.” This inventory, as they describe it, “included a total of 10,664 plots randomly distributed throughout the forested area of Catalonia,” where Scots pine was present in 30.2 percent of the plots and was the dominant tree species in 18.4 percent of them.
The five researchers’ work revealed, in their words, “an overall increase of 84% in Scots pine BAI [basal area increment] during the 20th century, consistent with most previous studies for temperate forests” and in harmony with the greening of the Earth that has accompanied the historical increase in the air’s CO2 content. They make a point of stating “this trend was associated with increased atmospheric CO2 concentration,” which they interpret to be “a fertilization effect,” while observing “there was also a marked increase in temperature across the study region (0.19°C per decade on average).”
Some people, however, worry rising temperatures will lead to greater respiration rates in the world’s forests. Bronson and Gower (2010), for example, write, “the boreal forest historically has been considered a carbon sink,” but “autotrophic respiration is more sensitive than photosynthesis to increases in temperature,” and therefore, in response to global warming “most models predict autotrophic respiration will increase at a greater rate than photosynthesis, which infers decreased carbon use efficiency and net primary production.” This in turn implies “a substantial increase in temperature could turn the boreal forest into a carbon source,” and this positive feedback phenomenon could lead to an intensification of the warming of the globe.
So the argument goes, but the study found otherwise. Working about 20 km south of Thompson, Manitoba, Canada (55°53’N, 98°20’W), within large enclosed greenhouse chambers containing black spruce trees (Picea mariana (Mill.) B.S.P.) and the majority of their fine roots, along with soil-heating cables that were used to warm air and soil temperatures about 5°C over ambient control temperatures, Bronson and Gower measured light-saturated net photosynthesis, foliage respiration, and stem respiration in heated and control forest plots during the 2005, 2006, and 2007 growing seasons. Throughout the study, “both the older foliage, which developed before the experiment, and the new foliage, developed during the experiment, had similar rates of light-saturated net photosynthesis, foliage respiration and stem respiration across all treatments.” This they write, “underscores the ability of black spruce to maintain homeostasis in a 5°C warmer environment.” In addition, while noting many global change models predict a doubling of respiration for every 10°C increase in temperature, Bronson and Gower state in the concluding sentence of their paper—and in no uncertain terms—that “the results from this and other whole-ecosystem warming experiments do not support this model assumption.”
In another study utilizing real-world measurements, Lewis et al. (2009) evaluated tropical forest inventory data, plant physiology experiments, ecosystem flux observations, earth observations, atmospheric measurements, and dynamic global vegetation models, which, “taken together,” in their words, “provide new opportunities to cross-validate results.”
According to the five researchers, both theory and experimental findings suggest that over the past several decades “plant photosynthesis should have increased in response to increasing CO2 concentrations, causing increased plant growth and forest biomass.” In this regard they find “long-term plot data collectively indicate an increase in carbon storage, as well as significant increases in tree growth, mortality, recruitment, and forest dynamism.” They also report that satellite measurements “indicate increases in productivity and forest dynamism” and that “five Dynamic Global Vegetation Models, incorporating plant physiology, competition, and dynamics, all predict increasing gross primary productivity, net primary productivity, and carbon storage when forced using late-twentieth century climate and atmospheric CO2 concentration data.” In addition, they state “the predicted increases in carbon storage via the differing methods are all of similar magnitude (0.2% to 0.5% per year).”
“Collectively,” Lewis et al. write, “these results point toward a widespread shift in the ecology of tropical forests, characterized by increased tree growth and accelerating forest dynamism, with forests, on average, getting bigger (increasing biomass and carbon storage).” These findings are just the opposite of what some researchers claim will result from the “twin evils” of rising air temperatures and CO2 concentrations. Instead of being the bane of Earth’s tropical forests, twentieth-century increases in air temperature and atmospheric CO2 concentrations have been a great boon to the trees of the tropics.
Also studying the effects of rising air temperature and CO2 concentration on tropical rainforests were Jaramillo et al. (2010), within a much different context: that of the Paleocene-Eocene Thermal Maximum (PETM) of some 56 million years ago, which they state was “one of the most abrupt global warming events of the past 65 million years (Kennett and Stott, 1991; Zachos et al., 2003; Westerhold et al., 2009).” It was driven, they remark, by “a massive release of 13C-depleted carbon (Pagani et al., 2006; Zeebe et al., 2009)” which led to “an approximate 5°C increase in mean global temperature in about 10,000 to 20,000 years (Zachos et al., 2003).” They note many people argue that during this warm period the Earth’s tropical ecosystems “suffered extensively because mean temperatures are surmised to have exceeded the ecosystems’ heat tolerance (Huber, 2008).”
To find out whether the ancient warming of the world truly constituted a major problem for the planet’s rainforests, the 29 researchers, hailing from eight countries, analyzed pollen and spore contents and the stable carbon isotopic composition of organic materials obtained from three tropical terrestrial PETM sites in eastern Colombia and western Venezuela. Contrary to the prevailing wisdom of the recent past, this work revealed that the onset of the PETM was “concomitant with an increase in diversity produced by the addition of many taxa (with some representing new families) to the stock of preexisting Paleocene taxa.” They further determined that this increase in biodiversity “was permanent and not transient.”
In discussing their findings, Jaramillo et al. write, “today, most tropical rainforests are found at mean annual temperatures below 27.5°C,” and they state several scientists have argued “higher temperatures could be deleterious to the health of tropical ecosystems.” In fact, they report that tropical warming during the PETM is believed to have produced intolerable conditions for tropical ecosystems, citing the writings of Huber (2008, 2009). Nevertheless, they reiterate that at the sites they studied, “tropical forests were maintained during the warmth of the PETM (~31° to 34°C),” and they state “it is possible that higher Paleocene CO2 levels (Royer, 2010) contributed to their success.” Such would indeed appear to be the case, in light of the well-established fact that most plants, including trees, tend to exhibit their greatest photosynthetic rates at ever-warmer temperatures as the air’s CO2 content continues to rise (Bjorkman et al., 1978; Nilsen et al., 1983; Jurik et al., 1984; Seeman et al., 1984; Harley et al., 1986; Stuhlfauth and Fock, 1990; McMurtrie et al., 1992; Sage et al., 1995; Ziska and Bunce, 1997; Cowling and Sage, 1998; Lewis et al., 2001; Roberntz, 2001; Borjigidai et al., 2006; Ghannoum et al., 2010).
In light of Jaramillo et al.’s findings, it is becoming increasingly clear that greater warmth and atmospheric CO2 concentrations are not “twin evils.” Quite to the contrary, they are just what the Earth’s ecosystems need in order to make them both more stable and more productive, characteristics essential for sustaining the still-expanding human population of the globe while preserving wildlife habitat.
Two other recent studies look beyond the present and contemplate still other factors of potential significance. Doherty et al. (2010) modeled future changes in land biogeochemistry and biogeography in the region bounded by 12.5°N, 12.5°S, 25°E, and 42.5°E, representing the whole of East Africa (Kenya, Tanzania, Uganda, Rwanda, Burundi, Ethiopia, and Somalia), plus portions of Central Africa (the Democratic Republic of Congo and Southern Sudan). They used 18 climate projections derived from nine general circulation models that figured prominently in the IPCC’s Fourth Assessment Report, employing the projections as input to the Lund-Potsdam-Jena dynamic global vegetation model that simulates changes in vegetation and ecosystem carbon cycling under future climate conditions, based on what they describe as “a coupled photosynthesis-hydrological scheme [that] computes gross primary productivity, plant respiration, and evapotranspiration on a daily time step based on the current climate, atmospheric CO2 concentration, vegetation structure and phenological state, and soil water content.”
Doherty et al. report “all simulations showed future increases in tropical woody vegetation over the region,” noting “regional increases in net primary productivity (18–36%) and total carbon storage (3–13 percent) by 2080–2099 compared with the present-day were common to all simulations,” and “seven out of nine simulations continued to show an annual net land carbon sink in the final decades of the 21st century because vegetation biomass continued to increase.” The researchers conclude, “overall, our model results suggest that East Africa, a populous and economically poor region, is likely to experience some ecosystem service benefits through increased precipitation, river runoff and fresh water availability,” and they state, “resulting enhancements in net primary productivity may lead to improved crop yields in some areas.” They specifically state their results “stand in partial contradiction of other studies that suggest possible negative consequences for agriculture, biodiversity and other ecosystem services caused by temperature increases.”
Hillstrom et al. (2010) note, “natural forest systems constitute a major portion of the world’s land area, and are subject to the potentially negative effects of both global climate change and invasion by exotic insects.” In this regard, they report “a suite of invasive weevils has become established in the northern hardwood forests of North America,” noting that how these insects will respond to continued increases in the air’s CO2 content is currently “unknown.” To examine this subject, they collected 200 mating pairs of Polydrusus sericeus weevils—which they describe as “the second most abundant invasive weevil species in northern hardwood forests”—from birch trees growing on the perimeter of the Aspen FACE facility, after which they fed them leaves taken from the birch, aspen, and maple trees growing within either the facility’s ambient-air rings or its CO2-enriched rings (maintained at a target concentration of 560 ppm) under controlled laboratory conditions throughout the summer of 2007, closely monitoring parameters related to weevil longevity and fecundity.
The five researchers, all from the University of Wisconsin’s Department of Entomology, report that feeding the weevils with foliage produced on trees in the CO2-enriched FACE plots had no effect on male longevity but reduced female longevity by 19 percent. Also, “Polydrusus sericeus egg production rate declined by 23% and total egg production declined by 29% for females fed foliage produced under elevated CO2 compared with ambient CO2.” Hillstrom et al. conclude, “concentrations of elevated CO2 above 500 ppm have the potential to decrease P. sericeus populations by reducing female longevity and fecundity,” which should benefit the northern hardwood forests of North America.
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