U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Results

Selected highlights of program research results are summarized below, along with program publications describing the research (ordered by publication date of the main papers). Many other scientific papers describing the program's research results are listed on the program publications page.


Climatic change is causing rapid shifts in plant distribution in California mountains.   Ongoing global warming is expected to shift the geographic distribution of plants as species expand into newly favorable areas and decline in increasingly hostile locations. A comparison of surveys of plant locations made in 1977 and 2006-2007 along a 2,314-meter elevational gradient in Southern California's Santa Rosa Mountains was used to assess whether plant geographic distributions are shifting now (Kelly and Goulden 2008). During the 30 years between the surveys the local climate warmed, precipitation variability increased, and the amount of snow decreased. Moreover, the average elevation of the dominant plant species along the transect rose in elevation by about 65 meters during the 30 years between the two surveys. That upslope change in plant distribution could not be attributed to changes in air pollution or fire frequency and appears to be a consequence of changes in regional climate.

Kelly AE, Goulden ML (2008) Rapid shifts in plant distribution with recent climate change. Proceedings of the National Academy of Sciences USA 105:11823-11826

Increased cold damage with warmer springs?   Plant ecologists have long been concerned with a seemingly paradoxical scenario in which warming (as a component of climatic change) may actually increase the risk of plant frost damage. The underlying hypothesis is that mild winters and warm, early springs, which are expected to occur as warming continues, may induce "premature" plant development in cold deciduous plants, resulting in exposure of vulnerable plant tissues and organs to subsequent late-spring frosts. The 2007 spring freeze in the eastern United States provided an excellent opportunity to evaluate this hypothesis and assess its potential consequences. In particular, the rapid prefreeze phenological advancement caused by unusually warm conditions was contrasted by Gu et al. (2008) with the dramatic (and unusual) regional-scale setback caused by a late-spring freeze. The widespread devastation of crops and natural vegetation occasioned by this combination of a warm early spring (and winter) followed by a late-spring freeze demonstrated the need to consider the possibility of future large fluctuations in spring temperatures as a real threat to terrestrial ecosystem structure and functioning in a warming climate.

Gu L, Hanson PJ, Post WM, Kaiser DP, Yang B, Nemani R, Pallardy SG, Meyers T (2008) The 2007 eastern US spring freeze: increased cold damage in a warming world? BioScience 58:253-262

Rising atmospheric CO2 concentration may not be all good news for crops.   It has been known for decades that yield of C3 crops increases with increased CO2 concentration (e.g., Amthor 2001), but a recent field experiment found that attack on soybeans by western corn rootworm, and by Japanese beetle, was increased with elevated CO2. A program-sponsored research project investigating the underlying cause of this increased insect attack in elevated CO2 reported that elevated CO2 caused downregulation of genes, and metabolic signaling systems, that help plants defend themselves against insects (Zavala et al. 2008). The researchers concluded that changes in the natural defense-signaling systems of crops caused by the ongoing increase in atmospheric CO2 concentration (which is caused mainly by fossil fuel use) has the potential to exacerbate pest problems in crops of the future.

Amthor JS (2001) Effects of atmospheric CO2 concentration on wheat yield: review of results from experiments using various approaches to control CO2 concentration. Field Crops Research 37:1-34

Zavala JA, Casteel CL, DeLucia EH, Berenbaum MR (2008) Anthropogenic increase in carbon dioxide compromises plant defense against invasive species. Proceedings of the National Academy of Sciences USA 105:5129-5133

Elevated CO2 concentration delays autumnal senescence of Populus leaves.   Based on two years of data from the long-term DOE elevated-CO2 experiment near Rhinelander, Wisconsin (i.e., the Rhinelander FACE experiment; see the 1997 Highlight below) and a similar European elevated-CO2 experiment, it was found that the elevated CO2 treatment (i.e., an about 45% increase in CO2 relative to the present atmosphere) delayed autumnal senescence in field-grown, cold-deciduous Populus trees (Taylor et al., 2008). The authors wrote that "leaf level photosynthetic activity and carbon uptake in elevated CO2 during the senescence period was also enhanced compared with ambient CO2." They concluded that the experimental "findings reveal a direct effect of rising atmospheric CO2, independent of temperature [increase caused by global warming] in delaying autumnal senescence for Populus, an important deciduous forest tree with implications for forest productivity and adaptation to a future high[er] CO2 world."

Taylor G, Tallis MJ, Giardina CP, Percy KE, Miglietta F, Gupta PS, Gioli B, Calfapietra C, Gielen B, Kubiske ME, Scarascia-Mugnozza GE, Kets K, Long SP, Karnosky DF (2008) Future atmospheric CO2 leads to delayed autumnal senescence. Global Change Biology 14:264-275


Climatic change may differentially affect establishment versus persistence of ragweed.   Until now, no data were available about potential effects of rising CO2 concentration or increased air temperature on the establishment and persistence of common ragweed (Ambrosia artemisiifolia L.) within a plant community following soil disturbance. To determine ragweed longevity, disturbed soil with a common seed bank population was exposed to an in situ temperature and CO2 concentration gradient along an urban-rural transect beginning in early 2002 (Ziska et al., 1007). Both temperature and CO2 concentration increased from the rural to the urban study sites, but no other consistent differences in meteorological variables (e.g., wind speed, humidity, solar radiation, and ozone concentration) along the transect were observed during the study period (i.e., 2002-2005). Initial ragweed biomass responded positively to increased temperature and CO2 concentration at the urban site, with peak biomass being observed by the end of 2003. By autumn 2004, however, and continuing through 2005, urban ragweed populations had dwindled to a few plants. The temporal decline in ragweed populations was not associated with increased disease, herbivory, or auto-allelopathy, but was part of a demographic reduction in the total number of annual plant species observed for the urban (i.e., high temperature and high O2) location. In a separate experiment, it was found that such a demographic shift is consistent with elevated-temperature and -CO2 induced increases in biomass and litter accumulation, with a subsequent reduction in germination/survival of annual plant species. The authors concluded that the "data indicate that [CO2]/temperature differences associated with urbanization may increase initial ragweed productivity and pollen production, but...long[er]-term...persistence of ragweed in the urban macro-environment may be dependent on other factors."

Ziska LH, George K, Frenz DA (2007) Establishment and persistence of common ragweed (Ambrosia artemisiifolia L.) in disturbed soil as a function of an urban--rural macro-environment. Global Change Biology 13:266-274


Nitrogen limitation constrains sustainability of grassland ecosystem response to elevated carbon dioxide concentration.   The ongoing increase in atmospheric CO2 concentration, caused in large part by fossil fuel use, generally stimulates primary production (plant growth) in terrestrial ecosystems. After 6 years of exposing experimental field plots in Minnesota (i.e., in the BioCON experiment) containing perennial grassland species to 560 ppm CO2, however, the positive response of plant biomass to elevated CO2 relative to ambient CO2 concentration (about 360 ppm) was progressively suppressed by low availability of ambient soil nitrogen (Reich et al., 2006). The positive response to elevated CO2 was greater in plots receiving nitrogen additions relative to ambient soil nitrogen plots. The authors concluded that since soil nitrogen limits plant growth in many terrestrial ecosystems, soil nitrogen supply "is probably an important constraint on global terrestrial [ecosystem] responses to elevated CO2."

Reich PB, Hobbie SE, Lee T, Ellsworth DS, West JB, Tilman D, Knops JMH, Naeem S, Trost J (2006) Nitrogen limitation constrains sustainability of ecosystem response to CO2. Nature 440:922-925

Ecosystem feedbacks to climatic change may enhance future global warming.   Historical evidence shows that atmospheric greenhouse gas concentrations increase during periods of warming, implying a positive feedback from ecosystems to global warming. The feedbacks for CO2 and CH4 were quantified by combining the mathematics of feedback with empirical ice-core information and general circulation model (GCM) climatic sensitivity by Torn and Harte (2006). They reported that the warming of 1.5-4.5 degrees C (2.7-8.1 degrees F) associated with anthropogenic doubling of CO2 in GCMs is amplified to a warming of 1.6-6.0 degrees C (2.9-10.8 degrees F) when ecosystem feedbacks are considered. They stated that there is growing experimental evidence that terrestrial ecosystems will amplify warming in the next century as a net result of ecological responses to changes in the length of growing seasons, changes in soil moisture, and reductions in permafrost. As a result, anthropogenic emissions that cause warming would result in higher final atmospheric greenhouse gas concentrations, and therefore more warming, than would be predicted in the absence of these ecosystem feedbacks. Because key ecological feedbacks to climatic change are unrepresented in GCMs, and because asymmetrical uncertainty from those feedbacks favors higher temperature, it is likely that the future will be warmer than implied by present GCMs.

Torn MS, Harte J (2006) Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming. Geophysical Research Letters 33:L10703, doi:10.1029/2005GL025540


Effects of elevated carbon dioxide and ozone concentrations on forest productivity are not additive.   In general, elevated CO2 concentration enhances tree growth and elevated O3 concentration limits tree growth. In spite of the fact that concentrations of both CO2 and O3 are increasing as a result of fossil fuel use, few field experiments have been conducted to determine the combined effects of increases in concentrations of both gases on tree growth. The DOE elevated-CO2 and elevated-O3 experiment near Rhinelander, Wisconsin (i.e., the Rhinelander FACE experiment; see the 1997 Highlight below) is an exception. It is the world's largest long-term field experimental study of the ecological effects of changes in atmospheric composition associated with fossil fuel use. A critical result of the study is that the effects of the combinaton of elevated CO2 and O3 were not additive relative to the effects of either gas alone on standing biomass of the constructed (model) northern hardwood forest stands used in the experiment (King et al., 2005). In aspen stands, aspen-birch stands, and aspen-maple stands, an about 50% increase in CO2 concentration stimulated standing biomass (the sum of dry mass of leaves, aboveground wood, and coarse and fine roots) after six years, relative to the ambient atmosphere. In the same stand types, an about 50% increase in O3 concentration limited standing biomass, realtive to ambient conditions. But in all three stand types, the combination of elevated CO2 and O3 had a negative effect on standing biomass relative to the hypothetical case of additive effects of elevated CO2 and O3 obtained in the single-gas treatments. This result reinforces the need to consider multiple factors in global-change ecosystem experiments because it can clearly be misleading to simply "add" results from single-factor experiments to project potential ecological effects of multiple environmental changes.

King JS, Kubiske ME, Pregitzer KS, Hendrey GR, McDonald EP, Giardina CP, Quinn VS, Karnosky DF (2005) Tropospheric O3 compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2. New Phytologist 168:623-635

Experiments using step-change increases in carbon dioxide concentration may cause unrealistic ecological responses.   Attempts to understand ecological effects of increasing atmospheric CO2 concentration usually involve exposing plants or ecosystems to elevated CO2 concentration imposed with a one-time step-change increase of 200 ppm or more. An assumption underlying this approach is that exposing ecosystems to a single-step increase in CO2 concentration will cause similar ecological responses to those of a gradual increase over several decades; the ongoing increases in atmospheric CO2 concentration are occurring gradually. Klironomos et al. (2005) tested this assumption on a mycorrhizal fungal community over a period of 6 years. In the experiment, CO2 was either increased abruptly, as is typical in elevated-CO2 experiments, or more gradually over 21 plant generations. The two approaches resulted in different structural and functional community responses to increased CO2. Some fungi were sensitive to the carbon pulse of the abrupt CO2-increase treatment. This resulted in an immediate decline in fungal species richness and a significant change in mycorrhizal functioning. The magnitude of changes in fungal diversity and functioning in response to gradually increasing CO2 concentration was smaller, and not significantly different than those observed with a constant (present ambient) CO2 concentration. The authors suggested that "studies may overestimate some community responses to increasing [CO2] because biota may be sensitive to ecosystem changes that occur as a result of abrupt increases".

Klironomos JN, Allen MF, Rillig MC, Piotrowski J, Makvandi-Nejad S, Wolfe BE, Powell JR (2005) Abrupt rise in atmospheric CO2 overestimates community response in a model plant--soil system. Nature 433:621-624


Warming might damage tropical forest productivity.   How tropical rainforests are responding to the ongoing global changes in atmospheric composition and climate is little studied and poorly understood. Although rising atmospheric CO2 concentration could enhance forest productivity, increased temperatures and drought are likely to diminish it. The limited field data published have produced conflicting views of the net effects of changes in climate and atmospheric composition on tropical forests. In a review of available data, Clark (2004) found that one set of studies indicates enhanced carbon uptake by tropical forests, but questions have arisen about these findings, and recent experiments with tropical-forest trees indicate carbon saturation of canopy leaves and no biomass increase resulting from elevated CO2 concentration. Other field observations indicate decreased forest productivity and increased tree mortality in recent years of high temperature and drought (strong El Nino episodes). Clark concluded that (a) in order to determine responses of tropical forests to present climatic variability, careful annual monitoring of ecosystem performance in representative forests will be required and (b) a more complete understanding of tropical rainforest carbon cycling is needed for determining whether these ecosystems are carbon sinks or sources now, and how this status might change during the next 100 years.

Clark DA (2004) Sources or sinks? The responses of tropical forests to current and future climate and atmospheric composition. Philosophical Transactions of the Royal Society of London, Series B 359:477-491

Ecosystem models may perform better as a group than as individuals.   Models represent our primary method for integration of small-scale, process-level phenomena into a comprehensive synthesis of forest-stand or ecosystem functioning. They also represent a key method for testing hypotheses about the effects of climatic change on ecosystems. An extensive model testing project carried out by Hanson et al. (2004) evaluated 13 forest-stand-level models varying in their spatial, mechanistic, and temporal complexity for their ability to simulate seasonal and interannual water and carbon fluxes in an upland, oak-dominated forest of eastern Tennessee (the study followed the framework established earlier by the program for boreal forests [Amthor et al., 2001]). Comparisons between model simulations and independent field observations were conducted for hourly, daily, and annual time steps. Data for the comparisons were obtained from a wide range of methods including: eddy covariance, sapflow, chamber-based soil respiration, biometric estimates of stand-level net primary production and growth, and soil water content by time or frequency domain reflectometry. A variety of goodness-of-fit statistics (bias, absolute bias, and model efficiency) were used to judge model performance. A key result was that a single model did not consistently perform the best at all time steps or for all variables considered. Intermodel comparisons showed good agreement for water cycle fluxes, but considerable disagreement among models for simulated carbon fluxes. The mean of all models was nearly always the best fit to the observations. Not surprisingly, models excluding key forest components or processes, such as roots or modeled soil water content, were unable to accurately simulate forest responses to short-term drought. Nevertheless, an inability to correctly capture short-term physiological processes under drought was not necessarily an indicator of poor annual water and carbon budget simulations. This was the case because droughts in the field were of short duration and thus had only a small cumulative effect on annual end points. Models using hourly time steps and detailed mechanistic processes, and having a realistic spatial representation of the forest ecosystem provided the best fit to observed data. Predictive ability of all the models deteriorated with drought, indicating that further work is needed to evaluate and improve ecosystem model performance under drought, and probably other environmental changes.

Amthor JS, Chen JM, Clein JS, Frolking SE, Goulden ML, Grant RF, Kimball JS, King AW, McGuire AD, Nikolov NT, Potter CS, Wang S, Wofsy SC (2001) Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: intermodel comparisons and relationships to field measurements. Journal of Geophysical Research 106:33,623-33,648

Hanson PJ, Amthor JS, Wullschleger SD, Wilson KB, Grant RF, Hartley A, Hui D, Hunt ER Jr, Johnson DW, Kimball JS, King AW, Luo Y, McNulty SG, Sun G, Thornton PE, Wang SS, Williams M, Baldocchi DD, Cushman RM (2004) Carbon and water cycle simulations for an upland oak forest using 13 stand-level models: intermodel comparisons and evaluations against independent measurements. Ecological Monographs 74:443-489

Elevated carbon dioxide and ozone concentrations may differentially affect selected groups of fauna in temperate forest soils.   Rising atmospheric CO2 concentrations might affect soil faunal abundance, and how increasing tropospheric O3 concentration might modify any effects of elevated CO2 are largely unknown. Loranger et al. (2004) used the program's Rhinelander FACE experiment (see the 1997 Highlight below) to assess independent and interactive effects of elevated CO2 and O3 concentrations on selected groups of soil fauna. After 4 years of experimental treatment, soil fauna were extracted from soils in plots exposed to the ambient atmosphere, to elevated CO2, to elevated O3, and to elevated CO2 plus elevated O3. Compared to the ambient plots, abundance of total soil fauna, Collembola, and Acari decreased significantly under elevated CO2. Abundance of Acari decreased significantly with elevated O3. Most importantly, perhaps, was the observation that faunal abundance was similar in the ambient plots and the combination of elevated CO2 and elevated O3. The individual negative effects of elevated CO2 and elevated O3 were somehow negated with exposure to increased concentrations of both gases. The authors concluded that soil faunal communities would probably change under elevated CO2 and elevated O3 in ways that cannot be predicted or explained by results from experiments involving each gas individually.

Loranger GI, Pregitzer KS, King JS (2004) Elevated CO2 and O3 concentrations differently affect selected groups of fauna in temperate forest soils. Soil Biology & Biochemistry 36:1521-1524


Increasing atmospheric ozone concentration may slow soil organic matter formation.   Fossil fuel use caused increases in tropospheric O3 concentration during the past century, and the concentration may continue to increase during the coming decades. In addition to limiting primary production in terrestrial ecosystems, increased O3 concentration might affect the quantity and quality of carbon inputs to soils, but data on the subject are limited. The Rhinelander FACE experiment (see the 1997 Highlight below) provided unique experimental data on effects of elevated O3 concentration on the formation rates of total and decay-resistant acid-insoluble soil carbon in experimental aspen (Populus tremuloides) stands and in mixed aspen-birch (Betula papyrifera) stands. With O3 concentrations increased about 50% above present ambient, the formation rates of total and acid-insoluble soil carbon were reduced by 50% relative to the amounts entering the soil when the forests were exposed to the ambient atmosphere (Loya et al., 2003). (All the experimental plots used in the study received about 50% increase in CO2 concentration relative to the ambient atmosphere; the isotopic signature in the added CO2 was used as a tracer of current inputs of carbon to the soil.) The results indicate that any future increases in tropospheric O3 concentration could limit the rate of soil organic carbon formation.

Loya WM, Pregitzer KS, Karberg NJ, King JS, Giardina CP (2003) Reduction of soil carbon formation by tropospheric ozone under increased carbon dioxide levels. Nature 425:705-707


Elevated ozone concentration may predispose aspen leaves to increased infection by rust.  

Karnosky DF, Percy KE, Xiang B, Callan B, Noormets A, Mankovska B, Hopkin A, Sober J, Jones W, Dickson RE, Isebrands JG (2002) Interacting elevated CO2 and tropospheric O3 predisposes aspen (Populus tremuloides Michx.) to infection by rust (Melampsora medusae f. sp. tremuloidae). Global Change Biology 8:329-338


Saplings, but not larger trees, may be sensitive to altered precipitation in eastern hardwood forests.   Global climatic change may cause changes in regional precipitation that have important implications for forest growth in the southern United States. In 1993, a stand-level experiment was initiated on Walker Branch Watershed, Tennessee, to study the sensitivity of forest saplings and large trees to changes in soil water content. Soil water content was manipulated by gravity-driven transfer of precipitation throughfall from a dry treatment plot (-33%) to a wet treatment plot (+33%). A control plot was included. Each plot was 6400 m2. Measurements of stem diameter and observations of mortality were made on large trees and saplings of Acer rubrum L., Cornus florida L., Liriodendron tulipifera L., Nyssa sylvatica Marsh, Quercus alba L. and Quercus prinus L. every 2 weeks during six growing seasons. Saplings of C. florida and A. rubrum grew faster and mortality was less on the wet plot compared with the dry and control plots, through 6 years of soil water manipulation. Conversely, diameter growth of large trees was unaffected by the treatments. However, tree diameter growth averaged across treatments was affected by year-to- year changes in soil water status. Growth in wet years was as much as 2-3 times greater than in dry years. Relationships between tree growth, phenology and soil water potential were consistent among species and quantitative expressions were developed for application in models. These field growth data indicate that differences in seasonal patterns of rainfall within and between years have greater impacts on growth than percentage changes in rainfall applied to all rainfall events.

Hanson PJ, Todd DE Jr, Amthor JS (2001) A six-year study of sapling and large-tree growth and mortality responses to natural and induced variability in precipitation and throughfall. Tree Physiology 21:345-358

Plant diversity enhances ecosystem response to elevated carbon dioxide concentration and nitrogen deposition.   Human actions are causing declines in plant biodiversity, increases in atmospheric CO2 concentrations and increases in nitrogen deposition; however, the interactive effects of these factors on ecosystem processes are unknown1, 2. Reduced biodiversity has raised numerous concerns, including the possibility that ecosystem functioning may be affected negatively1, 2, 3, 4, which might be particularly important in the face of other global changes5, 6. Here we present results of a grassland field experiment in Minnesota, USA, that tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric CO2 concentrations and nitrogen deposition. The study experimentally controlled plant diversity (1, 4, 9 or 16 species), soil nitrogen (unamended versus deposition of 4 g of nitrogen per m2 per yr) and atmospheric CO2 concentrations using free-air CO2 enrichment (ambient, 368 mol mol-1, versus elevated, 560 mol mol-1). We found that the enhanced biomass accumulation in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.

Reich PB, Knops J, Tilman D, Craine J, Ellsworth D, Tjoelker M, Lee T, Wedin D, Naeem S, Bahauddin D, Hendrey G, Jose S, Wrage K, Goth J, Bengston W (2001) Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition. Nature 410:809-812; 411:824


Plant respiration rate is not directly affected by elevated carbon dioxide concentration.   Dozens of earlier publications indicated that elevated CO2 concentration directly inhibts plant respiration rate. The resulting implication was that increasing atmospheric CO2 concentration would give rise to a negative feedback to that CO2 increase because of reduced plant respiration (global annual plant respiration may release 10 times as much CO2 to the atmosphere as fossil fuel buring). In program research challenging this conclusion, Amthor (2000b) found that any direct inhibition of plant respiration rate by elevated CO2 concentration was small or nonexistent (earlier reports may have been based on measurement system imperfections). Later program research supported this finding that there is unlikely to be a negative feedback on atmospheric CO2 concentration increase brought about by a direct inhibition of plant respiration (e.g., Amthor et al., 2001; Tjoelker et al., 2001; Davey et al., 2004). Although plant respiration rate is not directly affected by atmospheric CO2 concentration, plant growth in elevated CO2 is expected to indirectly affect plant respiration rate through effects on plant growth rate, plant size, and tissue composition as indicated by other program output (Amthor 2000a).

Amthor JS (2000a) The McCree--de Wit--Penning de Vries--Thornley respiration paradigms: 30 years later. Annals of Botany 86:1-20

Amthor JS (2000b) Direct effect of elevated CO2 on nocturnal in situ leaf respiration in nine temperate deciduous tree species is small. Tree Physiology 20:139-144

Amthor JS, Koch GW, Willms JR, Layzell DB (2001) Leaf O2 uptake in the dark is independent of coincident CO2 partial pressure. Journal of Experimental Botany 52:2235-2238

Davey PA, Hunt S, Hymus GJ, DeLucia EH, Drake BG, Karnosky DF, Long SP (2004) Respiratory oxygen uptake is not decreased by an instantaneous elevation of [CO2], but is increased with long-term growth in the field at elevated [CO2]. Plant Physiology 134:520-527

Tjoelker MG, Oleksyn J, Lee TD, Reich PB (2001) Direct inhibition of leaf dark respiration by elevated CO2 is minor in 12 grassland species. New Phytologist 150:419-424

Tree photosynthesis and respiration acclimates to experimental warming.   Physiological acclimation and genotypic adaptation to prevailing temperatures may influence forest responses to future climatic warming. We examined photosynthetic and respiratory responses of sugar maple (Acer saccharum Marsh.) from two portions of the species' range for evidence of both phenomena in a laboratory study with seedlings. A field study was also conducted to assess the impacts of temperature acclimation on saplings subjected to an imposed temperature manipulation (4 C above ambient temperature). The two seedling populations exhibited more evidence of physiological acclimation to warming than of ecotypic adaptation, although respiration was less sensitive to short-term warming in the southern population than in the northern population. In both seedling populations, thermal compensation increased photosynthesis by 14% and decreased respiration by 10% in the warm-acclimated groups. Saplings growing in open-top field chambers at ambient temperature and 4 C above ambient temperature showed evidence of temperature acclimation, but photosynthesis did not increase in response to the 4 C warming. On the contrary, photosynthetic rates measured at the prevailing chamber temperature throughout three growing seasons were similar, or lower (12% lower on average) in saplings maintained at 4 C above ambient temperature compared with saplings maintained at ambient temperature. However, the long-term photosynthetic temperature optimum for saplings in the field experiment was higher than it was for seedlings in either the 27 or the 31 C growth chamber. Respiratory acclimation was also evident in the saplings in the field chambers. Saplings had similar rates of respiration in both temperature treatments, and respiration showed little dependence on prevailing temperature during the growing season. We conclude that photosynthesis and respiration in sugar maple have the potential for physiological acclimation to temperature, but exhibit a low degree of genetic adaptation. Some of the potential for acclimation to a 4 C increase above a background of naturally fluctuating temperatures may be offset by differences in water relations, and, in the long term, may be obscured by the inherent variability in rates under field conditions. Nevertheless, physiologically based models should incorporate seasonal acclimation to temperature and permit ecotypic differences to influence model outcomes for those species with high genetic differentiation between regions

Gunderson CA, Norby RJ, Wullschleger SD (2000) Acclimation of photosynthesis and respiration to simulated climatic warming in northern and southern populations of Acer saccharum: laboratory and field evidence. Tree Physiology 20:87-96


A possible tradeoff between root growth versus mycorrhizal hyphae growth in response to long-term elevated carbon dioxide concentration.  

Rillig MC, Field CB, Allen MF (1999) Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands. Oecologia 119:572-577



Northern hardwood forest free-air carbon dioxide and ozone enrichment experiment implemented.   Design and construction of the world's largest long-term study of ecological effects of changes in atmospheric composition (both CO2 and O3) was completed. The state-of-the-science open air exposure system will be used to examine the effects of about 50% increases in both CO2 and O3 concentrations (relative to present ambient) on growth, productivity, trophic interactions, and numerous ecosystem processes in constructed northern hardwood stands (near Rhinelander, Wisconsin). The experimental design is a full factorial including three replicate exposure rings for each of the following treatments: ambient air, elevated CO2, elevated O3, and the combined elevated CO2 and elevated O3 treatment. Each ring is populated with three constructed populations of trees. The eastern half of each ring includes a mixture of aspen (Populus tremuloides) clones, the northwestern quarter includes a mixture of aspen and maple (Acer saccharum), and the southwest quarter a mixture of aspen and birch (Betula papyrifera). Treatments will occur during daylight hours from aspen bud break to leaf drop and when wind speeds are greater than 0.5 m/sec and less than 4 m/sec. Fumigation with O3 will not occur when leaves are wet for any reason (rain, fog, or dew events) or when predicted daily maximum temperatures are less than 15 degrees Celsius. The experimental treatments began in 1998 and are expected to be maintained through 2008. A detailed description of the experiment was later presented by Dickson et al. (2000).

Dickson RE, Lewin KF, Isebrands JG, Coleman MD, Heilman WE, Riemenschneider DE, Sober J, Host GE, Zak DR, Hendrey GR, Pregitzer KS, Karnosky DF (2000) Forest Atmosphere Carbon Transfer and Storage (FACTS-II) The Aspen Free-air CO2 and O3 Enrichment (FACE) project: An Overview. USDA Forest Service, North Central Research Station, General Technical Report NC-214, 68 p


Climatic change might increase geographic ranges of cultivated CAM plants.   The economic importance of cultivated Crassulacean acid metabolism (CAM) plant species, which are relatively well adapted to arid regions, may be limited by both climate and atmospheric CO2 concentration. The effects of low temperature and CO2 concentration on the cultivated CAM species Agave salmiana (Agavaceae), Opuntia ficus-indica (Cactaceae), and Stenocereus queretaroensis (Cactaceae) were studied in controlled-environment laboratory chambers (Nobel, 1996). The observed low-temperature sensitivity of the species indicated that rising temperature would expand the geographic range within which they might be successfully cultivated. In addition, a doubling of atmospheric CO2 concentration increased CO2 uptake about 36% in the A. salmiana and S. queretaroensis studied, which might also increase the range within which they might be cultivated. The author concluded that "rising air temperatures and increasing atmospheric CO2 concentrations accompanying global climate change should increase the opportunities for cultivation of agaves, cacti and other CAM plants of potential economic importance in arid and semi-arid regions."

Nobel PS (1996) Responses of some North American CAM plants to freezing temperatures and doubled CO2 concentrations: implications of global climate change for extending cultivation. Journal of Arid Environments 34:187-196

Elevated carbon dioxide concentration affects soil microbial amount, but not composition.   Using open-top chambers to double the CO2 concentration around Populus grandidentata seedlings, Zak et al. (1996) quantified the amount and community composition of rhizosphere and non-rhizosphere bacteria, actinomycetes, and fungi. After one growing season the proportions of bacterial, actinomycetal, and fungal phospholipid fatty acids (or PLFAs, which were used as markers of different types of soil microorganisms) were not affected by twice-ambient compared to ambient CO2 concentration treatments. Although the elevated CO2 treatment significantly increased the amount of soil microbial biomass carbon in both rhizosphere and nonrhizosphere soil, the authors concluded that there was evidence for shifts in microbial community composition, at least during the first year of an elevated CO2 exposure.

Zak DR, Ringelberg D, Pregitzer KS, Randlett DL, White DW, Curtis PS (1996) Soil microbial communities beneath Populus grandidentata Michx. grown under elevated atmospheric CO2. Ecological Applications 6:257-262


Species differences affect model projections of forest response to elevated CO2 concentration.   An ecosystem model was used to explores how the response of a temperate forest ecosystem to elevated (twice ambient) CO2 concentration might depend on species diversity and community change (Bolker et al., 1995). A detailed, field-calibrated model of forest dynamics was combined with glasshouse data on the range of seedling biomass growth response to doubled CO2 concentration. To isolate the effects of community change in the response to elevated CO2 the regular model, allowing dynamic community change, was compared with runs of a reduced model that held species composition static by using a single tree species with average parameters. Simulations that allowed community change (compared with holding species composition constant) showed a roughly 30% additional increase in total basal area over time scales of 50-150 years. Although the model omitted many possible feedbacks and mechanisms associated with ecosystem responses to changes in atmospheric composition, it nonetheless indicated that there could be a large potential effect of changes in species composition. That result reinforced the possible importance of species diversity to ecosystem functioning over time scales relevant to ongoing changes in atmospheric composition associated with fossil fuel use.

Bolker BM, Pacala SW, Bazzaz FA, Canham CD, Levin SA (1995) Species diversity and ecosystem response to carbon dioxide fertilization: conclusions from a temperate forest model. Global Change Biology 1:373-381

Elevated CO2 affects fine-root turnover of aspen trees.   Effects of twice-ambient atmospheric CO2 concentration on the growt hand turnover of fine roots of aspen was studied below open-top chambers, using microvideo and iamge analysis technology (Pregitzer et al., 1995). Doubling CO2 concentration increased rates of both fine root production and mortality in aspen cuttings. Rates of fine root mortality also increased substantially as soil nitrogen availability was increased, in both CO2 treatments. Nitrogen greatly influenced the proportional partitioning of carbon between leaves and find rots. The authors concluded that the amount of available nitrogen in the soil appeared to be the most important factor regulating fine root demography in the aspen genotype studied.

Pregitzer KS, Zak DR, Curtis PS, Kubiske ME, Teeri JA, Vogel CS (1995) Atmospheric CO2, soil nitrogen and turnover of fine roots. New Phytologist 129:579-585



Throughfall Displacement Experiment (TDE) implemented in eastern Tennessee.   The program sponsored the design and construction of the largest long-term field study of effects of altered precipitation on ecosystem structure and functioning -- the forest stand-level experimental manipulation of throughfall amount on the Walker Branch Watershed at the DOE Oak Ridge National Laboratory in eastern Tennessee (Turner et al., 1993). The experiment (which was conducted continuously for 14 years, beginning in 1993) included three square 0.64-ha (i.e., 80 x 80 m) plots placed in an aggrading hardwood forest. One plot received 67% of ambient througfall amount, one plot received the ambient amount of throughfall, and one plot received 133% of ambient throughfall amount. The treatments resulted in significant summer soil water content differences between plots, which served as a surrogate for altered precipitation amount. The experiment was conducted to study potential effects of possible future changes in precipitation amount (which might accompany future global warming) on the structure and functioning of eastern deciduous forests. Over the course of the experiment, effects of the treatments on tree growth, tree physiology, tree biochemistry, sapling growth and survival, forest floor dynamics, soil water and chemistry, and fungal and arthropod growth and activity were examined. A comprehensive analysis of results of the first 8 years of the experiment was presented in Hanson and Wullschleger (2003).

Hanson PJ, Wullschleger SD (editors) (2003) North American Temperate Deciduous Forest Responses to Changing Precipitation Regimes. Ecological Studies, Vol. 166. Springer, New York, 472 p

Turner RS, Hanson PJ, Huston MA, Garten CT Jr, Mulholland PJ (1993) A large-scale throughfall manipulation experiment on Walker Branch Watershed. In: L Rasmussen, T Brydges, P Mathy (eds) Experimental Manipulations of Biota and Biogeochemical Cycling in Ecosystems: Approach -- Methodologies -- Findings. Office for Official Publications of the European Communities, Luxembourg, 96-105