Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of [CO2] Are Closely Coupled with Decreases in Ecosystem Evapotranspiration
TL;DR: The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased gs of upper canopy leaves at elevated [CO2] results in decreased transfer of water vapor to the atmosphere.
Abstract: Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO2 ([CO2]) have been shown to decrease stomatal conductance (gs) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant, and canopy microclimate levels may feed back on ET, it is not certain that a decrease in gs will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO2] on gs at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (approximately 375 μmol CO2 mol−1 air) and elevated [CO2] (approximately 550 μmol mol−1) using free air CO2 enrichment. ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO2] caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with gs of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in gs results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO2]. The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased gs of upper canopy leaves at elevated [CO2] results in decreased transfer of water vapor to the atmosphere.
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TL;DR: Some of the lessons learned from the long-term investment in Free-Air CO(2) Enrichment experiments are described, where many of these lessons have been most clearly demonstrated in crop systems, and have important implications for natural systems.
Abstract: Plant responses to the projected future levels of CO2 were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO2 were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO2 Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO2 on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO2 under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO2 stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO2 improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO2 stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO2 does not directly stimulate C4 photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO2 in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.
1,377 citations
Cites background from "Decreases in Stomatal Conductance o..."
...A meta-analysis of SoyFACE results indicates an average 16% increase in Asat across a variety of soybean cultivars, which is substantially smaller than suggested by chamber studies (Fig....
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...Maize grown at SoyFACE in a rain-fed experiment allowed comparison between an ‘average’ year (2002) that included periods of drought stress versus an ‘atypical’ year (2004) in which the crop experienced no drought stress across the entire season (Leakey et al., 2006)....
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...Boxes overlapping 0 indicate no significant change at elevated [CO2]. at Biology Library on 13 July 2009 http://jxb.oxfordjournals.orgDownloaded from Vc,max at SoyFACE, which, in turn, drove a decrease in the Vc,max:Jmax inferring a shift in resource investment away from Rubisco (Bernacchi et al., 2005)....
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...…acclimation to [CO2] (Leakey et al., 2006a) means that the relationship of gs to ET would be similar for plants et al. (2000) and Kimball et al. (1995), cotton from Hunsaker et al. (1994) and Kimball et al. (1994), sorghum from Triggs et al. (2004), and soybean from Bernacchi et al. (2007)....
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...The mechanism of soybean respiratory responses to growth at elevated [CO2] has been investigated in two studies that combined molecular, biochemical, and physiological analyses of plants at the SoyFACE experiment in which the findings were consistent with the hypotheses of Williams and Farrar (1990)....
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TL;DR: In this paper, the impact of temperature, CO 2, and ozone on agronomic crops and the implications for crop production are discussed and a review of the impact on agricultural crops is presented.
Abstract: Changes in temperature, CO 2 , and precipitation under the scenarios of climate change for the next 30 yr present a challenge to crop production. This review focuses on the impact of temperature, CO 2 , and ozone on agronomic crops and the implications for crop production. Understanding these implications for agricultural crops is critical for developing cropping systems resilient to stresses induced by climate change. There is variation among crops in their response to CO 2 , temperature, and precipitation changes and, with the regional differences in predicted climate, a situation is created in which the responses will be further complicated. For example, the temperature effects on soybean [Glycine max (L.) Merr.] could potentially cause yield reductions of 2.4% in the South but an increase of 1.7% in the Midwest. The frequency of years when temperatures exceed thresholds for damage during critical growth stages is likely to increase for some crops and regions. The increase in CO 2 contributes significantly to enhanced plant growth and improved water use efficiency (WUE); however, there may be a downscaling of these positive impacts due to higher temperatures plants will experience during their growth cycle. A challenge is to understand the interactions of the changing climatic parameters because of the interactions among temperature, CO 2 , and precipitation on plant growth and development and also on the biotic stresses of weeds, insects, and diseases. Agronomists will have to consider the variations in temperature and precipitation as part of the production system if they are to ensure the food security required by an ever increasing population.
1,146 citations
TL;DR: A substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades is found, and a partial closure of stomata is suggested to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels.
Abstract: Terrestrial plants remove CO2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves. The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon. Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange. We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata-small pores on the leaf surface that regulate gas exchange-to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation-climate models.
955 citations
TL;DR: In this article, a meta-analysis synthesizes the research to date on rice responses to two elements of global change, rising atmospheric carbon dioxide concentration ([CO2]) and rising tropospheric ozone concentration ([O3]).
Abstract: Rice is arguably the most important food source on the planet and is consumed by over half of the world’s population. Considerable increases in yield are required over this century to continue feeding the world’s growing population. This meta-analysis synthesizes the research to date on rice responses to two elements of global change, rising atmospheric carbon dioxide concentration ([CO2]) and rising tropospheric ozone concentration ([O3]). On an average, elevated [CO2] (627 ppm) increased rice yields by 23%. Modest increases in grain mass and larger increases in panicle and grain number contributed to this response. The response of rice to elevated [CO2] varied with fumigation technique. The more closely the fumigation conditions mimicked field conditions, the smaller was the stimulation of yield by elevated [CO2]. Free air concentration enrichment (FACE) experiments showed only a 12% increase in rice yield. The rise in atmospheric [CO2] will be accompanied by increases in tropospheric O3 and temperature. When compared with rice grown in charcoal-filtered air, rice exposed to 62 ppb O3 showed a 14% decrease in yield. Many determinants of yield, including photosynthesis, biomass, leaf area index, grain number and grain mass, were reduced by elevated [O3]. While there have been too few studies of the interaction of CO2 and O3 for meta-analysis, the interaction of temperature and CO2 has been studied more widely. Elevated temperature treatments negated any enhancement in rice yield at elevated [CO2], which suggests that identifying high temperature tolerant germplasm will be key to realizing yield benefits in the future.
457 citations
Cites background from "Decreases in Stomatal Conductance o..."
...Such high spikelet temperatures might be exacerbated by reductions in transpiration and therefore evaporative cooling of the leaf canopy at elevated [CO2] ( Bernacchi et al., 2007 )....
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TL;DR: Water use efficiency (WUE) is defined as the amount of carbon assimilated as biomass or grain produced per unit of water used by the crop and there are opportunities to enhance WUE through crop selection and cultural practices to offset the impact of a changing climate.
Abstract: Water use efficiency (WUE) is defined as the amount of carbon assimilated as biomass or grain produced per unit of water used by the crop. One of the primary questions being asked is how plants will respond to a changing climate with changes in temperature, precipitation, and carbon dioxide (CO2) that affect their WUE At the leaf level, increasing CO2 increases WUE until the leaf is exposed to temperatures exceeded the optimum for growth (i.e., heat stress) and then WUE begins to decline. Leaves subjected to water deficits (i.e., drought stress) show varying responses in WUE. The response of WUE at the leaf level is directly related to the physiological processes controlling the gradients of CO2 and H2O, e.g., leaf:air vapor pressure deficits, between the leaf and air surrounding the leaf. There a variety of methods available to screen genetic material for enhanced WUE under scenarios of climate change. When we extend from the leaf to the canopy, then the dynamics of crop water use and biomass accumulation have to consider soil water evaporation rate, transpiration from the leaves, and the growth pattern of the crop. Enhancing WUE at the canopy level can be achieved by adopting practices that reduce the soil water evaporation component and divert more water into transpiration which can be through crop residue management, mulching, row spacing, and irrigation. Climate change will affect plant growth, but we have opportunities to enhance WUE through crop selection and cultural practices to offset the impact of a changing climate.
382 citations
Cites background from "Decreases in Stomatal Conductance o..."
...Free –air CO2 experiments conducted in Illinois on soybean grown at 375 and 550 µmol mol−1 revealed an ET decrease of 9–16% with the differences caused variation in temperature among the growing seasons (Bernacchi et al., 2007)....
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...The effect of increased CO2 on seasonal crop water use was observed by Bernacchi et al. (2007) when they found the control plots extracted the available soil water and the crop become water limited....
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References
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TL;DR: The results from this review may provide the most plausible estimates of how plants in their native environments and field-grown crops will respond to rising atmospheric [CO(2)]; but even with FACE there are limitations, which are discussed.
Abstract: Contents
Summary 1
I. What is FACE? 2
II. Materials and methods 2
III. Photosynthetic carbon uptake 3
IV. Acclimation of photosynthesis 6
V. Growth, above-ground production and yield 8
VI. So, what have we learned? 10
Acknowledgements 11
References 11
Appendix 1. References included in the database for meta-analyses 14
Appendix 2. Results of the meta-analysis of FACE effects 18
Summary
Free-air CO2 enrichment (FACE) experiments allow study of the effects of elevated [CO2] on plants and ecosystems grown under natural conditions without enclosure. Data from 120 primary, peer-reviewed articles describing physiology and production in the 12 large-scale FACE experiments (475–600 ppm) were collected and summarized using meta-analytic techniques. The results confirm some results from previous chamber experiments: light-saturated carbon uptake, diurnal C assimilation, growth and above-ground production increased, while specific leaf area and stomatal conductance decreased in elevated [CO2]. There were differences in FACE. Trees were more responsive than herbaceous species to elevated [CO2]. Grain crop yields increased far less than anticipated from prior enclosure studies. The broad direction of change in photosynthesis and production in elevated [CO2] may be similar in FACE and enclosure studies, but there are major quantitative differences: trees were more responsive than other functional types; C4 species showed little response; and the reduction in plant nitrogen was small and largely accounted for by decreased Rubisco. The results from this review may provide the most plausible estimates of how plants in their native environments and field-grown crops will respond to rising atmospheric [CO2]; but even with FACE there are limitations, which are also discussed.
3,140 citations
TL;DR: Although trends agree with parallel summaries of enclosure studies, important quantitative differences emerge that have important implications both for predicting the future terrestrial biosphere and understanding how crops may need to be adapted to the changed and changing atmosphere.
Abstract: Atmospheric CO(2) concentration ([CO(2)]) is now higher than it was at any time in the past 26 million years and is expected to nearly double during this century. Terrestrial plants with the C(3) photosynthetic pathway respond in the short term to increased [CO(2)] via increased net photosynthesis and decreased transpiration. In the longer term this increase is often offset by downregulation of photosynthetic capacity. But much of what is currently known about plant responses to elevated [CO(2)] comes from enclosure studies, where the responses of plants may be modified by size constraints and the limited life-cycle stages that are examined. Free-Air CO(2) Enrichment (FACE) was developed as a means to grow plants in the field at controlled elevation of CO(2) under fully open-air field conditions. The findings of FACE experiments are quantitatively summarized via meta-analytic statistics and compared to findings from chamber studies. Although trends agree with parallel summaries of enclosure studies, important quantitative differences emerge that have important implications both for predicting the future terrestrial biosphere and understanding how crops may need to be adapted to the changed and changing atmosphere.
1,566 citations
"Decreases in Stomatal Conductance o..." refers background in this paper
...With very few exceptions, decreased stomatal conductance (gs) is one of the most consistent and conserved responses of leaves to growth at elevated [CO2] (Curtis, 1996; Lee et al., 2001; Medlyn et al., 2001; Zheng and Peng, 2001; Ainsworth et al., 2002; Long et al., 2004; Ainsworth and Long, 2005)....
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TL;DR: Modern schemes incorporate biogeochemical and ecological knowledge and, when coupled with advanced climate and ocean models, will be capable of modeling the biological and physical responses of the Earth system to global change, for example, increasing atmospheric carbon dioxide.
Abstract: Atmospheric general circulation models used for climate simulation and weather forecasting require the fluxes of radiation, heat, water vapor, and momentum across the land-atmosphere interface to be specified. These fluxes are calculated by submodels called land surface parameterizations. Over the last 20 years, these parameterizations have evolved from simple, unrealistic schemes into credible representations of the global soil-vegetation-atmosphere transfer system as advances in plant physiological and hydrological research, advances in satellite data interpretation, and the results of large-scale field experiments have been exploited. Some modern schemes incorporate biogeochemical and ecological knowledge and, when coupled with advanced climate and ocean models, will be capable of modeling the biological and physical responses of the Earth system to global change, for example, increasing atmospheric carbon dioxide.
1,390 citations
"Decreases in Stomatal Conductance o..." refers background in this paper
...Soybean represents a large amount of the land cover in the Midwest Corn Belt, and, therefore, any decrease in ET compensated by increased Tc and H is likely to have a more profound influence on atmospheric conditions (e.g. Sellers et al., 1997)....
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...Most of this ecosystem is rain fed, and, since it is located in a continental interior, any direct effect of rising concentration of CO2 [CO2] on its transpiration could have a large impact on the regional climate (Sellers et al., 1997)....
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...However, all of these experiments concerned irrigated or flooded systems in which the potential feedback from vegetation to the climate would be expected to be low compared to rain-fed areas in continental interiors (Sellers et al., 1997), such as the Midwestern United States....
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TL;DR: An overview of micrometeorological theory and the different micromETeorological techniques available to make flux measurements is provided.
Abstract: Ecologists are expected to play an important role in future studies of the biosphere/atmosphere exchange of materials associated with the major biogeochemical cycles and climate. Most studies of material exchange reported in the ecological literature have relied on chamber techniques. Micrometeorological techniques provide an alternative means of measuring these exchange rates and are expected to be used more often in future ecological studies, since they have many advantages over the chamber techniques. In this article we will provide an overview of micrometeorological theory and the different micrometeorological techniques available to make flux measurements.
1,258 citations
"Decreases in Stomatal Conductance o..." refers methods in this paper
...Other meteorological methods that provide more direct measures of ET, such as eddy covariance or flux gradient analysis (Baldocchi et al., 1988), require a much larger fetch than can be provided using FACE (Kimball et al., 1999; Triggs et al., 2004)....
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Pennsylvania State University1, University of Southampton2, Yale University3, University of Arizona4, Goddard Institute for Space Studies5, Cooperative Institute for Research in Environmental Sciences6, University of Chicago7, University of Washington8, University of Bern9, University of California, San Diego10
TL;DR: Policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.
Abstract: Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.
1,218 citations
"Decreases in Stomatal Conductance o..." refers background in this paper
...Decreases in ET in continental regions have been identified as a potential amplifier for abrupt climate change (Alley et al., 2003)....
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