Direct and indirect effects of elevated atmospheric CO2 on net ecosystem production in a Chesapeake Bay tidal wetland

The rapid increase in atmospheric CO₂ concentrations (C_a) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO₂ between the atmosphere and the biosphere (NEE), under both ambient and elevated C_a. Here we report data on annual sums of CO₂ (NEE_net) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C₃ photosynthetic pathway)‐ and Spartina patens (C₄ photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated C_a (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated C_a on seasonally integrated CO₂ assimilation (NEE_net = NEE_day + NEE_night, kg C m^?2 y^?1) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE_net averaged 1.9 kg m^?2 y^?1 in ambient C_a and 2.5 kg m^?2 y^?1 in elevated C_a, for the C₃‐dominated community. Similarly, elevated C_a significantly (P < 0.01) increased carbon uptake in the C₄‐dominated community, as NEE_net averaged 1.5 kg m^?2 y^?1 in ambient C_a and 1.7 kg m^?2 y^?1 in elevated C_a. This resulted in an average CO₂ stimulation of 32% and 13% of seasonally integrated NEE_net for the C₃‐ and C₄‐dominated communities, respectively. Increased NEE_day was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated C_a, while decreased NEE_night was associated with lower canopy nitrogen content. These results suggest that rising C_a may increase carbon assimilation in both C₃‐ and C₄‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

Evapotranspiration and water use efficiency in a Chesapeake Bay wetland under carbon dioxide enrichment

Wetlands evapotranspire more water than other ecosystems, including agricultural, forest and grassland ecosystems. However, the effects of elevated atmospheric carbon dioxide (CO₂) concentration (C_a) on wetland evapotranspiration (ET) are largely unknown. Here, we present data on 12 years of measurements of ET, net ecosystem CO₂ exchange (NEE), and ecosystem water use efficiency (EWUE, i.e. NEE/ET) at 13:00–15:00 hours in July and August for a Scirpus olneyi (C3 sedge) community and a Spartina patens (C4 grass) community exposed to ambient and elevated (ambient+340 μmol mol^?1) C_a in a Chesapeake Bay wetland. Although a decrease in stomatal conductance at elevated C_a in the S. olneyi community was counteracted by an increase in leaf area index (LAI) to some extend, ET was still reduced by 19% on average over 12 years. In the S. patens community, LAI was not affected by elevated C_a and the reduction of ET was 34%, larger than in the S. olneyi community. For both communities, the relative reduction in ET by elevated C_a was directly proportional to precipitation due to a larger reduction in stomatal conductance in the control plants as precipitation decreased. NEE was stimulated about 36% at elevated C_a in the S. olneyi community but was not significantly affected by elevated C_a in S. patens community. A negative correlation between salinity and precipitation observed in the field indicated that precipitation affected ET through altered salinity and interacted with growth C_a. This proposed mechanism was supported by a greenhouse study that showed a greater C_a effect on ET in controlled low salinity conditions compared with high salinity. In spite of the differences between the two communities in their responses to elevated C_a, EWUE was increased about 83% by elevated C_a in both the S. olneyi and S. patens communities. These findings suggest that rising C_a could have significant impacts on the hydrologic cycles of coastal wetlands.     

Does deep soil N availability sustain long‐term ecosystem responses to elevated CO2?

A scrub‐oak woodland has maintained higher aboveground biomass accumulation after 11 years of atmospheric CO₂ enrichment (ambient +350 μmol CO₂ mol^?1), despite the expectation of strong nitrogen (N) limitation at the site. We hypothesized that changes in plant available N and exploitation of deep sources of inorganic N in soils have sustained greater growth at elevated CO₂. We employed a suite of assays performed in the sixth and 11th year of a CO₂ enrichment experiment designed to assess soil N dynamics and N availability in the entire soil profile. In the 11th year, we found no differences in gross N flux, but significantly greater microbial respiration (P≤0.01) at elevated CO₂. Elevated CO₂ lowered extractable inorganic N concentrations (P=0.096) considering the whole soil profile (0–190 cm). Conversely, potential net N mineralization, although not significant in considering the entire profile (P=0.460), tended to be greater at elevated CO₂. Ion‐exchange resins placed in the soil profile for approximately 1 year revealed that potential N availability at the water table was almost 3 × greater than found elsewhere in the profile, and we found direct evidence using a ¹⁵N tracer study that plants took up N from the water table. Increased microbial respiration and shorter mean residence times of inorganic N at shallower depths suggests that enhanced SOM decomposition may promote a sustained supply of inorganic N at elevated CO₂. Deep soil N availability at the water table is considerable, and provides a readily available source of N for plant uptake. Increased plant growth at elevated CO₂ in this ecosystem may be sustained through greater inorganic N supply from shallow soils and N uptake from deep soil.     

Acclimation of photosynthesis,respiration and ecosystem carbon flux of a wetland on Chesapeake Bay,Maryland to elevated atmospheric CO2 concentration

Acclimation of photosynthesis and respiration in shoots and ecosystem carbon dioxide fluxes to rising atmospheric carbon dioxide concentration (C _a ) was studied in a brackish wetland. Open top chambers were used to create test atmospheres of normal ambient and elevated C _a (=normal ambient + 34 Pa CO₂) over mono-specific stands of the C₃ sedge Scirpus olneyi, the dominant C₃ species in the wetland ecosystem, throughout each growing season since April of 1987. Acclimation of photosynthesis and respiration were evaluated by measurements of gas exchange in excised shoots. The impact of elevated C _a on the accumulation of carbon in the ecosystem was determined by ecosystem gas exchange measurements made using the open top chamber as a cuvette.Elevated C _a increased carbohydrate and reduced Rubisco and soluble protein concentrations as well as photosynthetic capacity(A) and dark respiration (R _d ; dry weight basis) in excised shoots and canopies (leaf area area basis) of Scirpus olneyi. Nevertheless, the rate of photosynthesis was stimulated 53% in shoots and 30% in canopies growing in elevated C _a compared to normal ambient concentration. Elevated C _a inhibited R _d measured in excised shoots (–19 to –40%) and in seasonally integrated ecosystem respiration (R _e ; –36 to –57%). Growth of shoots in elevated C _a was stimulated 14–21%, but this effect was not statistically significant at peak standing biomass in midseason. Although the effect of elevated C _a on growth of shoots was relatively small, the combined effect of increased number of shoots and stimulation of photosynthesis produced a 30% stimulation in seasonally integrated gross primary production (GPP). The stimulation of photosynthesis and inhibition of respiration by elevated C _a increased net ecosystem production (NEP=GPP–R _e ) 59% in 1993 and 50% in 1994. While this study consistently showed that elevated C _a produced a significant increase in NEP, we have not identified a correspondingly large pool of carbon below ground.     

Intensified inundation shifts a freshwater wetland from a CO2 sink to a source

Climate change has altered global precipitation patterns and has led to greater variation in hydrological conditions. Wetlands are important globally for their soil carbon storage. Given that wetland carbon processes are primarily driven by hydrology, a comprehensive understanding of the effect of inundation is needed. In this study, we evaluated the effect of water level (WL) and inundation duration (ID) on carbon dioxide (CO₂) fluxes by analysing a 10‐year (2008–2017) eddy covariance dataset from a seasonally inundated freshwater marl prairie in the Everglades National Park. Both gross primary production (GPP) and ecosystem respiration (ER) rates showed declines under inundation. While GPP rates decreased almost linearly as WL and ID increased, ER rates were less responsive to WL increase beyond 30 cm and extended inundation periods. The unequal responses between GPP and ER caused a weaker net ecosystem CO₂ sink strength as inundation intensity increased. Eventually, the ecosystem tended to become a net CO₂ source on a daily basis when either WL exceeded 46 cm or inundation lasted longer than 7 months. Particularly, with an extended period of high‐WLs in 2016 (i.e., WL remained >40 cm for >9 months), the ecosystem became a CO₂ source, as opposed to being a sink or neutral for CO₂ in other years. Furthermore, the extreme inundation in 2016 was followed by a 4‐month postinundation period with lower net ecosystem CO₂ uptake compared to other years. Given that inundation plays a key role in controlling ecosystem CO₂ balance, we suggest that a future with more intensive inundation caused by climate change or water management activities can weaken the CO₂ sink strength of the Everglades freshwater marl prairies and similar wetlands globally, creating a positive feedback to climate change.     

开垦对黄河三角洲湿地净生态系统CO2交换的影响

近年来, 由于对湿地的不合理利用, 自然湿地被大面积地垦殖为农田, 导致湿地生态系统碳循环的模式发生改变, 从而影响了湿地生态系统碳汇功能。该研究通过涡度相关法, 对山东省东营市黄河三角洲芦苇(Phragmites australis)湿地和开垦多年的棉花(Gossypium spp.)农田的净生态系统CO₂交换(NEE)进行了对比观测, 以探讨该地区典型生态系统NEE的变化规律及其影响因子, 揭示开垦对芦苇湿地NEE和碳汇功能的影响。结果表明: 在生长季, 湿地和农田生态系统NEE的日平均值各月均呈明显的“U”型变化曲线, 非生长季NEE的变幅很小。生长季湿地生态系统日最大净吸收值和释放值分别为16.04 g CO₂·m^-2·d^-1(8月17日)和14.95 g CO₂·m^-2·d^-1(8月9日); 农田生态系统日最大净吸收值和释放值分别为18.99 g CO₂·m^-2·d^-1 (8月22日)和12.23 g CO₂·m^-2·d^-1 (7月29日)。生长季白天两个生态系统NEE与光合有效辐射(PAR)之间呈直角双曲线关系; 非生长季NEE主要受土壤温度(T_s)的影响; 生态系统生长季夜间NEE受T_s和土壤含水量(SWC)的共同影响; 湿地和农田的生态系统呼吸熵(Q₁₀)分别为2.30和3.78。2011年生长季, 黄河三角洲湿地和农田生态系统均表现为CO₂的汇, 总净固碳量分别为780.95和647.35 g CO₂·m^-2, 开垦降低了湿地的碳吸收能力; 而在2011年非生长季, 黄河三角洲湿地和农田生态系统均表现为CO₂的源, CO₂总释放量分别为181.90和111.55 g CO₂·m^-2。全年湿地和农田生态系统总净固碳量分别为599.05和535.80 g CO₂·m^-2。     

Temperature and precipitation controls over leaf‐ and ecosystem‐level CO2 flux along a woody plant encroachment gradient

Conversion of grasslands to woodlands may alter the sensitivity of CO₂ exchange of individual plants and entire ecosystems to air temperature and precipitation. We combined leaf‐level gas exchange and ecosystem‐level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large‐scale vegetative shifts. Optimum temperatures (T_opt) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (Ω₅₀). Under dry pre‐ and postmonsoon conditions, leaf‐level Ω₅₀ in C₃ shrubs were two‐to‐three times that of C₄ grasses, but under moist monsoon conditions, leaf‐level Ω₅₀ was similar between growth forms. At the ecosystems‐scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water‐limited periods, woody plants assimilated larger amounts of carbon. This higher carbon‐assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C₄ grasses to C₃ shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.     

Photosynthetic responses of two eucalypts to industrial‐age changes in atmospheric [CO2] and temperature

The unabated rise in atmospheric [CO₂] is associated with increased air temperature. Yet, few CO₂‐enrichment studies have considered pre‐industrial [CO₂] or warming. Consequently, we quantified the interactive effects of growth [CO₂] and temperature on photosynthesis of faster‐growing Eucalyptus saligna and slower‐growing E. sideroxylon. Well‐watered and ‐fertilized tree seedlings were grown in a glasshouse at three atmospheric [CO₂] (290, 400, and 650 µL L^?1), and ambient (26/18 °C, day/night) and high (ambient + 4 °C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO₂] and temperature treatments with few interactive effects. Light‐saturated photosynthesis (A_sat) and light‐ and [CO₂]‐saturated photosynthesis (A_max) increased by ～50% and ～10%, respectively, with each step‐increase in growth [CO₂], underpinned by a corresponding 6–11% up‐regulation of maximal electron transport rate (J_max). Maximal carboxylation rate (V_cmax) was not affected by growth [CO₂]. Thermal photosynthetic acclimation occurred such that A_sat and A_max were similar in ambient‐ and high‐temperature‐grown plants. At high temperature, the thermal optimum of A_sat increased by 2–7 °C across [CO₂] treatments. These results are the first to suggest that photosynthesis of well‐watered and ‐fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO₂] in a future, warmer climate.     

Responses of terrestrial ecosystems to temperature and precipitation change: a meta‐analysis of experimental manipulation

Global mean temperature is predicted to increase by 2–7 °C and precipitation to change across the globe by the end of this century. To quantify climate effects on ecosystem processes, a number of climate change experiments have been established around the world in various ecosystems. Despite these efforts, general responses of terrestrial ecosystems to changes in temperature and precipitation, and especially to their combined effects, remain unclear. We used meta‐analysis to synthesize ecosystem‐level responses to warming, altered precipitation, and their combination. We focused on plant growth and ecosystem carbon (C) balance, including biomass, net primary production (NPP), respiration, net ecosystem exchange (NEE), and ecosystem photosynthesis, synthesizing results from 85 studies. We found that experimental warming and increased precipitation generally stimulated plant growth and ecosystem C fluxes, whereas decreased precipitation had the opposite effects. For example, warming significantly stimulated total NPP, increased ecosystem photosynthesis, and ecosystem respiration. Experimentally reduced precipitation suppressed aboveground NPP (ANPP) and NEE, whereas supplemental precipitation enhanced ANPP and NEE. Plant productivity and ecosystem C fluxes generally showed higher sensitivities to increased precipitation than to decreased precipitation. Interactive effects of warming and altered precipitation tended to be smaller than expected from additive, single‐factor effects, though low statistical power limits the strength of these conclusions. New experiments with combined temperature and precipitation manipulations are needed to conclusively determine the importance of temperature–precipitation interactions on the C balance of terrestrial ecosystems under future climate conditions.     

Canopy photosynthesis of crops and native plant communities exposed to long-term elevated CO2

Abstract. There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO₂: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO₂ increased canopy photosynthesis in all cases. The relative effect of CO₂ was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO₂ were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO₂ treatment to date, monospecific stands of a C₃ sedge, Scirpus olneyi (Grey), and a C₄ grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO₂ concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO₂ exchange per unit green biomass (NCE_b) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO₂ increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.     

Climatic role of terrestrial ecosystem under elevated CO2: a bottom‐up greenhouse gases budget

The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO₂) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO₂. We show that elevated CO₂ significantly stimulates plant C pool (NPP) by 20%, soil CO₂ fluxes by 24%, and methane (CH₄) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH₄ uptake of upland soils by 3.8%. Elevated CO₂ causes insignificant increases in soil nitrous oxide (N₂O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO₂‐induced increase in GHG emissions may decline with CO₂ enrichment levels. An elevated CO₂‐induced rise in soil CH₄ and N₂O emissions (2.76 Pg CO₂‐equivalent year^?1) could negate soil C enrichment (2.42 Pg CO₂ year^?1) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO₂ year^?1) under elevated CO₂. Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO₂ might have been largely offset by its induced increases in soil GHGs source strength.     

阴天和晴天对黄河三角洲芦苇湿地净生态系统CO2交换的影响

云量以及大气气溶胶含量变化引起的阴天和晴天会对局地的微气候环境产生综合效应, 影响地面接收的太阳辐射强度, 同时引起环境因子的变化, 最终对净生态系统CO₂交换(NEE)产生影响。该文通过涡度相关系统以及微气象梯度观测系统, 对黄河三角洲芦苇(Phragmites australis)湿地NEE以及环境要素进行了观测。在自然条件下选择12对相邻阴天和晴天数据, 在生物要素(生物量、叶面积指数)、土壤水分以及养分特征保持不变的前提下, 揭示了阴天和晴天变化对湿地生态系统NEE的光响应和温度响应的影响。结果表明: 12对阴天和晴天生态系统NEE的日平均动态均呈“U”型曲线, 但阴天NEE的变幅较小。晴天条件下湿地生态系统NEE的日均值显著高于阴天(p < 0.01)。阴天和晴天湿地生态系统NEE与光合有效辐射(PAR)之间均呈直角双曲线关系, 但晴天条件下, 最大光合速率(A_max)显著大于阴天(p < 0.01), 同时白天生态系统呼吸(R_eco,daytime)也显著大于阴天(p < 0.01)。不论阴天还是晴天, R_eco,daytime与气温均呈显著的指数关系。晴天湿地生态系统呼吸的温度敏感系数Q₁₀ (5.5)远大于阴天(1.9)。阴天和晴天昼间PAR差值以及气温差值对NEE差值的协同影响达到63%。     

Gross primary production responses to warming,elevated CO2, and irrigation: quantifying the drivers of ecosystem physiology in a semiarid grassland

Determining whether the terrestrial biosphere will be a source or sink of carbon (C) under a future climate of elevated CO₂ (eCO₂) and warming requires accurate quantification of gross primary production (GPP), the largest flux of C in the global C cycle. We evaluated 6 years (2007–2012) of flux‐derived GPP data from the Prairie Heating and CO₂ Enrichment (PHACE) experiment, situated in a grassland in Wyoming, USA. The GPP data were used to calibrate a light response model whose basic formulation has been successfully used in a variety of ecosystems. The model was extended by modeling maximum photosynthetic rate (A_max) and light‐use efficiency (Q) as functions of soil water, air temperature, vapor pressure deficit, vegetation greenness, and nitrogen at current and antecedent (past) timescales. The model fits the observed GPP well (R² = 0.79), which was confirmed by other model performance checks that compared different variants of the model (e.g. with and without antecedent effects). Stimulation of cumulative 6‐year GPP by warming (29%, P = 0.02) and eCO₂ (26%, P = 0.07) was primarily driven by enhanced C uptake during spring (129%, P = 0.001) and fall (124%, P = 0.001), respectively, which was consistent across years. Antecedent air temperature (Tair_ant) and vapor pressure deficit (VPD_ant) effects on A_max (over the past 3–4 days and 1–3 days, respectively) were the most significant predictors of temporal variability in GPP among most treatments. The importance of VPD_ant suggests that atmospheric drought is important for predicting GPP under current and future climate; we highlight the need for experimental studies to identify the mechanisms underlying such antecedent effects. Finally, posterior estimates of cumulative GPP under control and eCO₂ treatments were tested as a benchmark against 12 terrestrial biosphere models (TBMs). The narrow uncertainties of these data‐driven GPP estimates suggest that they could be useful semi‐independent data streams for validating TBMs.     

Water availability affects seasonal CO2‐induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO₂ (eCO₂) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (g_s) and increased soil water content (V_SWC) and second, through increased leaf internal CO₂ (C_i) and decreased stomatal limitations (S_lim). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C₃ herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO₂‐induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol^?1 increase in atmospheric CO₂ across seasons. This eCO₂‐induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily V_SWC. The highest photosynthetic enhancement by eCO₂ (>30%) was observed during the most water‐limited period, for example, with V_SWC <0.07 in this sandy surface soil. Under eCO₂ there was neither a significant decrease in g_s in the three herbaceous species, nor increases in V_SWC, indicating no “water‐savings effect” of eCO₂. Periods of low V_SWC showed lower g_s (less than ≈ 0.12 mol m^?2 s^?1), higher relative S_lim (>30%) and decreased C_i under the ambient CO₂ concentration (aCO₂), with leaf photosynthesis strongly carboxylation‐limited. The alleviation of S_lim by eCO₂ was facilitated by increasing C_i, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO₂, controls g_s and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO₂ has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the “water‐savings effect” usually invoked in grasslands.     

Autumn warming reduces the CO2 sink of a black spruce forest in interior Alaska based on a nine‐year eddy covariance measurement

Nine years (2003–2011) of carbon dioxide (CO₂) flux were measured at a black spruce forest in interior Alaska using the eddy covariance method. Seasonal and interannual variations in the gross primary productivity (GPP) and ecosystem respiration (RE) were associated primarily with air temperature: warmer conditions enhanced GPP and RE. Meanwhile, interannual variation in annual CO₂ balance was controlled predominantly by RE, and not GPP. During these 9 years of measurement, the annual CO₂ balance shifted from a CO₂ sink to a CO₂ source, with a 9‐year average near zero. The increase in autumn RE was associated with autumn warming and was mostly attributed to a shift in the annual CO₂ balance. The increase in autumn air temperature (0.22 °C yr^?1) during the 9 years of study was 15 times greater than the long‐term warming trend between 1905 and 2011 (0.015 °C yr^?1) due to decadal climate oscillation. This result indicates that most of the shifts in observed CO₂ fluxes were associated with decadal climate variability. Because the natural climate varies in a cycle of 10–30 years, a long‐term study covering at least one full cycle of decadal climate oscillation is important to quantify the CO₂ balance and its interaction with the climate.     

Biomass production in a nitrogen-fertilized,tallgrass prairie ecosystem exposed to ambient and elevated levels of CO2

Increased biomass production in terrestrial ecosystems with elevated atmospheric CO₂ may be constrained by nutrient limitations as a result of increased requirement or reduced availability caused by reduced turnover rates of nutrients. To determine the short-term impact of nitrogen (N) fertilization on plant biomass production under elevated CO₂, we compared the response of N-fertilized tallgrass prairie at ambient and twice-ambient CO₂ levels over a 2-year period. Native tallgrass prairie plots (4.5 m diameter) were exposed continuously (24 h) to ambient and twice-ambient CO₂ from 1 April to 26 October. We compared our results to an unfertilized companion experiment on the same research site. Above- and belowground biomass production and leaf area of fertilized plots were greater with elevated than ambient CO₂ in both years. The increase in biomass at high CO₂ occurred mainly aboveground in 1991, a dry year, and belowground in 1990, a wet year. Nitrogen concentration was lower in plants exposed to elevated CO₂, but total standing crop N was greater at high CO₂. Increased root biomass under elevated CO₂ apparently increased N uptake. The biomass production response to elevated CO₂ was much greater on N-fertilized than unfertilized prairie, particularly in the dry year. We conclude that biomass production response to elevated CO₂ was suppressed by N limitation in years with below-normal precipitation. Reduced N concentration in above- and belowground biomass could slow microbial degradation of soil organic matter and surface litter, thereby exacerbating N limitation in the long term.     

Temperate forest responses to carbon dioxide, temperature and nitrogen: a model analysis

The ITE Edinburgh Forest Model, which describes diurnal and seasonal changes in the pools and fluxes of C, N and water in a fully coupled forest–soil system, was parametrized to simulate a managed conifer plantation in upland Britain. The model was used to examine (i) the transient effects on forest growth of an IS92a scenario of increasing [CO₂] and temperature over two future rotations, and (ii) the equilibrium (sustainable) effects of all combinations of increases in [CO₂] from 350 to 550 and 750 μmol mol^?1, mean annual temperature from 7.5 to 8.5 and 9.5°C and annual inputs of 20 or 40 kg N ha^?1. Changes in underlying processes represented in the model were then used to explain the responses. Eight conclusions were supported by the model for this forest type and climate.

• 1 Increasing temperatures above 3°C alone may cause forest decline owing to water stress.
• 2 Elevated [CO₂] can protect trees from water stress that they may otherwise suffer in response to increased temperature.
• 3 In N-limiting conditions, elevated [CO₂] can increase allocation to roots with little increase in leaf area, whereas in N-rich conditions elevated [CO₂] may not increase allocation to roots and generally increases leaf area.
• 4 Elevated [CO₂] can decrease water use by forests in N-limited conditions and increase water use in N-rich conditions.
• 5 Elevated [CO₂] can increase forest productivity even in N-limiting conditions owing to increased N acquisition and use efficiency.
• 6 Rising temperatures (along with rising [CO₂]) may increase or decrease forest productivity depending on the supply of N and changes in water stress.
• 7 Gaseous losses of N from the soil can increase or decrease in response to elevated [CO₂] and temperature.
• 8 Projected increases in [CO₂] and temperature (IS92a) are likely to increase net ecosystem productivity and hence C sequestration in temperate forests.

     

Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature

In recent years, increased awareness of the potential interactions between rising atmospheric CO₂ concentrations ([ CO₂ ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO₂ ] manipulation, and compares it with those obtained in single factor [ CO₂ ] and temperature manipulation experiments. Across all combined elevated [ CO₂ ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO₂ ]‐only treatment than to those in the warming‐only treatment. In contrast to warming‐only experiments, both the combined and the [ CO₂ ]‐only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO₂ ]‐only treatment, possibly due to the warming‐induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO₂ ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO₂ ] and warming, i.e. the response to the combined treatment was usually less‐than‐additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long‐term) multifactor manipulation experiments. Because single factor CO₂ responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.     

Testing simulations of intra‐ and inter‐annual variation in the plant production response to elevated CO2 against measurements from an 11‐year FACE experiment on grazed pasture

Ecosystem models play a crucial role in understanding and evaluating the combined impacts of rising atmospheric CO₂ concentration and changing climate on terrestrial ecosystems. However, we are not aware of any studies where the capacity of models to simulate intra‐ and inter‐annual variation in responses to elevated CO₂ has been tested against long‐term experimental data. Here we tested how well the ecosystem model APSIM/AgPasture was able to simulate the results from a free air carbon dioxide enrichment (FACE) experiment on grazed pasture. At this FACE site, during 11 years of CO₂ enrichment, a wide range in annual plant production response to CO₂ (?6 to +28%) was observed. As well as running the full model, which includes three plant CO₂ response functions (plant photosynthesis, nitrogen (N) demand and stomatal conductance), we also tested the influence of these three functions on model predictions. Model/data comparisons showed that: (i) overall the model over‐predicted the mean annual plant production response to CO₂ (18.5% cf 13.1%) largely because years with small or negative responses to CO₂ were not well simulated; (ii) in general seasonal and inter‐annual variation in plant production responses to elevated CO₂ were well represented by the model; (iii) the observed CO₂ enhancement in overall mean legume content was well simulated but year‐to‐year variation in legume content was poorly captured by the model; (iv) the best fit of the model to the data required all three CO₂ response functions to be invoked; (v) using actual legume content and reduced N fixation rate under elevated CO₂ in the model provided the best fit to the experimental data. We conclude that in temperate grasslands the N dynamics (particularly the legume content and N fixation activity) play a critical role in pasture production responses to elevated CO₂, and are processes for model improvement.