Response of wheat growth,grain yield and water use to elevated CO2 under a Free‐Air CO2 Enrichment (FACE) experiment and modelling in a semi‐arid environment

The response of wheat crops to elevated CO₂ (eCO₂) was measured and modelled with the Australian Grains Free‐Air CO₂ Enrichment experiment, located at Horsham, Australia. Treatments included CO₂ by water, N and temperature. The location represents a semi‐arid environment with a seasonal VPD of around 0.5 kPa. Over 3 years, the observed mean biomass at anthesis and grain yield ranged from 4200 to 10 200 kg ha^?1 and 1600 to 3900 kg ha^?1, respectively, over various sowing times and irrigation regimes. The mean observed response to daytime eCO₂ (from 365 to 550 μmol mol^?1 CO₂) was relatively consistent for biomass at stem elongation and at anthesis and LAI at anthesis and grain yield with 21%, 23%, 21% and 26%, respectively. Seasonal water use was decreased from 320 to 301 mm (P = 0.10) by eCO₂, increasing water use efficiency for biomass and yield, 36% and 31%, respectively. The performance of six models (APSIM‐Wheat, APSIM‐Nwheat, CAT‐Wheat, CROPSYST, OLEARY‐CONNOR and SALUS) in simulating crop responses to eCO₂ was similar and within or close to the experimental error for accumulated biomass, yield and water use response, despite some variations in early growth and LAI. The primary mechanism of biomass accumulation via radiation use efficiency (RUE) or transpiration efficiency (TE) was not critical to define the overall response to eCO₂. However, under irrigation, the effect of late sowing on response to eCO₂ to biomass accumulation at DC65 was substantial in the observed data (~40%), but the simulated response was smaller, ranging from 17% to 28%. Simulated response from all six models under no water or nitrogen stress showed similar response to eCO₂ under irrigation, but the differences compared to the dryland treatment were small. Further experimental work on the interactive effects of eCO₂, water and temperature is required to resolve these model discrepancies.     

Response of maize biomass and soil water fluxes on elevated CO2 and drought—From field experiments to process‐based simulations

The rising concentration of atmospheric carbon dioxide (CO₂) is known to increase the total aboveground biomass of several C3 crops, whereas C4 crops are reported to be hardly affected when water supply is sufficient. However, a free‐air carbon enrichment (FACE) experiment in Braunschweig, Germany, in 2007 and 2008 resulted in a 25% increased biomass of the C4 crop maize under restricted water conditions and elevated CO₂ (550 ppm). To project future yields of maize under climate change, an accurate representation of the effects of eCO₂ and drought on biomass and soil water conditions is essential. Current crop growth models reveal limitations in simulations of maize biomass under eCO₂ and limited water supply. We use the coupled process‐based hydrological‐plant growth model Catchment Modeling Framework‐Plant growth Modeling Framework to overcome this limitation. We apply the coupled model to the maize‐based FACE experiment in Braunschweig that provides robust data for the investigation of combined CO₂ and drought effects. We approve hypothesis I that CO₂ enrichment has a small direct‐fertilizing effect with regard to the total aboveground biomass of maize and hypothesis II that CO₂ enrichment decreases water stress and leads to higher yields of maize under restricted water conditions. Hypothesis III could partly be approved showing that CO₂ enrichment decreases the transpiration of maize, but does not raise soil moisture, while increasing evaporation. We emphasize the importance of plant‐specific CO₂ response factors derived by use of comprehensive FACE data. By now, only one FACE experiment on maize is accomplished applying different water levels. For the rigorous testing of plant growth models and their applicability in climate change studies, we call for datasets that go beyond single criteria (only yield response) and single effects (only elevated CO₂).     

Responses to atmospheric CO2 concentrations in crop simulation models: a review of current simple and semicomplex representations and options for model development

Elevated atmospheric CO₂ concentrations ([CO₂]) cause direct changes in crop physiological processes (e.g. photosynthesis and stomatal conductance). To represent these CO₂ responses, commonly used crop simulation models have been amended, using simple and semicomplex representations of the processes involved. Yet, there is no standard approach to and often poor documentation of these developments. This study used a bottom‐up approach (starting with the APSIM framework as case study) to evaluate modelled responses in a consortium of commonly used crop models and illuminate whether variation in responses reflects true uncertainty in our understanding compared to arbitrary choices of model developers. Diversity in simulated CO₂ responses and limited validation were common among models, both within the APSIM framework and more generally. Whereas production responses show some consistency up to moderately high [CO₂] (around 700 ppm), transpiration and stomatal responses vary more widely in nature and magnitude (e.g. a decrease in stomatal conductance varying between 35% and 90% among models was found for [CO₂] doubling to 700 ppm). Most notably, nitrogen responses were found to be included in few crop models despite being commonly observed and critical for the simulation of photosynthetic acclimation, crop nutritional quality and carbon allocation. We suggest harmonization and consideration of more mechanistic concepts in particular subroutines, for example, for the simulation of N dynamics, as a way to improve our predictive understanding of CO₂ responses and capture secondary processes. Intercomparison studies could assist in this aim, provided that they go beyond simple output comparison and explicitly identify the representations and assumptions that are causal for intermodel differences. Additionally, validation and proper documentation of the representation of CO₂ responses within models should be prioritized.     

Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus‐limited Eucalyptus woodland

Free‐air CO₂ enrichment (FACE) experiments have demonstrated increased plant productivity in response to elevated (e)CO₂, with the magnitude of responses related to soil nutrient status. Whilst understanding nutrient constraints on productivity responses to eCO₂ is crucial for predicting carbon uptake and storage, very little is known about how eCO₂ affects nutrient cycling in phosphorus (P)‐limited ecosystems. Our study investigates eCO₂ effects on soil N and P dynamics at the EucFACE experiment in Western Sydney over an 18‐month period. Three ambient and three eCO₂ (+150 ppm) FACE rings were installed in a P‐limited, mature Cumberland Plain Eucalyptus woodland. Levels of plant accessible nutrients, evaluated using ion exchange resins, were increased under eCO₂, compared to ambient, for nitrate (+93%), ammonium (+12%) and phosphate (+54%). There was a strong seasonality to responses, particularly for phosphate, resulting in a relatively greater stimulation in available P, compared to N, under eCO₂ in spring and summer. eCO₂ was also associated with faster nutrient turnover rates in the first six months of the experiment, with higher N (+175%) and P (+211%) mineralization rates compared to ambient rings, although this difference did not persist. Seasonally dependant effects of eCO₂ were seen for concentrations of dissolved organic carbon in soil solution (+31%), and there was also a reduction in bulk soil pH (‐0.18 units) observed under eCO₂. These results demonstrate that CO₂ fertilization increases nutrient availability – particularly for phosphate – in P‐limited soils, likely via increased plant belowground investment in labile carbon and associated enhancement of microbial turnover of organic matter and mobilization of chemically bound P. Early evidence suggests that there is the potential for the observed increases in P availability to support increased ecosystem C‐accumulation under future predicted CO₂ concentrations.     

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment,independent of nitrogen supply

Uncertainty about long‐term leaf‐level responses to atmospheric CO₂ rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO₂ (eCO₂) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO₂ on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO₂ Enrichment experiment, BioCON. Monocultures of species belonging to C₃ grasses, C₄ grasses, forbs, and legumes were exposed to two levels of CO₂ and nitrogen supply in factorial combinations over 21 years. eCO₂ increased photosynthesis by 12.9% on average in C₃ species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO₂ was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO₂ was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C₄ grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO₂ primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO₂ in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO₂ may allow plants to better withstand drought.     

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.     

On the causes of trends in the seasonal amplitude of atmospheric CO2

No consensus has yet been reached on the major factors driving the observed increase in the seasonal amplitude of atmospheric CO₂ in the northern latitudes. In this study, we used atmospheric CO₂ records from 26 northern hemisphere stations with a temporal coverage longer than 15 years, and an atmospheric transport model prescribed with net biome productivity (NBP) from an ensemble of nine terrestrial ecosystem models, to attribute change in the seasonal amplitude of atmospheric CO₂. We found significant (p < .05) increases in seasonal peak‐to‐trough CO₂ amplitude (AMP_P‐T) at nine stations, and in trough‐to‐peak amplitude (AMP_T‐P) at eight stations over the last three decades. Most of the stations that recorded increasing amplitudes are in Arctic and boreal regions (>50°N), consistent with previous observations that the amplitude increased faster at Barrow (Arctic) than at Mauna Loa (subtropics). The multi‐model ensemble mean (MMEM) shows that the response of ecosystem carbon cycling to rising CO₂ concentration (eCO₂) and climate change are dominant drivers of the increase in AMP_P‐T and AMP_T‐P in the high latitudes. At the Barrow station, the observed increase of AMP_P‐T and AMP_T‐P over the last 33 years is explained by eCO₂ (39% and 42%) almost equally than by climate change (32% and 35%). The increased carbon losses during the months with a net carbon release in response to eCO₂ are associated with higher ecosystem respiration due to the increase in carbon storage caused by eCO₂ during carbon uptake period. Air‐sea CO₂ fluxes (10% for AMP_P‐T and 11% for AMP_T‐P) and the impacts of land‐use change (marginally significant 3% for AMP_P‐T and 4% for AMP_T‐P) also contributed to the CO₂ measured at Barrow, highlighting the role of these factors in regulating seasonal changes in the global carbon cycle.     

Accumulation of soil carbon under elevated CO2 unaffected by warming and drought

Elevated atmospheric CO₂ concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO₂ (eCO₂) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO₂, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m² higher across all treatment combinations with eCO₂ compared to ambient CO₂ treatments (equal to an increase of 0.120 ± 0.043 kg C m^?2 year^?1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO₂ may be as high as 0.177 ± 0.070 kg C m^?2 year^?1. Furthermore, the response to eCO₂ was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO₂ observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO₂ concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO₂, warming, and changes in precipitation patterns, are, therefore, urgently needed.     

Impacts of long‐term elevated atmospheric CO2 concentrations on communities of arbuscular mycorrhizal fungi

The ecological impacts of long‐term elevated atmospheric CO₂ (eCO₂) levels on soil microbiota remain largely unknown. This is particularly true for the arbuscular mycorrhizal (AM) fungi, which form mutualistic associations with over two‐thirds of terrestrial plant species and are entirely dependent on their plant hosts for carbon. Here, we use high‐resolution amplicon sequencing (Illumina, HiSeq) to quantify the response of AM fungal communities to the longest running (>15 years) free‐air carbon dioxide enrichment (FACE) experiment in the Northern Hemisphere (GiFACE); providing the first evaluation of these responses from old‐growth (>100 years) semi‐natural grasslands subjected to a 20% increase in atmospheric CO₂. eCO₂ significantly increased AM fungal richness but had a less‐pronounced impact on the composition of their communities. However, while broader changes in community composition were not observed, more subtle responses of specific AM fungal taxa were with populations both increasing and decreasing in abundance in response to eCO₂. Most population‐level responses to eCO₂ were not consistent through time, with a significant interaction between sampling time and eCO₂ treatment being observed. This suggests that the temporal dynamics of AM fungal populations may be disturbed by anthropogenic stressors. As AM fungi are functionally differentiated, with different taxa providing different benefits to host plants, changes in population densities in response to eCO₂ may significantly impact terrestrial plant communities and their productivity. Thus, predictions regarding future terrestrial ecosystems must consider changes both aboveground and belowground, but avoid relying on broad‐scale community‐level responses of soil microbes observed on single occasions.     

Short‐term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration

Projections of future climate are highly sensitive to uncertainties regarding carbon (C) uptake and storage by terrestrial ecosystems. The Eucalyptus Free‐Air CO₂ Enrichment (EucFACE) experiment was established to study the effects of elevated atmospheric CO₂ concentrations (eCO₂) on a native mature eucalypt woodland with low fertility soils in southeast Australia. In contrast to other FACE experiments, the concentration of CO₂ at EucFACE was increased gradually in steps above ambient (+0, 30, 60, 90, 120, and 150 ppm CO₂ above ambient of ~400 ppm), with each step lasting approximately 5 weeks. This provided a unique opportunity to study the short‐term (weeks to months) response of C cycle flux components to eCO₂ across a range of CO₂ concentrations in an intact ecosystem. Soil CO₂ efflux (i.e., soil respiration or R_soil) increased in response to initial enrichment (e.g., +30 and +60 ppm CO₂) but did not continue to increase as the CO₂ enrichment was stepped up to higher concentrations. Light‐saturated photosynthesis of canopy leaves (A_sat) also showed similar stimulation by elevated CO₂ at +60 ppm as at +150 ppm CO₂. The lack of significant effects of eCO₂ on soil moisture, microbial biomass, or activity suggests that the increase in R_soil likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in R_soil suggests that under eCO_2, additional photosynthate was produced, transported belowground, and respired. The consequences of this increased belowground activity and whether it is sustained through time in mature ecosystems under eCO₂ are a priority for future research.     

Biomass responses in a temperate European grassland through 17 years of elevated CO2

Future increase in atmospheric CO₂ concentrations will potentially enhance grassland biomass production and shift the functional group composition with consequences for ecosystem functioning. In the “GiFACE” experiment (Giessen Free Air Carbon dioxide Enrichment), fertilized grassland plots were fumigated with elevated CO₂ (eCO₂) year‐round during daylight hours since 1998, at a level of +20% relative to ambient concentrations (in 1998, aCO₂ was 364 ppm and eCO₂ 399 ppm; in 2014, aCO₂ was 397 ppm and eCO₂ 518 ppm). Harvests were conducted twice annually through 23 years including 17 years with eCO₂ (1998 to 2014). Biomass consisted of C3 grasses and forbs, with a small proportion of legumes. The total aboveground biomass (TAB) was significantly increased under eCO₂ (p = .045 and .025, at first and second harvest). The dominant plant functional group grasses responded positively at the start, but for forbs, the effect of eCO₂ started out as a negative response. The increase in TAB in response to eCO₂ was approximately 15% during the period from 2006 to 2014, suggesting that there was no attenuation of eCO₂ effects over time, tentatively a consequence of the fertilization management. Biomass and soil moisture responses were closely linked. The soil moisture surplus (c. 3%) in eCO₂ manifested in the latter years was associated with a positive biomass response of both functional groups. The direction of the biomass response of the functional group forbs changed over the experimental duration, intensified by extreme weather conditions, pointing to the need of long‐term field studies for obtaining reliable responses of perennial ecosystems to eCO₂ and as a basis for model development.     

Seasonal responses of terrestrial ecosystem water‐use efficiency to climate change

Ecosystem water‐use efficiency (EWUE) is an indicator of carbon–water interactions and is defined as the ratio of carbon assimilation (GPP) to evapotranspiration (ET). Previous research suggests an increasing long‐term trend in annual EWUE over many regions and is largely attributed to the physiological effects of rising CO₂. The seasonal trends in EWUE, however, have not yet been analyzed. In this study, we investigate seasonal EWUE trends and responses to various drivers during 1982–2008. The seasonal cycle for two variants of EWUE, water‐use efficiency (WUE, GPP/ET), and transpiration‐based WUE (WUE_t, the ratio of GPP and transpiration), is analyzed from 0.5° gridded fields from four process‐based models and satellite‐based products, as well as a network of 63 local flux tower observations. WUE derived from flux tower observations shows moderate seasonal variation for most latitude bands, which is in agreement with satellite‐based products. In contrast, the seasonal EWUE trends are not well captured by the same satellite‐based products. Trend analysis, based on process‐model factorial simulations separating effects of climate, CO₂, and nitrogen deposition (NDEP), further suggests that the seasonal EWUE trends are mainly associated with seasonal trends of climate, whereas CO₂ and NDEP do not show obvious seasonal difference in EWUE trends. About 66% grid cells show positive annual WUE trends, mainly over mid‐ and high northern latitudes. In these regions, spring climate change has amplified the effect of CO₂ in increasing WUE by more than 0.005 gC m⁻² mm⁻¹ yr⁻¹ for 41% pixels. Multiple regression analysis further shows that the increase in springtime WUE in the northern hemisphere is the result of GPP increasing faster than ET because of the higher temperature sensitivity of GPP relative to ET. The partitioning of annual EWUE to seasonal components provides new insight into the relative sensitivities of GPP and ET to climate, CO_2, and NDEP.     

Biomass increases attributed to both faster tree growth and altered allometric relationships under long‐term carbon dioxide enrichment at a temperate forest

Increases in atmospheric carbon dioxide (CO₂) concentrations are expected to lead to increases in the rate of tree biomass accumulation, at least temporarily. On the one hand, trees may simply grow faster under higher CO₂ concentrations, preserving the allometric relations that prevailed under lower CO₂ concentrations. Alternatively, the allometric relations themselves may change. In this study, the effects of elevated CO₂ (eCO₂) on tree biomass and allometric relations were jointly assessed. Over 100 trees, grown at Duke Forest, NC, USA, were harvested from eight plots. Half of the plots had been subjected to CO₂ enrichment from 1996 to 2010. Several subplots had also been subjected to nitrogen fertilization from 2005 to 2010. Allometric equations were developed to predict tree height, stem volume, and aboveground biomass components for loblolly pine (Pinus taeda L.), the dominant tree species, and broad‐leaved species. Using the same diameter‐based allometric equations for biomass, it was estimated that plots with eCO₂ contained 21% more aboveground biomass, consistent with previous studies. However, eCO₂ significantly affected allometry, and these changes had an additional effect on biomass. In particular, P. taeda trees at a given diameter were observed to be taller under eCO₂ than under ambient CO₂ due to changes in both the allometric scaling exponent and intercept. Accounting for allometric change increased the treatment effect of eCO₂ on aboveground biomass from a 21% to a 27% increase. No allometric changes for the nondominant broad‐leaved species were identified, nor were allometric changes associated with nitrogen fertilization. For P. taeda, it is concluded that eCO₂ affects allometries, and that knowledge of allometry changes is necessary to accurately compute biomass under eCO₂. Further observations are needed to determine whether this assessment holds for other taxa.     

Alteration of forest succession and carbon cycling under elevated CO2

Regenerating forests influence the global carbon (C) cycle, and understanding how climate change will affect patterns of regeneration and C storage is necessary to predict the rate of atmospheric carbon dioxide (CO₂) increase in future decades. While experimental elevation of CO₂ has revealed that young forests respond with increased productivity, there remains considerable uncertainty as to how the long‐term dynamics of forest regrowth are shaped by elevated CO₂ (eCO₂). Here, we use the mechanistic size‐ and age‐ structured Ecosystem Demography model to investigate the effects of CO₂ enrichment on forest regeneration, using data from the Duke Forest Free‐Air Carbon dioxide Enrichment (FACE) experiment, a forest chronosequence, and an eddy‐covariance tower for model parameterization and evaluation. We find that the dynamics of forest regeneration are accelerated, and stands consistently hit a variety of developmental benchmarks earlier under eCO₂. Because responses to eCO₂ varied by plant functional type, successional pathways, and mature forest composition differed under eCO₂, with mid‐ and late‐successional hardwood functional types experiencing greater increases in biomass compared to early‐successional functional types and the pine canopy. Over the simulation period, eCO₂ led to an increase in total ecosystem C storage of 9.7 Mg C ha^‐1. Model predictions of mature forest biomass and ecosystem–atmosphere exchange of CO₂ and H₂O were sensitive to assumptions about nitrogen limitation; both the magnitude and persistence of the ecosystem response to eCO₂ were reduced under N limitation. In summary, our simulations demonstrate that eCO₂ can result in a general acceleration of forest regeneration while altering the course of successional change and having a lasting impact on forest ecosystems.     

Understorey productivity in temperate grassy woodland responds to soil water availability but not to elevated [CO2]

Rising atmospheric [CO₂] and associated climate change are expected to modify primary productivity across a range of ecosystems globally. Increasing aridity is predicted to reduce grassland productivity, although rising [CO₂] and associated increases in plant water use efficiency may partially offset the effect of drying on growth. Difficulties arise in predicting the direction and magnitude of future changes in ecosystem productivity, due to limited field experimentation investigating climate and CO₂ interactions. We use repeat near‐surface digital photography to quantify the effects of water availability and experimentally manipulated elevated [CO₂] (eCO₂) on understorey live foliage cover and biomass over three growing seasons in a temperate grassy woodland in south‐eastern Australia. We hypothesised that (i) understorey herbaceous productivity is dependent upon soil water availability, and (ii) that eCO₂ will increase productivity, with greatest stimulation occurring under conditions of low water availability. Soil volumetric water content (VWC) determined foliage cover and growth rates over the length of the growing season (August to March), with low VWC (<0.1 m³ m^?3) reducing productivity. However, eCO₂ did not increase herbaceous cover and biomass over the duration of the experiment, or mitigate the effects of low water availability on understorey growth rates and cover. Our findings suggest that projected increases in aridity in temperate woodlands are likely to lead to reduced understorey productivity, with little scope for eCO₂ to offset these changes.     

Informing climate models with rapid chamber measurements of forest carbon uptake

Models predicting ecosystem carbon dioxide (CO₂) exchange under future climate change rely on relatively few real‐world tests of their assumptions and outputs. Here, we demonstrate a rapid and cost‐effective method to estimate CO₂ exchange from intact vegetation patches under varying atmospheric CO₂ concentrations_. We find that net ecosystem CO₂ uptake (NEE) in a boreal forest rose linearly by 4.7 ± 0.2% of the current ambient rate for every 10 ppm CO₂ increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clear short‐term NEE response to fertilization in such an N‐limited system is inconsistent with the instantaneous downregulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with considerable empirical support – diversion of excess carbon to storage compounds – into an existing earth system model brings the model output into closer agreement with our field measurements. A global simulation incorporating this modified model reduces a long‐standing mismatch between the modeled and observed seasonal amplitude of atmospheric CO₂. Wider application of this chamber approach would provide critical data needed to further improve modeled projections of biosphere–atmosphere CO₂ exchange in a changing climate.     

Forest fine-root production and nitrogen use under elevated CO2: contrasting responses in evergreen and deciduous trees explained by a common principle

Despite the importance of nitrogen (N) limitation of forest carbon (C) sequestration at rising atmospheric CO₂ concentration, the mechanisms responsible are not well understood. To elucidate the interactive effects of elevated CO₂ (eCO₂) and soil N availability on forest productivity and C allocation, we hypothesized that (1) trees maximize fitness by allocating N and C to maximize their net growth and (2) that N uptake is controlled by soil N availability and root exploration for soil N. We tested this model using data collected in Free‐Air CO₂ Enrichment sites dominated by evergreen (Pinus taeda; Duke Forest) and deciduous [Liquidambar styraciflua; Oak Ridge National Laboratory (ORNL)] trees. The model explained 80–95% of variation in productivity and N‐uptake data among eCO₂, N fertilization and control treatments over 6 years. The model explains why fine‐root production increased, and why N uptake increased despite reduced soil N availability under eCO₂ at ORNL and Duke. In agreement with observations at other sites, the model predicts that soil N availability reduced below a critical level diminishes all eCO₂ responses. At Duke, a negative feedback between reduced soil N availability and N uptake prevented progressive reduction in soil N availability at eCO₂. At ORNL, soil N availability progressively decreased because it did not trigger reductions in N uptake; N uptake was maintained at ORNL through a large increase in the production of fast turnover fine roots. This implies that species with fast root turnover could be more prone to progressive N limitation of carbon sequestration in woody biomass than species with slow root turnover, such as evergreens. However, longer term data are necessary for a thorough evaluation of this hypothesis. The success of the model suggests that the principle of maximization of net growth to control growth and allocation could serve as a basis for simplification and generalization of larger scale forest and ecosystem models, for example by removing the need to specify parameters for relative foliage/stem/root allocation.     

Dynamics of soil CO2 efflux under varying atmospheric CO2 concentrations reveal dominance of slow processes

We evaluated the effect on soil CO₂ efflux (F_CO2) of sudden changes in photosynthetic rates by altering CO₂ concentration in plots subjected to +200 ppmv for 15 years. Five‐day intervals of exposure to elevated CO₂ (eCO₂) ranging 1.0–1.8 times ambient did not affect F_CO2. F_CO2 did not decrease until 4 months after termination of the long‐term eCO₂ treatment, longer than the 10 days observed for decrease of F_CO2 after experimental blocking of C flow to belowground, but shorter than the ~13 months it took for increase of F_CO2 following the initiation of eCO₂. The reduction of F_CO2 upon termination of enrichment (~35%) cannot be explained by the reduction of leaf area (~15%) and associated carbohydrate production and allocation, suggesting a disproportionate contraction of the belowground ecosystem components; this was consistent with the reductions in base respiration and F_CO2‐temperature sensitivity. These asymmetric responses pose a tractable challenge to process‐based models attempting to isolate the effect of individual processes on F_CO2.