The response of terrestrial ecosystems to increased atmospheric CO2 concentrations is controversial and not yet fully understood, with previous large‐scale forest manipulation experiments exhibiting ...contrasting responses. Although there is consensus that increased CO2 has a relevant effect on instantaneous processes such as photosynthesis and transpiration, there are large uncertainties regarding the fate of extra assimilated carbon in ecosystems. Filling this research gap is challenging because tracing the movement of new carbon across ecosystem compartments involves the study of multiple processes occurring over a wide range of timescales, from hours to millennia. We posit that a comprehensive quantification of the effect of increased CO2 must answer two interconnected questions: How much and for how long is newly assimilated carbon stored in ecosystems? Therefore, we propose that the transit time distribution of carbon is the key concept needed to effectively address these questions. Here, we show how the transit time distribution of carbon can be used to assess the fate of newly assimilated carbon and the timescales at which it is cycled in ecosystems. We use as an example a transit time distribution obtained from a tropical forest and show that most of the 60% of fixed carbon is respired in less than 1 year; therefore, we infer that under increased CO2, most of the new carbon would follow a similar fate unless increased CO2 would cause changes in the rates at which carbon is cycled and transferred among ecosystem compartments. We call for a more frequent adoption of the transit time concept in studies seeking to quantify the ecosystem response to increased CO2.
Terrestrial ecosystems remove CO2 from the atmosphere via photosynthesis and convert it into biomass and soil organic matter. This carbon is returned to the atmosphere via decomposition and respiration, processes that depend on climatic conditions, microbial community structure and function, and nutrient availability. When ecosystems are exposed to increased CO2, the photosynthesis rate increases, but it is not clear how much carbon remains in the system and for how long. The transit time concept can be used to assess the fate of newly assimilated carbon and the timescales at which it is cycled in ecosystems. Modified after Steiner (2008).
At leaf level, elevated atmospheric CO2 concentration (eCO2) results in stimulation of carbon net assimilation and reduction of stomatal conductance. However, a comprehensive understanding of the ...impact of eCO2 at larger temporal (seasonal and annual) and spatial (from leaf to whole‐tree) scales is still lacking. Here, we review overall trends, magnitude and drivers of dynamic tree responses to eCO2, including carbon and water relations at the leaf and the whole‐tree level. Spring and early season leaf responses are most susceptible to eCO2 and are followed by a down‐regulation towards the onset of autumn. At the whole‐tree level, CO2 fertilization causes consistent biomass increments in young seedlings only, whereas mature trees show a variable response. Elevated CO2‐induced reductions in leaf stomatal conductance do not systematically translate into limitation of whole‐tree transpiration due to the unpredictable response of canopy area. Reduction in the end‐of‐season carbon sink demand and water‐limiting strategies are considered the main drivers of seasonal tree responses to eCO2. These large temporal and spatial variabilities in tree responses to eCO2 highlight the risk of predicting tree behavior to eCO2 based on single leaf–level point measurements as they only reveal snapshots of the dynamic responses to eCO2.
This review describes overall trends, magnitude and drivers of leaf‐scale and whole‐tree‐scale responses to rising atmospheric CO2 concentration. Results encourage frequent monitoring, as single point measurements provide a limited snapshot of dynamic tree functioning under future CO2 growing conditions.
► Data from a FACE experiment are compared with an ecohydrological model. ► Modeled differences between CO2 scenarios are within the uncertainty of observations. ► Changes between CO2 scenarios in ...terms of water and energy fluxes are small. ► Testing the carbon allocation is hampered by current accuracy of field data.
Projections of the future carbon and water cycles rely on knowledge on how forests will respond to rising atmospheric CO2. Experiments with elevated CO2 are logistically challenging and carbon pools and fluxes are difficult to measure and upscale due to their spatiotemporal heterogeneity. Therefore, it is important to combine the knowledge derived from experimental results with modeling. Here, we systematically compare data from a free air CO2 enrichment (FACE) experiment in a mature deciduous forest in Switzerland with realizations from an ecohydrological model (Tethys–Chloris). We test whether a mechanistic ecohydrological model is able to simulate physiological plant responses under ambient and elevated CO2 concentration. We overcome measurement limitations by quantifying differences in response to ambient and elevated CO2 over ten years. The reliability of model realizations is demonstrated by comparing simulations with field observations of stomatal conductance, sap flow, leaf and fruit litter, and stem growth. The model successfully captures the observed CO2-induced difference in stomatal conductance and transpiration and its sensitivity to atmospheric demand, as well as qualitative changes in soil moisture. The simulated differences between CO2 scenarios generally fall within the uncertainty of experimental observations, both for the carbon and water balance. Simulated total evapotranspiration is 2.8% (18mmyr−1) lower and soil moisture 1.2% higher in the CO2-enriched scenario. Latent and sensible heat are modified by ca. 1Wm−2. Net primary production is simulated to increase by 19.8% and allocation to stem growth is 53gCyr−1m−2 higher in the elevated CO2 scenario, which represents the limit of the detection threshold of the experiment. Results show that while ecohydrological models can be used to reliably simulate multi-year energy, water, and carbon fluxes at the stand level, testing carbon allocation remains critical with current accuracy of field measurements. Uncertainties due to the simplified carbon allocation scheme are shown to be more significant for carbon than for energy and water fluxes. Generally, we conclude that for this type of forest, differences in annual energy and water fluxes induced by elevated CO2 are likely to be less than 10%.
This paper provides a systematic review of studies assessing the impact of climate change on crop yields in southern Africa. Moreover, it synthesises the current knowledge of the impact of elevated ...ambient CO
2
levels (eCO
2
) and temperatures on physiological processes, and the application of this knowledge in mechanistic crop models. While eCO
2
evidently has a strong impact on photosynthesis and crop water use, it is uncertain how this will work out for the climatic and crop management conditions prevailing in southern Africa. The impact of heat stress on crop reproductive processes and the process of transpiration cooling mitigating heat stress are poorly represented in models, while both process are relevant given the climatic conditions prevailing in southern Africa. Twenty studies assessing the impact of climate change on future yields of crops, mostly maize, have been retrieved. The results suggest that potato, Bambara groundnut and sugarcane yields may improve. No consistent trends for maize and sorghum could be identified. While yield predictions are obviously context-specific, large uncertainties related to climate predictions and crop models imply results should be treated with caution. Suggestions are made for field experimentation and the improved application of crop models for climate change research in the region.
Soil filamentous fungi play a prominent role in regulating ecosystem functioning in terrestrial ecosystems. This necessitates understanding their responses to climate change drivers in order to ...predict how nutrient cycling and ecosystem services will be influenced in the future. Here, we provide a quantitative synthesis of ten studies on soil fungal community responses to elevated CO
2
. Many of these studies reported contradictory diversity responses. We identify the duration of the study as an influential parameter that determines the outcome of experimentation. Our analysis reconciles the existing globally distributed experiments on fungal community responses to elevated CO
2
and provides a framework for comparing results of future CO
2
enrichment studies.
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.
The extent to which greater net primary productivity (NPP) will be sustained as the atmospheric CO2concentration increases will depend, in part, on the long-term supply of N for plant growth. Over a ...two-year period, we used common field and laboratory methods to quantify microbial N, gross N mineralization, microbial N immobilization, and specific microbial N immobilization in three free-air CO2enrichment experiments (Duke Forest, Oak Ridge, Rhinelander). In these experiments, elevated atmospheric CO2has increased the input of above- and belowground litter production, which fuels heterotrophic metabolism in soil. Nonetheless, we found no effect of atmospheric CO2concentration on any microbial N cycling pool or process, indicating that greater litter production had not initially altered the microbial supply of N for plant growth. Thus, we have no evidence that changes in plant litter production under elevated CO2will initially slow soil N availability and produce a negative feedback on NPP. Understanding the time scale over which greater plant production modifies microbial N demand lies at the heart of our ability to predict long-term changes in soil N availability and hence whether greater NPP will be sustained in a$CO_{2}-enriched$atmosphere.
Climate change is evident and increases of carbon dioxide concentration (CO2), temperature and extreme weather events are predicted. To predict the effects of such changes on carbon (C) cycling, the ...processes and mechanisms determining the magnitude of C storage and fluxes must be well understood. The biggest challenge is nowadays to quantify belowground components of the C-cycle. Soil respiration accounts for ∼70% of total annual ecosystem respiration. However, the CO2 flux from soil originates from several sources, such as root respiration, rhizomicrobial respiration, mineralization of litter and mineralization of soil organic matter (SOM). Increasing atmospheric CO2 concentrations will generally increase plant growth, thus C-input to soil. This higher C-input will be accompanied by higher SOM mineralization due to warming. However, mineralization of more stable pools may be affected more by warming compared to mineralization of labile pools. The importance of cropland management is demonstrated in a model scenario. Crop residue incorporation increased C-storage in the soil markedly. However, under the assumption of a higher temperature sensitivity of mineralization of stable C-pools the net-sink of C under recommended management practice is severely reduced. Precise predictions are hampered due to the lack of quantitative, mechanistic knowledge. It is discussed that a more interdisciplinary scientific approach will increase the speed in generating urgently needed understanding of belowground processes of C-cycling.
High temperature and accompanying high vapor pressure deficit often stress plants without causing distinctive changes in plant canopy structure and consequential spectral signatures. Sun‐induced ...chlorophyll fluorescence (SIF), because of its mechanistic link with photosynthesis, may better detect such stress than remote sensing techniques relying on spectral reflectance signatures of canopy structural changes. However, our understanding about physiological mechanisms of SIF and its unique potential for physiological stress detection remains less clear. In this study, we measured SIF at a high‐temperature experiment, Temperature Free‐Air Controlled Enhancement, to explore the potential of SIF for physiological investigations. The experiment provided a gradient of soybean canopy temperature with 1.5, 3.0, 4.5, and 6.0°C above the ambient canopy temperature in the open field environments. SIF yield, which is normalized by incident radiation and the fraction of absorbed photosynthetically active radiation, showed a high correlation with photosynthetic light use efficiency (r = 0.89) and captured dynamic plant responses to high‐temperature conditions. SIF yield was affected by canopy structural and plant physiological changes associated with high‐temperature stress (partial correlation r = 0.60 and −0.23). Near‐infrared reflectance of vegetation, only affected by canopy structural changes, was used to minimize the canopy structural impact on SIF yield and to retrieve physiological SIF yield (ΦF) signals. ΦF further excludes the canopy structural impact than SIF yield and indicates plant physiological variability, and we found that ΦF outperformed SIF yield in responding to physiological stress (r = −0.37). Our findings highlight that ΦF sensitively responded to the physiological downregulation of soybean gross primary productivity under high temperature. ΦF, if reliably derived from satellite SIF, can support monitoring regional crop growth and different ecosystems' vegetation productivity under environmental stress and climate change.
The study investigated soybean responses to high‐temperature stress, which is a major threat to crop yield in the U.S. Corn Belt. We utilized an unprecedented experiment, Temperature Free‐Air‐Controlled Enhancement experiment, which enabled detailed investigation by providing four levels of canopy temperature increments. Using advanced hyperspectral remote sensing technique, we collected sun‐induced chlorophyll fluorescence (SIF) data and demonstrated that SIF has the unique capability of quantifying crop physiological variability under high‐temperature stress (i.e., depressions in instantaneous photosynthetic rates per unit leaf area). The unique strength of SIF for quantifying physiological status is expected to contribute to improving vegetation productivity quantification.
PDF
