B. El Masri, C. Schwalm, D.N. Huntzinger, J. Mao, X. Shi, C. Peng, J.B. Fisher, A.K. Jain, H. Tian, B. Poulter and A.M. Michalak
Two important indicators for understanding terrestrial ecosystems responses to a changing climate are carbon use efficiency (the rate of net carbon uptake to gross carbon uptake) and water use efficiency (amount of water a terrestrial ecosystem uses relative to the carbon gained). The ability of models to accurately estimate these factors has implications for how well terrestrial ecosystems’ response to climate change can be predicted. A comparison of estimates from ten terrestrial ecosystem models to global flux tower observations revealed that modeled increases in water use efficiency due to CO2 fertilization (increased photosynthesis due to increased levels of CO2 in the atmosphere) and nitrogen deposition were consistent with experimental studies, but that modeled carbon use efficiency was less so. Discrepancies were attributed to how different models accounted for net primary production and autotrophic respiration, nitrogen cycling, and carbon allocation. This study highlights the importance of understanding the impact of model assumptions and parameterizations on predictions of how terrestrial ecosystems will respond to climate change.
Figure: Effect of environmental variables on the annual changes in WUE (water use efficiency) from 1901–2010. (a) climate effect on models WUE calculated as the difference between SG1-RG1 simulations, (b) CO2 fertilization effect on models WUE calculated as the difference between SG3-SG2, (c) N deposition effect on models WUE calculated as the difference between BG1-SG3, and (d) the interactions of climate, CO2 fertilization, and N deposition (BG1 simulation) effects on models WUE calculated as the net change in WUE relative to year 1901 (See Supplementary Information).
Terrestrial ecosystems carbon and water cycles are tightly coupled through photosynthesis and evapotranspiration processes. The ratios of carbon stored to carbon uptake and water loss to carbon gain are key ecophysiological indicators essential to assess the magnitude and response of the terrestrial plant to the changing climate. Here, we use estimates from 10 terrestrial ecosystem models to quantify the impacts of climate, atmospheric CO2 concentration, and nitrogen (N) deposition on water use efficiency (WUE), and carbon use efficiency (CUE). We find that across models, WUE increases over the 20th Century particularly due to CO2 fertilization and N deposition and compares favorably to experimental studies. Also, the results show a decrease in WUE with climate for the last 3 decades, in contrasts with up-scaled flux observations that demonstrate a constant WUE. Modeled WUE responds minimally to climate with modeled CUE exhibiting no clear trend across space and time. The divergence between simulated and observationally-constrained WUE and CUE is driven by modeled NPP and autotrophic respiration, nitrogen cycle, carbon allocation, and soil moisture dynamics in current ecosystem models. We suggest that carbon-modeling community needs to reexamine stomatal conductance schemes and the soil-vegetation interactions for more robust modeling of carbon and water cycles.
El Masri, B., C. Schwalm, D.N. Huntzinger, J. Mao, X. Shi, C. Peng, J.B. Fisher, A.K. Jain, H. Tian, B. Poulter, A.M. Michalak (2019) "Carbon and Water Use Efficiencies: A Comparative Analysis of Ten Terrestrial Ecosystem Models under Changing Climate," Scientific Reports, 9, 14680, doi:10.1038/s41598-019-50808-7.