Image courtesy of Oak Ridge National Laboratory
A free-air CO2 enrichment (FACE) site at Oak Ridge National Laboratory. Researchers are using data from this and other FACE sites to improve climate model predictions of plant responses to elevated atmospheric carbon dioxide.
Different ecosystem models vary widely in predicting the effect of elevated atmospheric carbon dioxide (CO2) concentrations on transpiration, or the passage of water from a plant’s roots, through its vascular system, and then to the atmosphere. To better understand and constrain this variability, forest carbon and water flux data from the free-air CO2 enrichment (FACE) experiments at Duke University and Oak Ridge National Laboratory were compared to simulations from 11 ecosystem models.
This study yields a framework for analyzing and interpreting model predictions of transpiration responses to elevated CO2. The results highlight important areas for immediate model improvement, hypotheses for experimental testing, and opportunities for data synthesis to significantly reduce discrepancies among models.
A primary objective of the study was to identify key underlying assumptions in model structure that cause differences in model predictions of transpiration and canopy water-use efficiency. Model-to-model and model-to-observations differences resulted from four main sets of assumptions: (1) the nature of stomatal response to elevated CO2, (2) the roles of leaves and the atmospheric boundary layer, (3) treatment of canopy interception, and (4) impact of soil moisture stress. The degree of coupling between carbon and water fluxes and the way that coupling is calculated are key assumptions that determine how well the models compare with observations.
Martin G. De Kauwe
Macquarie University, Department of Biological Sciences
New South Wales 2109, Australia
Research was supported by the National Center for Ecological Analysis and Synthesis, a Center funded by the National Science Foundation (grant #EF-0553768); the University of California, Santa Barbara; and the state of California. The Oak Ridge and Duke FACE sites and additional synthesis activities were supported by the DOE Office of Science Biological and Environmental Research (BER) program. Duke FACE research was supported by BER’s Terrestrial Carbon Processes program (FACE, DE-FG02-95ER62083).
Kauwe, M.G., et al. “Forest water use and water use efficiency at elevated CO2: a model-data intercomparison at two contrasting temperate forest FACE sites.” Glob. Chang. Biol. 19(6), 1759–1779 (2013). [DOI: 10.1111/gcb.12164].
University, DOE Laboratory