Forest Water Use and Water-Use Efficiency at Two FACE Sites


Predicted responses of transpiration to elevated atmospheric CO2 concentrations are highly variable among process-based models. 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. A primary objective 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 key sets of assumptions: (1) the nature of the stomatal response to elevated CO2; (2) the roles of the leaf and atmospheric boundary layer; (3) the treatment of canopy interception; and (4) the impact of soil moisture stress. The degree of coupling between carbon and water fluxes, and how that coupling is calculated, is one of the key assumptions that determines how well the models compare with observations. This study yields a framework for analyzing and interpreting model predictions of transpiration responses to elevated CO2. This approach highlights key areas for immediate model improvement, hypotheses for experimental testing, and opportunities for data synthesis to significantly reduce discrepancies among models.


Kauwe, M. G., B. E. Medlyn, S. Zaehle, A. P. Walker, M. C. Dietze, T. Hickler, A. K. Jain, Y. Luo, W. J. Parton, C. Prentice, B. Smith, P. E. Thornton, S. Wang, Y. P. Wang, D. Wårlind, E. S. Weng, K. Y. Crous, D. S. Ellsworth, P. J. Hanson, H. Seok-Kim, J. M. Warren, R. Oren, and R. J. Norby. 2013. “Forest Water Use and Water Use Efficiency at Elevated CO2: A Model-Data Intercomparison at Two Contrasting Temperate Forest FACE Sites,” Global Change Biology, DOI: 10.1111/gcb.12164.