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TitleForest water use and water use efficiency at elevated CO2: a model-data intercomparison at two contrasting temperate forest FACE sites
AuthorDe Kauwe, M G; Medlyn, B E; Zaehle, S; Walker, A P; Dietze, M C; Hickler, T; Jain, A K; Luo, Y; Parton, W J; Prentice, I C; Smith, B; Thornton, P E; Wang, S; Wang, Y -P; Wårlind, D; Weng, E; Crous, K Y; Ellsworth, D S; Hanson, P J; Kim, H -S; Warren, J M; Oren, R; Norby, R J
SourceGlobal Change Biology vol. 19, 2013 p. 1759-1779,
Alt SeriesEarth Sciences Sector, Contribution Series 20140170
PublisherBlackwell Publishing Ltd
Mediapaper; on-line; digital
File formatpdf
Subjectsenvironmental geology; hydrogeology; surface waters; carbon dioxide; environmental analysis; environmental studies; environmental impacts; climate; climate effects; vegetation; models; climate change
Illustrationstables; plots; histograms
ProgramAquifer Assessment & support to mapping, Groundwater Geoscience
AbstractPredicted responses of transpiration to elevated atmospheric CO2 concentration (eCO2) are highly variable amongst process-based models. To better understand and constrain this variability amongst models, we conducted an intercomparison of 11 ecosystem models applied to data from two forest free-air CO2 enrichment (FACE) experiments at Duke University and Oak Ridge National Laboratory. We analysed model structures to identify the key underlying assumptions causing differences in model predictions of transpiration and canopy water use efficiency. We then compared the models against data to identify model assumptions that are incorrect or are large sources of uncertainty. We found that model-to-model and model-to-observations differences resulted from four key sets of assumptions, namely (i) the nature of the stomatal response to elevated CO2 (coupling between photosynthesis and stomata was supported by the data); (ii) the roles of the leaf and atmospheric boundary layer (models which assumed multiple conductance terms in series predicted more decoupled fluxes than observed at the broadleaf site); (iii) the treatment of canopy interception (large intermodel variability, 2-15%); and (iv) the impact of soil moisture stress (process uncertainty in how models limit carbon and water fluxes during moisture stress). Overall, model predictions of the CO2 effect on WUE were reasonable (intermodel l = approximately 28% 10%) compared to the observations (l = approximately 30% 13%) at the well-coupled coniferous site (Duke), but poor (intermodel l = approximately 24% 6%; observations l = approximately 38% 7%) at the broadleaf site (Oak Ridge). The study yields a framework for analysing and interpreting model predictions of transpiration responses to eCO2, and highlights key improvements to these types of models.
Summary(Plain Language Summary, not published)
The Free-Air CO2 Enrichment, FACE, is a large international program jointly conducted by more than fifteen countries. The goal of FACE science is to study the effects of elevated CO2 in the atmosphere on ecosystems functions, services, and the water cycles. The FACE-model Interaction project, led by Oak Ridge National Laboratory (ORNL) and supported by US Department of Energy (DOE), is to benchmark ecosystem response models with the FACE data. ORNL selected 11 ecosystem models from 6 countries to conduct this project. The EALCO model developed in CCRS is one of the 11 models and it is the only one from Canada and the only remote sensing-based model. This paper presents part of results from this international collaboration. It focuses on studying the forest water use and water use efficiency at elevated atmospheric CO2 concentration.