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Zusammenfassung:
Predicted 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.
