Forest water use and water use efficiency at elevated CO₂: a model‐data intercomparison at two contrasting temperate forest FACE sites

Abstract

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 μ = approximately 28% ± 10%) compared to the observations (μ = approximately 30% ± 13%) at the well-coupled coniferous site (Duke), but poor (intermodel μ = approximately 24% ± 6%; observations μ = 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.

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

“…Other models impose biochemical limitations and indirectly reduce stomatal conductance by reducing A as soil moisture stress increases (the models explicitly scale g w with A; otherwise, water-use efficiency would decrease rather than increase with increased biochemical limitation on A). Neither approach can entirely replicate observations (Damour et al, 2010;Egea et al, 2011;De Kauwe et al, 2013), and possibly both diffusive and biochemical limitations must be considered (Zhou et al, 2013). There is also uncertainty about the form of the soil moisture stress function (Verhoef and Egea, 2014).…”

Plants and Drought in a Changing Climate

“…The changes in WUE found in two forested FACE experiments show increases of ~30% in WUE associated with an increase of ~200ppm in CO 2 . However the Oak Ridge and Duke FACE experiments differed in the cause for the WUE change, with either decreasing (Oak Ridge) or little change in transpiration (Duke) paired with large (Oak Ridge) to moderate (Duke) increases in photosynthesis [52][53][54]. FACE experiments are some of the few direct experiments available to test our understanding of whole ecosystem responses to CO 2 and provide invaluable insight into multiple competing responses within ecosystems including deserts, croplands, and agricultural crops [55].…”

“…To asses the water budget and thus the availably of water to plants we need to not only understand how WUE responds to CO 2 and other factors, but also how transpiration and photosynthesis will change in response to elevated CO 2 independent of one another. Climate models do a reasonable job representing the order of magnitude of changes in WUE found in observations over the historical period at forested sites [12], however it is possible that they are still failing to represent the individual changes in photosynthesis and transpiration [52]. The changes in WUE found in two forested FACE experiments show increases of ~30% in WUE associated with an increase of ~200ppm in CO 2 .…”

“…Experiments that use direct manipulation of atmospheric CO 2 concentrations in the field, called Free Air CO 2 Enrichment experiments (FACE), also find increases in WUE, but do not necessarily see decreases in transpiration [52]. To asses the water budget and thus the availably of water to plants we need to not only understand how WUE responds to CO 2 and other factors, but also how transpiration and photosynthesis will change in response to elevated CO 2 independent of one another.…”

Transient dynamics of terrestrial carbon storage: mathematical foundation and its applications

“…For example, incorporating an increasing number of processes that influence the C cycle may represent the real-world phenomena more realistically but makes the models more complex and less tractable. MIPs have effectively revealed the extent of the differences between model predictions (Schwalm et al, 2010;Keenan et al, 2012;De Kauwe et al, 2013) but provide limited insights into sources of model differences (see Medlyn et al, 2015). The physical emulators make data assimilation computationally feasible for global C cycle models (Hararuk et al, , 2015 and thus offer the possibility to generate independent yet constrained estimates of global land C sequestration to be compared with the indirect estimate from the airborne fraction of C emission and ocean uptake.…”