title: 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.
authors.affiliation: Department of Biological Sciences, Macquarie University, North Ryde, New South Wales, 2109, Australia. mdekauwe@gmail.com
authors.forename: Martin G
authors.initials: MG
authors.lastname: De Kauwe
authors.forename: Belinda E
authors.initials: BE
authors.lastname: Medlyn
authors.forename: Sönke
authors.initials: S
authors.lastname: Zaehle
authors.forename: Anthony P
authors.initials: AP
authors.lastname: Walker
authors.forename: Michael C
authors.initials: MC
authors.lastname: Dietze
authors.forename: Thomas
authors.initials: T
authors.lastname: Hickler
authors.forename: Atul K
authors.initials: AK
authors.lastname: Jain
authors.forename: Yiqi
authors.initials: Y
authors.lastname: Luo
authors.forename: William J
authors.initials: WJ
authors.lastname: Parton
authors.forename: I Colin
authors.initials: IC
authors.lastname: Prentice
authors.forename: Benjamin
authors.initials: B
authors.lastname: Smith
authors.forename: Peter E
authors.initials: PE
authors.lastname: Thornton
authors.forename: Shusen
authors.initials: S
authors.lastname: Wang
authors.forename: Ying-Ping
authors.initials: YP
authors.lastname: Wang
authors.forename: David
authors.initials: D
authors.lastname: Wårlind
authors.forename: Ensheng
authors.initials: E
authors.lastname: Weng
authors.forename: Kristine Y
authors.initials: KY
authors.lastname: Crous
authors.forename: David S
authors.initials: DS
authors.lastname: Ellsworth
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Hanson
authors.forename: Hyun-
authors.initials: H
authors.lastname: Seok Kim
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Warren
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Norby
mesh_terms: D002245:Carbon Dioxide / Q000032:analysis*; D008962:Models, Theoretical; D014867:Water
chemical_list: D014867:Water; D002245:Carbon Dioxide