title: Seasonal responses of terrestrial ecosystem water-use efficiency to climate change.
abstract: Ecosystem water-use efficiency (EWUE) is an indicator of carbon-water interactions and is defined as the ratio of carbon assimilation (GPP) to evapotranspiration (ET). Previous research suggests an increasing long-term trend in annual EWUE over many regions and is largely attributed to the physiological effects of rising CO2 . The seasonal trends in EWUE, however, have not yet been analyzed. In this study, we investigate seasonal EWUE trends and responses to various drivers during 1982-2008. The seasonal cycle for two variants of EWUE, water-use efficiency (WUE, GPP/ET), and transpiration-based WUE (WUEt , the ratio of GPP and transpiration), is analyzed from 0.5° gridded fields from four process-based models and satellite-based products, as well as a network of 63 local flux tower observations. WUE derived from flux tower observations shows moderate seasonal variation for most latitude bands, which is in agreement with satellite-based products. In contrast, the seasonal EWUE trends are not well captured by the same satellite-based products. Trend analysis, based on process-model factorial simulations separating effects of climate, CO2 , and nitrogen deposition (NDEP), further suggests that the seasonal EWUE trends are mainly associated with seasonal trends of climate, whereas CO2 and NDEP do not show obvious seasonal difference in EWUE trends. About 66% grid cells show positive annual WUE trends, mainly over mid- and high northern latitudes. In these regions, spring climate change has amplified the effect of CO2 in increasing WUE by more than 0.005 gC m(-2)  mm(-1)  yr(-1) for 41% pixels. Multiple regression analysis further shows that the increase in springtime WUE in the northern hemisphere is the result of GPP increasing faster than ET because of the higher temperature sensitivity of GPP relative to ET. The partitioning of annual EWUE to seasonal components provides new insight into the relative sensitivities of GPP and ET to climate, CO2, and NDEP.
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.
authors.forename: Mengtian
authors.initials: M
authors.lastname: Huang
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.|Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100085, China.
authors.forename: Shilong
authors.initials: S
authors.lastname: Piao
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.
authors.forename: Zhenzhong
authors.initials: Z
authors.lastname: Zeng
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.|LSCE, UMR CEA-CNRS, Bat. 709, CE, L'Orme des Merisiers, F-91191, Gif-sur-Yvette, France.
authors.forename: Shushi
authors.initials: S
authors.lastname: Peng
authors.affiliation: LSCE, UMR CEA-CNRS, Bat. 709, CE, L'Orme des Merisiers, F-91191, Gif-sur-Yvette, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Ciais
authors.affiliation: CSIRO Land and Water Flagship, GPO Box 1666, Canberra, ACT, 2601, Australia.
authors.forename: Lei
authors.initials: L
authors.lastname: Cheng
authors.affiliation: Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN, 37831, USA.
authors.forename: Jiafu
authors.initials: J
authors.lastname: Mao
authors.affiliation: Institute on Ecosystems and the Department of Ecology, Montana State University, Bozeman, MT, 59717, USA.
authors.forename: Ben
authors.initials: B
authors.lastname: Poulter
authors.affiliation: Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN, 37831, USA.
authors.forename: Xiaoying
authors.initials: X
authors.lastname: Shi
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.
authors.forename: Yitong
authors.initials: Y
authors.lastname: Yao
authors.affiliation: Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Yang
authors.affiliation: CSIRO Ocean and Atmosphere Flagship, PMB 1, Aspendale, Vic., 3195, Australia.
authors.forename: Yingping
authors.initials: Y
authors.lastname: Wang
mesh_terms: D002244:Carbon / Q000032:analysis; D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000032:analysis; D057231:Climate Change; D017753:Ecosystem; D008962:Models, Theoretical; D009584:Nitrogen / Q000032:analysis; D018526:Plant Transpiration; D012621:Seasons; D014867:Water; D058438:Water Cycle
chemical_list: D014867:Water; D002245:Carbon Dioxide; D002244:Carbon; D009584:Nitrogen
keywords: CO2 enrichment; climate change; process-based model; seasonal variation; terrestrial ecosystem; water-use efficiency