title: Spatial variability and temporal trends in water-use efficiency of European forests.
abstract: The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data.
authors.affiliation: Paul Scherrer Institut, Villigen, Switzerland.
authors.forename: Matthias
authors.initials: M
authors.lastname: Saurer
authors.forename: Renato
authors.initials: R
authors.lastname: Spahni
authors.forename: David C
authors.initials: DC
authors.lastname: Frank
authors.forename: Fortunat
authors.initials: F
authors.lastname: Joos
authors.forename: Markus
authors.initials: M
authors.lastname: Leuenberger
authors.forename: Neil J
authors.initials: NJ
authors.lastname: Loader
authors.forename: Danny
authors.initials: D
authors.lastname: McCarroll
authors.forename: Mary
authors.initials: M
authors.lastname: Gagen
authors.forename: Ben
authors.initials: B
authors.lastname: Poulter
authors.forename: Rolf T W
authors.initials: RT
authors.lastname: Siegwolf
authors.forename: Laia
authors.initials: L
authors.lastname: Andreu-Hayles
authors.forename: Tatjana
authors.initials: T
authors.lastname: Boettger
authors.forename: Isabel
authors.initials: I
authors.lastname: Dorado Liñán
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Fairchild
authors.forename: Michael
authors.initials: M
authors.lastname: Friedrich
authors.forename: Emilia
authors.initials: E
authors.lastname: Gutierrez
authors.forename: Marika
authors.initials: M
authors.lastname: Haupt
authors.forename: Emmi
authors.initials: E
authors.lastname: Hilasvuori
authors.forename: Ingo
authors.initials: I
authors.lastname: Heinrich
authors.forename: Gerd
authors.initials: G
authors.lastname: Helle
authors.forename: Håkan
authors.initials: H
authors.lastname: Grudd
authors.forename: Risto
authors.initials: R
authors.lastname: Jalkanen
authors.forename: Tom
authors.initials: T
authors.lastname: Levanič
authors.forename: Hans W
authors.initials: HW
authors.lastname: Linderholm
authors.forename: Iain
authors.initials: I
authors.lastname: Robertson
authors.forename: Eloni
authors.initials: E
authors.lastname: Sonninen
authors.forename: Kerstin
authors.initials: K
authors.lastname: Treydte
authors.forename: John S
authors.initials: JS
authors.lastname: Waterhouse
authors.forename: Ewan J
authors.initials: EJ
authors.lastname: Woodley
authors.forename: Peter M
authors.initials: PM
authors.lastname: Wynn
authors.forename: Giles H F
authors.initials: GH
authors.lastname: Young
mesh_terms: D057486:Carbon Cycle / Q000502:physiology*; D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes / Q000032:analysis; D057231:Climate Change; D005060:Europe; D065928:Forests; D005843:Geography; D008962:Models, Theoretical; D013997:Time Factors; D014197:Trees / Q000254:growth & development*; D058438:Water Cycle / Q000502:physiology*
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide
keywords: carbon isotope discrimination; climate change; dynamic vegetation model; tree rings