title: Elevated CO₂ increases tree-level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites.
abstract: Elevated CO₂ increases intrinsic water use efficiency (WUE(i) ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO₂ Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO₂ effects on stomatal conductance were explored using δ¹⁸O as a proxy. Across all the sites, elevated CO₂ increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO₂ and climate impacts on forest productivity, carbon and water balances.
authors.affiliation: Environmental Science Department, Second University of Naples, 81100, Caserta, Italy.|Centre for Bio-Archeology and Ecology, Ecole Pratique des Hautes Etudes (PALECO EPHE), Institut de Botanique, University of Montpellier 2, F-34090, Montpellier, France.
authors.forename: Giovanna
authors.initials: G
authors.lastname: Battipaglia
authors.affiliation: PSI Paul Scherrer Institute, 5232, Villigen, Switzerland.
authors.forename: Matthias
authors.initials: M
authors.lastname: Saurer
authors.affiliation: WSL Swiss Federal Institute for Forest, Snow and Landscape Research, 8903, Birmensdorf, Switzerland.
authors.forename: Paolo
authors.initials: P
authors.lastname: Cherubini
authors.affiliation: IBAF-Institute of agro-environmental and Forest Biology, CNR, Porano, Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Calfapietra
authors.affiliation: Department of Microbiology and Plant Biology, University of Oklahoma, Norman, OK, USA.
authors.forename: Heather R
authors.initials: HR
authors.lastname: McCarthy
authors.affiliation: Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, TN, USA.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Norby
authors.affiliation: Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO, USA.
authors.forename: M
authors.initials: M
authors.lastname: Francesca Cotrufo
mesh_terms: D000388:Air / Q000032:analysis*; D002245:Carbon Dioxide / Q000494:pharmacology*; D002247:Carbon Isotopes; D002980:Climate; D007558:Italy; D009657:North Carolina; D010103:Oxygen Isotopes; D010313:Partial Pressure; D010788:Photosynthesis / Q000187:drug effects; D054046:Plant Stomata / Q000187:drug effects / Q000502:physiology; D012044:Regression Analysis; D013714:Tennessee; D013997:Time Factors; D014197:Trees / Q000187:drug effects* / Q000502:physiology*; D014867:Water / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D014867:Water; D002245:Carbon Dioxide