The WUEtree project focuses on "water use efficiency (WUE)" and the functional related traits because the challenge of many tree plantation programs, facing the global changes, is to improve the tree adaptation and the biomass production without increasing water extraction. The project responds to BIOADAPT in developing research on adaptation mechanisms and is situated at the interface of the two themes proposed by the call: “biological adaptive response of living organisms facing global change” and “methods and selection tools to adapt organisms to global changes”. WUEtree tests the general hypothesis that WUE exhibits significant variability within tree population and that it can be improved in breeding programs without compromising the biomass production. WUEtree aims at understanding the biological basis of WUE and its genetic and environmental determinisms. The ultimate goal is to define an ideotype and a selection methodology combining phenotypic and molecular information. To reach this objective we develop with a 36 month research program, through four scientific tasks (tasks 2 to 5) and one coordination and management task (task 1). The task 2 consists in understanding the functional relationship between WUE measured at leaf level (WI) and tree level (Wp) with ?13C and other related ecophysiological components (leaf traits, transpiration, etc..). It seeks to predict WUE at leaf and individual tree level using ?13C and related traits. The task 3 estimates the genetic and environmental variance components of ?13C and related traits and the correlations with biomass using quantitative genetic models. The task 4 is complementary to task 3 and seeks to understand, using association studies and fine mapping strategy, which genomic regions and which genes underlie ?13C and associated traits. In task3 and task 4 the genotype by environment interaction is evaluated using appropriate field designs showing different stress levels. The task 5 uses tasks 2, 3 and 4 information to test selection methodologies, such as genomic selection, associating both phenotypic and genomic data. The objective is to provide new statistical models to accurately rank genotypes. WUEtree is conducted with two tree model species, the hybrid Eucalyptus urophylla x grandis and Pinus pinaster, and relies on original experimental field trials established respectively, in the Republic of the Congo and in the south west of France. The expected results are a better understanding of the biological basis of WUE and of the genetic and environmental determinants. This new knowledge will be used for defining and selecting ideotypes presenting optimal WUE and productivity for marginal zones. The four partners, UMR 1134 AGAP CIRAD-INRA-SUPAGRO “genetic improvement and adaptation of tropical and Mediterranean plants”, its sub-contractor CRDPI “Centre de Recherche sur la Durabilité et la Productivité des Plantations Industrielles, Republic of the Congo”, UMR1137 EEF INRA-Université de Lorraine "Ecologie et Ecophysiologie Forestières”, UMR 111 Eco&Sols Montpellier SupAgro–CIRAD–INRA–IRD “Ecologie Fonctionnelle & Biogéochimie des Sols & Agroécosystèmes” and UMR 1202 BIOGECO INRA-Université de Bordeaux 1 “Biodiversité Gènes et Communautés” exhibit strong complementary skills combining genetics and biostatistics (AGAP/BIOGECO), tree ecophysiology (EEF/BIOGECO), process-based modelling (Eco&Sols/EEF), tree genomics and genome analyses (AGAP/BIOGECO) and field trial management (CRDPI/BIOGECO). The position of AGAP, Eco&sols and of BIOGECO in institutions associating research and companies involved in tree plantations (“Eucalyptus Fiber of the Congo” with the CRDPI), (GIS “Pin maritime du Futur” consortium funded by its memberships (INRA, FCBA, CRPF, CPFA and ONF), offers opportunities for the project to significantly impact the Eucalyptus and maritime pine breeding programmes.