Increasing water stress is emerging as a global phenomenon, and is anticipated to have a marked impact on forest function. The role of tree functional strategies is pivotal in regulating forest fitness and their ability to cope with water stress. However, how the functional strategies found at the tree or species level scale up to characterise forest communities and their variation across regions is not yet well-established. By combining eight water-stress-related functional traits with forest inventory data from the USA and Europe, we investigated the community-level trait coordination and the biogeographic patterns of trait associations for woody plants, and analysed the relationships between the trait associations and climate factors. We find that the trait associations at the community level are consistent with those found at the species level. Traits associated with acquisitive-conservative strategies forms one dimension of variation, while leaf turgor loss point, associated with stomatal water regulation strategy, loads along a second dimension. Surprisingly, spatial patterns of community-level trait association are better explained by temperature than by aridity, suggesting a temperature-driven adaptation. These findings provide a basis to build predictions of forest response under water stress, with particular potential to improve simulations of tree mortality and forest biomass accumulation in a changing climate.

Birmingham Institute of Forest Research University of Birmingham B15 2TT Birmingham UK

Centre for Ecology Evolution and Environmental Changes Azorean Biodiversity Group CHANGE Global Change and Sustainability Institute and Universidade dos Açores Faculty of Agricultural Sciences and Environment PT 9700 042 Angra do Heroísmo Azores Portugal

Department of Botany and Biodiversity Research University of Vienna Rennweg 14 1030 Vienna Austria

Department of Forest Resource Management Swedish University of Agricultural Sciences SE901 83 Umeå Sweden

Department of Physical Geography and Ecosystem Science Lund University Sölvegatan 12 22362 Lund Sweden

Forest and Natural Resources Research Centre Taxus IT ul Płomyka 56A 02 491 Warszawa Poland

Global Change Research Institute of the Czech Academy of Sciences Bělidla 986 4b 603 00 Brno Czech Republic

IFER Institute of Forest Ecosystem Research Cs Armady 655 254 01 Jilove u Prahy Czech Republic

Institute for Environmental Futures School of Geography Geology and the Environment University of Leicester LE1 7RH Leicester UK

National Centre for Earth Observation University of Leicester LE4 5SP Leicester UK

Natural Resources Institute Finland Latokartanonkaari 9 00790 Helsinki Finland

Optics of Photosynthesis Laboratory Institute for Atmospheric and Earth System Research Forest Sciences Viikki Plant Science Centre University of Helsinki Helsinki 00014 Finland

School of Geography Earth and Environmental Sciences University of Birmingham B15 2TT Birmingham UK

The United States Department of Agriculture Forest Service Northern Research Station NH 03824 Durham USA

Univ Grenoble Alpes INRAE LESSEM F 38402 St Martin d'Hères France

Universidad de Alcalá Departamento de Ciencias de la Vida Grupo de Ecología y Restauración Forestal 28805 Alcalá de Henares Spain

Universidad de Alcalá Departamento de Geología Geografía y Medio Ambiente Grupo de Investigación en Teledetección Ambiental 28801 Alcalá de Henares Madrid Spain

Wageningen University and Research Wageningen Environmental Research Droevendaalsesteeg 3 6708PB Wageningen The Netherlands

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