Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.

Beijing Research and Development Centre for Grass and Environment Beijing Academy of Agriculture and Forestry Sciences 100097 Beijing China

Brandenburg State Forestry Center of Excellence Eberswalde Germany

Chair of Forest Growth and Dendroecology University of Freiburg 79085 Freiburg Germany

Chair of Forest Growth and Woody Biomass Production TU Dresden 01737 Tharandt Germany

Climate Dynamics and Landscape Evolution Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences 14473 Potsdam Germany

CREAF E08193 Bellaterra Catalonia Spain

DendroGreif Institute for Botany and Landscape Ecology University Greifswald 17487 Greifswald Germany

Departamento de Ecología Centro de Investigaciones sobre Desertificación 46113 Moncada Valencia Spain

Department for Geography University of Bonn 53115 Bonn Germany

Department of Botany Faculty of Science University of South Bohemia České Budějovice Czech Republic

Department of Botany University of Innsbruck 6020 Innsbruck Austria

Department of Forest Botany Dendrology and Geobiocoenology Faculty of Forestry and Wood Technology Mendel University in Brno 613 00 Brno Czech Republic

Department of Forest Ecology Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague 165 00 Prague Czech Republic

Department of Forest Ecology Faculty of Forestry and Wood Technology Mendel University in Brno 613 00 Brno Czech Republic

Department of Geomatics Forest Research Institute 05 090 Raszyn Poland

Department of Land Environment Agriculture and Forestry University of Padua Padua Italy

Department of Natural Forests Forest Research Institute 17 230 Białowieża Poland

Department of Wood Science and Technology Faculty of Forestry and Wood Technology Mendel University in Brno 613 00 Brno Czech Republic

Environmental Measuring Systems Ltd 621 00 Brno Czech Republic

European Forest Institute Resilience Programme 53113 Bonn Germany

Forest Ecology and Forest Management Wageningen University and Research 6700 AA Wageningen The Netherlands

Forestry and Game Management Research Institute 252 02 Jíloviště Czech Republic

Geography Department Humboldt University 12489 Berlin Germany

Global Change Research Institute of the Czech Academy of Sciences 603 00 Brno Czech Republic

Grupo de Investigación Sistemas Naturales e Historia Forestal Universidad Politécnica de Madrid 28040 Madrid Spain

ICREA 08010 Barcelona Spain

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

Institute for Alpine Environment Eurac Research 39100 Bozen Bolzano Italy

Institute for Atmospheric and Earth System Research Forest Sciences Faculty of Agriculture and Forestry University of Helsinki 00014 Helsinki Finland

Institute for Atmospheric and Earth System Research Physics Faculty of Science University of Helsinki 00014 Helsinki Finland

Institute for environmental studies Faculty of Science Charles University Praha Czech Republic

Institute of Biology University of Hohenheim Stuttgart Germany

Institute of Botany of the Czech Academy of Sciences Průhonice Czech Republic

Institute of Forest Ecology Slovak Academy of Sciences 96053 Zvolen Slovakia

Izmir Katip Çelebi University Faculty of Forestry Çigli Izmir Turkey

Laboratory for Climatology and Remote Sensing Faculty of Geography 35032 Marburg Germany

Laboratory for Wood Anatomy and Dendrochronology Department of Geology Lund University Lund Sweden

Laboratory of Plant Ecology Department of Plants and Crops Faculty of Bioscience Engineering Ghent University 9000 Ghent Belgium

Laboratory of Tree Ring Research University of Arizona Tucson AZ 85721 USA

National Forest Centre Forest Research Institute 96001 Zvolen Slovakia

National Museum of Natural Sciences CSIC 28006 Madrid Spain

Natural Sciences Unit German Archaeological Institute 14195 Berlin Germany

Office National des Forêts Département Recherche Développement et Innovation 77300 Fontainebleau France

School of Natural Resources and the Environment University of Arizona Tucson AZ 85721 USA

Siberian Federal University 660041 Krasnoyarsk Russia

Southern Swedish Forest Research Centre Swedish University of Agricultural Sciences 230 53 Alnarp Sweden

Southwest Anatolia Forest Research Institute Antalya Turkey

Swiss Federal Institute for Forest Snow and Landscape Research WSL 8903 Birmensdorf Switzerland

Swiss Federal Research Institute WSL Insubric Ecosystems Research Group 6593 Cadenazzo Switzerland

Technical University in Zvolen Faculty of Forestry 96001 Zvolen Slovakia

Trinity College Dublin School of Natural Sciences Botany Department Dublin Ireland

UCD Earth Institute University College Dublin Belfield Dublin Ireland

UCD Forestry School of Agriculture and Food Science University College Dublin Dublin Ireland

UCD School of Biology and Environmental Science University College Dublin Belfield Dublin Ireland

Universitat Autònoma de Barcelona E08193 Bellaterra Catalonia Spain

Universite de Bordeaux INRAE BIOGECO 33615 Pessac France

Université de Lorraine AgroParisTech INRAE SILVA F 54000 Nancy France

Université Paris Saclay CNRS AgroParisTech Ecologie Systématique et Evolution 91405 Orsay France

Wageningen Environmental Research Wageningen University and Research 6700 AA Wageningen The Netherlands

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