Forests are undergoing increasing risks of drought-induced tree mortality. Species replacement patterns following mortality may have a significant impact on the global carbon cycle. Among major hardwoods, deciduous oaks (Quercus spp.) are increasingly reported as replacing dying conifers across the Northern Hemisphere. Yet, our knowledge on the growth responses of these oaks to drought is incomplete, especially regarding post-drought legacy effects. The objectives of this study were to determine the occurrence, duration, and magnitude of legacy effects of extreme droughts and how that vary across species, sites, and drought characteristics. The legacy effects were quantified by the deviation of observed from expected radial growth indices in the period 1940-2016. We used stand-level chronologies from 458 sites and 21 oak species primarily from Europe, north-eastern America, and eastern Asia. We found that legacy effects of droughts could last from 1 to 5 years after the drought and were more prolonged in dry sites. Negative legacy effects (i.e., lower growth than expected) were more prevalent after repetitive droughts in dry sites. The effect of repetitive drought was stronger in Mediterranean oaks especially in Quercus faginea. Species-specific analyses revealed that Q. petraea and Q. macrocarpa from dry sites were more negatively affected by the droughts while growth of several oak species from mesic sites increased during post-drought years. Sites showing positive correlations to winter temperature showed little to no growth depression after drought, whereas sites with a positive correlation to previous summer water balance showed decreased growth. This may indicate that although winter warming favors tree growth during droughts, previous-year summer precipitation may predispose oak trees to current-year extreme droughts. Our results revealed a massive role of repetitive droughts in determining legacy effects and highlighted how growth sensitivity to climate, drought seasonality and species-specific traits drive the legacy effects in deciduous oak species.

Botanical Garden Institute of the Far Eastern Branch of the Russian Academy of Sciences Russia; Siberian Federal University Krasnoyarsk Russia

DeLaWi Tree Ring Analyses 51570 Windeck Germany

DendroGreif Institute of Botany and Landscape Ecology University of Greifswald Soldmannstr 15 17487 Greifswald Germany

Departamento Biología Animal Parasitología Ecología Edafología y Química Agrícola University Salamanca 37007 Salamanca Spain

Departamento de Ciencia y Tecnología Agroforestal y Genética Universidad de Castilla La Mancha Albacete Spain

Departamento de Sistemas y Recursos Naturales E T S 1 Montes Forestal y del Medio Natural Universidad Politécnica de Madrid Madrid Spain

Departement Recherche et Développement ONF Office National des Fôrets Batiment B Boulevard de Constance Fontainebleau F 77300 France

Department of Geography Institute of Ecology and Earth Sciences University of Tartu Vanemuise 46 Tartu Estonia

ETH Zurich Department of Environmental Systems Science Institute of Terrestrial Ecosystems Universitätstrasse 22 8092 Zurich Switzerland

Faculty of Forest and Environment Eberswalde University for Sustainable Development 16225 Eberswalde Germany

Federal Scientific Center of the East Asia Terrestrial Biodiversity Far Eastern Branch of Russian Academy of Sciences 690022 Vladivostok Russia

Global Change Research Institute of the Czech Academy of Sciences Brno Czech Republic; Department of Wood Science and Wood Technology Mendel University in Brno Brno Czech Republic

Harvard Forest 324 N Main St Petersham MA 01366 USA

Institute of Botany The Czech Academy of Sciences Třeboň Czech Republic

Institute of Botany The Czech Academy of Sciences Třeboň Czech Republic; Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague Kamýcká 129 Suchdol 165 21 Prague 6 Czech Republic

Institute of Botany The Czech Academy of Sciences Třeboň Czech Republic; Faculty of Science University of South Bohemia České Budějovice Czech Republic

Instituto Pirenaico de Ecología Avda Montañana 1005 Apdo 202 Zaragoza E 50192 Spain

Instituto Pirenaico de Ecología Avda Montañana 1005 Apdo 202 Zaragoza E 50192 Spain; Scuola di Scienze Agrarie Forestali Alimentari e Ambientali Università della Basilicata Potenza Italy

National Institute for Research and Development in Forestry Marin Dracea Eroilor 128 077190 Voluntari Romania

Southern Swedish Research Center Swedish University of Agricultural Sciences Alnarp Sweden; Institut de recherche sur les forêts Université du Québec en Abitibi Témiscamingue Québec Canada

Technische Universität München TUM School of Life Sciences Freising Germany; Technische Universität München Institute for Advanced Study Garching Germany

Thünen Institute of Forest Ecosystems Alfred Moeller Str 1 Haus 41 42 16225 Eberswalde Germany

Université de Lorraine AgroParisTech INRAE Silva 54000 Nancy France

WSL Swiss Federal Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland

WSL Swiss Federal Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland; Department of Biological Evolution Ecology and Environmental Sciences University of Barcelona 08028 Barcelona Spain

WSL Swiss Federal Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland; ETH Zurich Department of Environmental Systems Science Institute of Terrestrial Ecosystems Universitätstrasse 22 8092 Zurich Switzerland

WSL Swiss Federal Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland; Forestry and Wood Technology Discipline Khulna University Khulna Bangladesh

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