Growth and resilience responses of Scots pine to extreme droughts across Europe depend on predrought growth conditions

Global climate change is expected to further raise the frequency and severity of extreme events, such as droughts. The effects of extreme droughts on trees are difficult to disentangle given the inherent complexity of drought events (frequency, severity, duration, and timing during the growing season). Besides, drought effects might be modulated by trees phenotypic variability, which is, in turn, affected by long-term local selective pressures and management legacies. Here we investigated the magnitude and the temporal changes of tree-level resilience (i.e., resistance, recovery, and resilience) to extreme droughts. Moreover, we assessed the tree-, site-, and drought-related factors and their interactions driving the tree-level resilience to extreme droughts. We used a tree-ring network of the widely distributed Scots pine (Pinus sylvestris) along a 2,800 km latitudinal gradient from southern Spain to northern Germany. We found that the resilience to extreme drought decreased in mid-elevation and low productivity sites from 1980 1999 to 2000 2011 likely due to more frequent and severe droughts in the later period. Our study showed that the impact of drought on tree-level resilience was not dependent on its latitudinal location, but rather on the type of sites trees were growing at and on their growth performances (i.e., magnitude and variability of growth) during the predrought period. We found significant interactive effects between drought duration and tree growth prior to drought, suggesting that Scots pine trees with higher magnitude and variability of growth in the long term are more vulnerable to long and severe droughts. Moreover, our results indicate that Scots pine trees that experienced more frequent droughts over the long-term were less resistant to extreme droughts. We, therefore, conclude that the physiological resilience to extreme droughts might be constrained by their growth prior to drought, and that more frequent and longer drought periods may overstrain their potential for acclimation. © 2020 The Authors. Global Change Biology published by John Wiley & Sons Ltd

The study was supported by a Marie Sk?odowska-Curie Individual Fellowship (PROJECT ID: 749051-REFOREST) to A.K.B. R.S-.S. was supported by Postdoctoral grant (IJCI-2015-25845, FEDER funds), J.J.C., J.C.L., and R.S-.S. were supported by the RTI2018-096884-B-C31 and RTI2018-096884-B-C33 projects (Ministry of Science, Innovation and Universities, Spain) and VULBOS project (UPO-1263216, FEDER Funds, Andalusia Regional Government, Consejer?a de Econom?a, Conocimiento, Empresas y Universidad 2014-2020), A.H. by PinCaR project (UHU-1266324, FEDER Funds, Andalusia Regional Government, Consejer?a de Econom?a, Conocimiento, Empresas y Universidad 2014-2020), and A.M. by the Bavarian Ministry of Science via the Bavarian Climate Research Network (bayklif). Part of the sampling was funded within the project DENDROKLIMA by the German Waldklimafond (FKZ 28W-C-4-077-01) and ST327 by the Bavarian State Ministry for Food, Agriculture, and Forestry. We thank Kerstin Treydte, Richard Peters, and Stefan Klesse for providing valuable supports on tree ring width detrending methods, David Forrester for providing information on site productivity index, Tanja Sanders for providing valuable comments on earlier version of the manuscript, and Landesforst Mecklenburg-Vorpommern, Landeskompetenzzentrum Forst Eberswalde, and Nordwestdeutsche Forstliche Versuchsanstalt for maintaining the intensive forest monitoring sites (Level II) used in this study, Javier Don?s (Director of Montes de Valsa?n), National Parks Autonomous Agency (OAPN), Junta de Castilla y Le?n forest guards and all the participants involved in the ?International Tree-Ring Summer School? celebrated in 2012 in Valsa?n (Segovia-Spain), and Ramzi Touchan and Dave Meko. We also thank all colleagues who were involved in field sampling and laboratory measurements for the other sites.