# Revealing legacy effects of extreme droughts on tree growth of oaks across the Northern Hemisphere

Arun Bose, Jiri Doležal, Daniel Scherrer, Jan Altman, Daniel Ziche, Elisabet Martínez-Sancho, Christof Bigler, Andreas Bolte, Michele Colangelo, Isabel Dorado-Liñán, et al.

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► To cite this version:

Arun Bose, Jiri Doležal, Daniel Scherrer, Jan Altman, Daniel Ziche, et al. Revealing legacy effects of extreme droughts on tree growth of oaks across the Northern Hemisphere. Science of the Total Environment, 2024, 926, pp.172049. 10.1016/j.scitotenv.2024.172049. hal-04684838

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Title: Revealing legacy effects of extreme droughts on tree growth of oaks across the

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Northern Hemisphere

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Author names and affiliations

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Arun K. Bose¹,²,*, Jiri Doležal³,⁴, Daniel Scherrer¹, Jan Altman³,⁵, Daniel Ziche⁶,

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Elisabet Martínez-Sancho⁷,¹, Christof Bigler⁸, Andreas Bolte⁹, Michele Colangelo¹⁰,¹¹, Isabel

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Dorado-Liñán¹², Igor Drobyshev¹³,¹⁴, Sophia Etzold¹, Patrick Fonti¹, Arthur Gessler¹,⁸, Tomáš

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Kolář¹⁵,¹⁶, Eva Koňasová¹⁵,¹⁶, Kiril Aleksandrovich Korznikov³, Francois Lebourgeois¹⁷,

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Manuel Esteban Lucas-Borja¹⁸, Annette Menzel¹⁹,²⁰, Burkhard Neuwirth²¹, Manuel Nicolas²²,

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Alexander Mikhaylovich Omelko²³, Neil Pederson²⁴, Any Mary Petritan²⁵, Andreas

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Rigling¹,⁸, Michal Rybníček¹⁵,¹⁶, Tobias Scharnweber²⁶, Jens Schröder⁶, Fernando Silla²⁷,

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Irena Sochová¹⁵,¹⁶, Kristina Sohar²⁸, Olga Nikolaevna Ukhvatkina²³, Anna Stepanovna

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Vozmishcheva²⁹,³⁰, Roman Zweifel¹ and J. Julio Camarero¹⁰

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¹WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Zürcherstrasse
111, CH-8903 Birmensdorf, Switzerland.

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²Forestry and Wood Technology Discipline, Khulna University, Khulna, Bangladesh.

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³Institute of Botany, The Czech Academy of Sciences, Třeboň, Czech Republic.

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⁴Faculty of Science, University of South Bohemia, České Budějovice, Czech Republic.

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⁵Faculty of Forestry and Wood Sciences, Czech University of Life Sciences Prague,
Kamýcká 129, 165 21 Prague 6 - Suchdol, Czech Republic

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⁶Faculty of Forest and Environment, Eberswalde University for Sustainable
Development, 16225 Eberswalde, Germany.

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⁷Department of Biological Evolution, Ecology and Environmental Sciences, University
of Barcelona, 08028 Barcelona, Spain

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⁸ETH Zurich, Department of Environmental Systems Science, Institute of Terrestrial Ecosystems (ITES), Universitätstrasse, 22, 8092 Zurich, Switzerland.

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⁹Thünen Institute of Forest Ecosystems, Alfred-Moeller-Str. 1, Haus 41/42, 16225 Eberswalde, Germany.

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¹⁰Instituto Pirenaico de Ecología (IPE-CSIC), Avda. Montañana 1005, Apdo. 202, Zaragoza, E-50192, Spain.

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¹¹Scuola di Scienze Agrarie, Forestali, Alimentari, e Ambientali, Università della Basilicata, Potenza, Italy.

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¹²Departamento de Sistemas y Recursos Naturales, E.T.S.I. Montes Forestal y del Medio Natural, Universidad Politécnica de Madrid (UPM), Madrid, Spain.

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¹³Southern Swedish Research Center, Swedish University of Agricultural Sciences, Alnarp, Sweden.

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¹⁴Institut de recherche sur les forêts, Université du Québec en Abitibi-Témiscamingue, Québec, Canada.

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¹⁵Global Change Research Institute of the Czech Academy of Sciences, Brno, Czech Republic.

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¹⁶Department of Wood Science and Wood Technology, Mendel University in Brno, Brno, Czech Republic.

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¹⁷Université de Lorraine, AgroParisTech, INRAE, Silva, 54000, Nancy, France.

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¹⁸Departamento de Ciencia y Tecnología Agroforestal y Genética, Universidad de Castilla La Mancha, Albacete, Spain.

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¹⁹Technische Universität München, TUM School of Life Sciences, Freising, Germany.

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²⁰Technische Universität München, Institute for Advanced Study, Garching, Germany.

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²¹DeLaWi Tree-Ring Analyses, 51570 Windeck, Germany.
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²²**Departement Recherche et Développement, ONF, Office National des Fôrets, Batiment B, Boulevard de Constance, Fontainebleau F-77300, France.**

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²³**Federal Scientific Center of the East Asia Terrestrial Biodiversity, Far Eastern Branch of Russian Academy of Sciences, 690022 Vladivostok, Russia.**

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²⁴**Harvard Forest, 324 N. Main St, Petersham, MA 01366, USA.**

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²⁵**National Institute for Research and Development in Forestry “Marin Dracea”, Eroilor 128, 077190 Voluntari, Romania.**

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²⁶**DendroGreif, Institute of Botany and Landscape Ecology, University of Greifswald, Soldmannstr. 15, 17487 Greifswald, Germany.**

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²⁷**Departamento Biología Animal, Parasitología, Ecología, Edafología y Química Agrícola, University Salamanca, 37007 Salamanca, Spain.**

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²⁸**Department of Geography, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, Tartu, Estonia.**

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²⁹**Botanical Garden-Institute of the Far Eastern Branch of the Russian Academy of Sciences, Russia.**

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³⁰**Siberian Federal University, Krasnoyarsk, Russia.**

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*Corresponding author: Arun K. Bose, WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland, Email:

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arun.bose@wsl.ch.

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71 **Abstract**

72 Forests are undergoing increasing risks of drought-induced tree mortality. Species 73 replacement patterns following mortality may have a significant impact on the global carbon 74 cycle. Among major hardwoods, deciduous oaks (*Quercus* spp.) are increasingly reported as 75 replacing dying conifers across the Northern Hemisphere. Yet, our knowledge on the growth 76 responses of these oaks to drought is incomplete, especially regarding post-drought legacy 77 effects. The objectives of this study were to determine the occurrence, duration, and 78 magnitude of legacy effects of extreme droughts and how that vary across species, sites, and 79 drought characteristics. The legacy effects were quantified by the deviation of observed from 80 expected radial growth indices in the period 1940-2016. We used stand-level chronologies 81 from 458 sites and 21 oak species primarily from Europe, north-eastern America, and eastern 82 Asia. We found that legacy effects of droughts could last from 1 to 5 years after the drought 83 and were more prolonged in dry sites. Negative legacy effects (i.e., lower growth than 84 expected) were more prevalent after repetitive droughts in dry sites. The effect of repetitive 85 drought was stronger in Mediterranean oaks especially of *Quercus faginea*. Species-specific 86 analyses revealed that *Q. petraea* and *Q. macrocarpa* from dry sites were more negatively 87 affected by the droughts while growth of several oak species from mesic sites increased 88 during post-drought years. Sites showing positive correlations to winter temperature showed 89 little to no growth depression after drought, whereas sites with a positive correlation to 90 previous summer water balance showed decreased growth. This may indicate that although 91 winter warming favors tree growth during droughts, previous-year summer precipitation may 92 predispose oak trees to current-year extreme droughts. Our results revealed a massive role of 93 repetitive droughts in determining legacy effects and highlighted how growth sensitivity to 94 climate, drought seasonality and species-specific traits drive the legacy effects in deciduous 95 oak species.
96 **Keywords**

97 Climate change; warming; repetitive droughts; legacy effects; acclimation; tree rings.

98 **Introduction**

99 Drought-induced tree mortality is increasing globally, with major consequences for
100 the structure and functioning of forests and their ecosystem services (Allen et al., 2015;
101 Hammond et al., 2022; Hartmann et al., 2022). Abrupt tree mortality during and after
102 droughts can alter community composition towards the dominance of drought-tolerant tree
103 and shrub species (Batllori et al., 2020; Etzold et al., 2019). In temperate regions, deciduous
104 oaks (Quercus spp.) can tolerate relatively extreme droughts because of their deep-
105 penetrating root systems and strong stomatal control, allowing them to maintain relatively
106 high predawn water potentials during drought (Abrams, 1990; Madrigal-González et al.,
107 2017). Further, deciduous oaks are usually ring-porous species producing large vessels in the
108 early growing season, allowing for increased hydraulic conductivity and stomatal
109 conductance (Martínez-Sancho et al., 2017; Peters et al., 2023), but such xylem is very
110 vulnerable to frost- and drought-induced xylem cavitation (Sperry et al., 1994). Several
111 recent studies reported a growing dominance of deciduous oak species after drought-induced
112 mortality events mainly affecting conifers (Galiano et al., 2010; Rigling et al., 2013; Vilà-
113 Cabrera et al., 2013), albeit oaks may also experience increased defoliation (Michel et al.,
114 2022) and dieback in response to severe dry spells (Camarero et al., 2021; Druckenbrod et
115 al., 2019). Since drought-stressed trees are vulnerable to secondary pathogens (Haavik et al.,
116 2015), drought-pathogen interactions can play a critical role in oak tree performance (Wood
117 et al., 2018) and post-drought recovery. Performance and sensitivity of tree species during
118 post-drought conditions will play an important role under future climatic conditions with
119 increased frequency of extreme climate events and are predicted to have a lasting effect on
120 ecosystem dynamics (Müller & Bahn, 2022) and associated changes in carbon storage and
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cycle (Schwalm *et al.*, 2017; Trugman *et al.*, 2018). Therefore, there is an urgent need to

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understand how previously drought-exposed trees would respond to future drought events to

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better mitigate potential adverse effects (Bose *et al.*, 2020; Gessler *et al.*, 2020).

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Post-drought growth recovery can be a result of species- or population-related

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response patterns (Kannenberg *et al.*, 2019b; Peltier & Ogle, 2019), and persistent functional

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adjustments during and after droughts can induce acclimation to future droughts (Zweifel *et al.*, 2020). Such an acclimation process is characterized as ecological legacy (Vilonen *et al.*, 2022), which can significantly alter the responses of trees even several years after the drought

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event (Anderegg *et al.*, 2015b; Ovenden *et al.*, 2021; Wu *et al.*, 2018). The alteration of tree

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growth due to legacies of past drought could occur through different sets of mechanisms

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(Bose *et al.*, 2022; Marqués *et al.*, 2022; Ogle *et al.*, 2015). For example, drought could

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induce crown dieback (Klesse *et al.*, 2022) or damage to water transport system (Anderegg *et al.*, 2015a). The repair of damaged organs after drought could occur at the expense of

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biomass growth (Kannenberg *et al.*, 2019b), which could result in negative drought legacies

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on radial growth (Anderegg *et al.*, 2015b; Huang *et al.*, 2018). The negative legacy effect on

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growth can endure several years depending on post-drought climatic conditions, the

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magnitude of drought-induced damages, and species-specific recovery capacities (Ruehr *et al.*, 2019). For example, based on a global analysis of tree-ring series, pine species

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experienced more persistent negative legacy effects of drought than oaks (Anderegg *et al.*, 2015b). Furthermore, the magnitude of these negative legacy effects was found to be higher

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in dry sites compared to mesic sites. Moreover, Huang *et al.* (2018) reported that negative

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legacy effects were more pervasive when droughts occurred in the dry compared to the wet

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season. Studying two widely distributed deciduous oak species across a latitudinal gradient in

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Europe, Bose *et al.* (2021b) identified slow recovery to spring droughts in *Q. petraea* but not

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in *Q. robur*. The later species even grew better during the 2nd and 3rd post-drought years

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compared to pre-drought years, indicating a positive legacy effect. The switch from negative to positive legacy effects can occur through changes in tree-to-tree competition within a forest stand (Cavin et al., 2013). The changes in competition may occur through drought-induced mortality or growth decline of drought-sensitive species or individuals favoring the growth of drought-tolerant species or individuals capable of taking water from deeper soil layers (Ripullone et al., 2020). These findings suggest that the magnitude of drought legacy effects vary between drought-tolerant and -intolerant species. However, we currently lack a large global-scale synthesis on drought tolerant species, especially how their responses evolve during the post-drought years and whether they show positive legacy effects i.e., higher growth than expected due to droughts.

Deep-rooted deciduous oaks are widely distributed in dry and mesic sites (Abrams, 1990) and considered drought tolerant (Arend et al., 2011; Dickson & Tomlinson, 1996). In north-eastern America, *Q. rubra*, *Q. montana*, *Q. stellata*, and *Q. velutina* showed little to no-legacy effects of droughts (Kannenberg et al., 2019a), though projections of future oak abundance are reputed to considerably decrease in the U.S. (Novick et al., 2022). Those ring-porous species can recover quickly by developing new xylem (because they keep only a small number of tree rings active and therefore significantly change the quality of their hydraulics every year). In contrast, the diffuse-porous species need a longer time to repair (if possible at all) drought-induced hydraulic damages since they maintain a higher number of tree rings in the sapwood area and usually have longer leaf lifespans (Zweifel & Sterck, 2018). In the Mediterranean region, *Q. pubescens* showed a progressive decline in resistance to extreme droughts in northern Spain (Camarero et al., 2021; González de Andrés et al., 2021), while drought-induced growth decline and tree mortality have been reported for *Q. pubescens*, *Q. frainetto*, and *Q. faginea* in Southern Italy and Eastern Spain (Colangelo et al., 2017; Corcuera et al., 2004; Ripullone et al., 2020). Studying a *Q. robur* population mixed with
Pinus sylvestris and Picea abies in southern Germany, Zang *et al.* (2012) reported a faster recovery of *Q. robur* compared to two other species. This result for *Q. robur* is consistent with those reported by Steckel *et al.* (2020) and Bose *et al.* (2021b). However, the later study showed a weaker recovery potential in *Q. petraea* than *Q. robur*. In a mixed-species forest, *Q. petraea* was not significantly affected by droughts but benefited from the release of competition through drought-induced mortality of neighboring *Fagus sylvatica* (Cavin *et al.*, 2013). Overall, the existing literature on deciduous oaks is primarily based on local to regional studies and indicates drought tolerance but also shows their vulnerability under extreme drought conditions. This inconsistencies in literature need to be addressed by examining the responses at a larger spatial scale and considering many oak species.

We used a network of tree-ring chronologies of deciduous oak species from 458 stands covering 21 species and a large part of their distribution in North America, Europe, and Asia. Our main objective was to determine the occurrence, duration, and magnitude of legacy effects on tree growth induced by extreme droughts. Specifically, we asked: (i) do legacy effects occur in deciduous oaks and at which extent? (ii) are legacy effects more prominent in dry than mesic sites irrespective of species? (iii) do legacy effects vary by drought seasonality, drought repetitiveness and by species, and can species-specific traits explain this? and (iv) do legacy effects of extreme droughts in deciduous oaks depend on long-term growth-climate sensitivity? We hypothesized that post-drought growth legacies are mainly negative, and their absolute magnitude peaks one year after drought, particularly in dry sites.

## 192 Methods

### 193 Study area

The study area spans from 122.03° W to 145.85°E and includes a latitudinal band from 32.21°N to 59.36 °N (Fig. 1), where oak forests are located in temperate and sub-
tropical (e.g., Mediterranean) ecosystems. Across the study sites, the climate varies largely (Salinger, 2005), with mean annual temperatures, total annual precipitation sum, and climatic water balance (precipitation-potential evapotranspiration, P-PET) for the years 1986–2015 ranging between 0.88 and 18.93 °C (mean 9.75 °C), 338 and 2,133 mm (mean 833 mm), and -36.28 and 131.0 mm (mean 15.08 mm), respectively. While temperatures increased by 0.85 °C during 1986-2015 compared to the period 1956-1985, precipitation and climatic water balance remained relatively stable.

## Ring width data

We compiled tree-ring width data (RW) of 21 deciduous *Quercus* species from 12,744 trees located in 458 sites in North America, Europe, and Eastern Asia (Fig. 1), covering a period approximately from 1940-2016. The RW data were obtained from the International Tree-Ring Data Bank (http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring) in November 2022 (n=117), from Bose *et al.* (2021b) (n=106), and from additional sites provided by the authors (n=239). Based on the geographic distribution (Fig. 1) and their climatic envelope (Fig. SM1), the 21 oak species were grouped as North American oaks, temperate European oaks, Mediterranean oaks, and Mongolian oaks (Table SM1). To reduce trends related to tree age and size, we detrended the raw ring-width (RW) series using a 30-year cubic smoothing spline with a 50% frequency cut-off (Cook & Kairiukstis, 1990). This detrending method retains 99% of the variance at 5 years and 50% of the variance at 15 years (Cook & Kairiukstis, 1990). We chose cubic spline method due to difference in age of the sampled trees. Although removing the effects of past forest management on tree-ring width chronologies is difficult with existing method, Esper *et al.* (2012) showed minor effects of harvesting on the climate-growth relationships in temperate forests. We then averaged the individual level residual series of ring-width indices (RWI) to obtain site-level standard chronologies.
221 Climate data and identification of droughts

222 We obtained site-specific climate data from the CHELSA V1.2 timeseries
223 (Climatologies at high resolution for the earth's land surface areas) for each site using the
224 latitude and longitude coordinates (Karger et al., 2017). Monthly precipitation sums and
225 monthly mean, maximum, and minimum air temperatures from January to December were
226 obtained for the period 1940-2016. We then calculated the climatic water balance (CWB) at
227 monthly, seasonal, and annual scales. The CWB represents precipitation minus potential
228 evapotranspiration (PET). The PET was obtained using the *Thornthwaite* function of the R
229 package SPEI (Begueria & Vicente-Serrano, 2013). Seasonal climatic variables were
230 computed by averaging (temperature) or summing (precipitation) monthly values (winter:
231 December–February; spring: March–May; summer: June–August; and autumn: September–
232 November). We identified extreme drought events at annual as well as at three seasonal
233 scales (i.e., previous winter (December–February), current spring (March–May), and current
234 summer (June–August)). CWB measures absolute climate water balance and were chosen to
235 characterize drought as it better captures the absolute water availability (Zang et al., 2019)
236 and was also used by previous studies (Anderegg et al., 2015b; Bose et al., 2021b). CWB that
237 exceeded 1.5 standard deviations towards negative values were used to define an extreme
238 drought “event”. The list of years when the extreme drought events occurred at annual and/or
239 seasonal scales is presented in Table SM2 in the supplementary materials. We characterized
240 each of those identified drought as (i) whether it has a repetitive drought (i.e., repetitive
241 within the 5-year period post of a drought), termed as “*has repetitive drought*”, (ii) whether it
242 is a repetitive drought termed as “*is repetitive drought*” and (iii) whether there were no other
243 droughts within the 5-year post of the drought event, termed as a “*single drought*”. We
244 characterized those three variables as binary variables (i.e., 0 or 1). The variable “*is repetitive
245 drought*” was not significant for any of the analyses performed (see below), therefore
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excluded from the analyses. The variable "single drought" was correlated with "has repetitive drought" and not considered in the final analyses.

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## Quantification of legacy effects

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We quantified the legacy effects of droughts as a departure of observed tree growth (RWI) from expected growth (based on the relationship between RWIs and climate)

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(Anderegg et al., 2015b). Considering the availability of climatic information from 1940 to 2016, we quantified legacy effects of all droughts occurred in this period. For each site, we first identified the most important climatic variable (i.e, CWB) based on the linear

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relationship between growth indices (RWI) and CWB. For that purpose, we considered monthly, seasonal, and annual CWB from previous year to current year growing season. For each site, the most important climatic variable (i.e., CWB) was then used to predict tree growth after each drought event. The difference between the observed and the predicted post-drought growth indices was used to determine the legacy effect. We determined legacy effects for a five-year period after the drought event. We compared the trend of observed and predicted growth irrespective of species (Fig. SM2) as well as separately for major species (Fig. SM3).

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## Statistical analyses

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We used linear-mixed effect models (Zuur et al., 2009) for detecting the effects of predictor variables on legacy effects. The values of the legacy effects were standardized for better quantifying the effect size of predictor variables and comparing effect sizes across predictor variables. The modelling was performed using the function *lme* of the R package *nlme* (Pinheiro et al., 2014; Pinheiro & Bates, 2000; R Development Core Team, 2022). For predictor variables, we considered site types (dry, moderate, and mesic), regional species groups (Mongolian oak, temperate European oaks, Mediterranean oaks and North American oaks), elevation, latitude, longitude, drought intensity, drought types (i.e., "has repetitive

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296 where the data was available. For each species, we summed the legacy effect values of 1-5-
297 post drought years and termed as “integrated legacy effects” similar to Anderegg et al.
298 (2015b). We then used linear regression to detect relationship between integrated legacy
299 effects and species-specific functional traits.

300 For detecting the role of drought timing or seasonality (Bose et al., 2021b;
301 D'Orangeville et al., 2018; Huang et al., 2018), we first quantified the legacy effects of all
302 seasonal droughts (Table SM2) occurred in the study period (1940-2016) following the same
303 methods explained above. We considered three seasons including the previous winter, current
304 spring, and current summer. The legacy effects were then modelled as a function of site
305 types, seasons, regional species groups, and years since droughts and the same random effect
306 structure (explained earlier).

## 307 Results

308 Repetitive droughts caused significant negative legacy effects (i.e., lower growth than
309 expected) in radial growth in dry (TAP 350-598 mm) and mesic sites (TAP >1000 mm) but
310 not in moderate sites (TAP 600-1000 mm) (Table SM3 and Fig. 2A). The negative legacy
311 effects of repetitive droughts were not fully recovered in dry sites and the size of negative
312 legacy effects was larger in dry sites compared to other mesic sites but not initial two-years
313 after droughts (Fig. 2A). However, negative legacy effects turned to positive (i.e., higher
314 growth than expected) both in mesic and moderate sites during 3-5-years after the non-
315 repetitive extreme droughts. Contrary to moderate and mesic sites, the initial non-significant
316 legacy effects turned to negative legacy effects in dry sites from the 3rd year after the non-
317 repetitive extreme droughts.

318 We detected significant negative effect of repetitive droughts irrespective of site-
319 specific growth-climate sensitivity (Table SM4). Significant negative legacy effects were
320 observed only in sensitive sites (where growth significantly correlated with CWB), and that
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occurred only in the first year after the drought event (Fig. 2B and Table SM4). The negative
legacy effects then turned to positive and stayed positive from 2 to 5 years after the droughts.

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Repetitive droughts caused significant negative legacy effects in Mediterranean oaks
and such negative legacy effects were not fully recovered during 1–5-year post-drought
period (Fig. 2C). However, non-repetitive droughts resulted significant positive legacy effects
in Mediterranean oaks and not in other species group (Fig. 2C). Negative legacy effects were
found in temperate European, North American, and Mongolian oaks, but only in the 1st year
after the extreme droughts (Fig. 2C). The frequency of negative legacy effects was lower
compared to positive legacy effects except in the first year after the drought event (Fig.
SM4).

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Species-specific analysis revealed that repetitive droughts had significant effects in
determining legacy effects of *Q. petraea* and *Q. faginea* but not of other species (Fig. 3). *Q.
petraea*, *Q. macrocarpa*, and *Q. faginea* could not fully recover the growth in dry sites after
repetitive droughts for the entire 1-5 years post-drought period. However, we observed
positive legacy effects in mesic sites for *Q. petraea*, and *Q. macrocarpa* (Fig. 3). Significant
difference between mesic and dry sites were observed only for *Q. macrocarpa*. The
differences between mesic and dry sites were tested only for four species (*Q. macrocarpa*, *Q.
petraea*, *Q. robur*, and *Q. faginea*) because of insufficient number of observations for other
species. Significant negative legacy effects that occurred during 1-2 years after the extreme
droughts were fully recovered for *Q. faginea* and *Q. rubra*. We did not observe significant
changes in legacy effects over 1-5 years after the drought event for *Q. robur* and *Q.
mongolica*. The magnitude of positive legacy effects was higher for *Q. faginea* as compared
to other species after non-repetitive droughts (Fig. 3). When we integrated the legacy effects
of 1-5 years by species, we detected significant relationship between integrated legacy effects

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and P50, SLA, and nitrogen mass, although the species-specific trait data was available for
only six oak species (Fig. 4).

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We found a strong positive relationship between the legacy effect and growth sensitivity to past-year winter temperature (Fig. 5A) and a negative relationship between the legacy effect and growth sensitivity to past-year summer water balance (Fig. 5B). These analyses revealed a switch from positive to negative legacy effects in dry sites while negative to positive legacy effects in mesic sites over the 5 years after the extreme droughts (Fig. 5A).

This switch has also been identified in Mediterranean oaks which showed negative legacy effects during the first year after the extreme droughts followed by positive legacy effects during later years (Fig. 5B). The effects of mean annual temperature on legacy effects also switched from negative to positive during 1-5 years after the extreme droughts (Fig. 5M).

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In mesic sites, significant negative legacy effects of winter droughts were found and lasted 1-4 years after the drought event while significant positive legacy effects of winter droughts were found in dry sites (Fig. 6A). Contrary to winter droughts, significant negative legacy effects of spring and summer droughts occurred in dry sites and lasted 1-5 years after the droughts. The size of negative legacy effects of winter droughts was larger in North American oaks compared to the other species groups (Fig. 6B). We observed non-significant differences among temperate European, Mediterranean, and Mongolian oaks when responding to winter droughts. Temperate European oaks were more negatively affected by spring droughts as the negative legacy effects were larger compared to other species groups. Spring droughts induced positive legacy effects in Mongolian oaks. Contrary to spring droughts, Mongolian oaks were more negatively affected by summer droughts compared to other species groups (Fig. 6B).

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Discussion

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While prior research has assessed the enduring impacts of severe droughts on tree species' radial growth at various geographical levels (Anderegg et al., 2015b; Bose et al., 2021b; Huang et al., 2018; Kannenberg et al., 2019a; Peltier et al., 2016; Wu et al., 2018), our study provides a first empirical confirmation for deciduous oak species, typically seen as resilient species to extreme drought events. We acknowledge that the spatial coverage of our data set is limited largely to temperate and Mediterranean biomes, although it encompasses substantial variation in climatic conditions within these regions. We showed that the magnitude and the direction (positive or negative) of legacy effects in deciduous oaks largely depend on species-specific functional traits, drought repetitiveness, drought seasonality growth sensitivity to the previous year climate, and site type related to long-term precipitation averages. Our results revealed significantly different post-drought growth trajectories between oak trees growing in mesic vs dry sites. We detected a strong legacy effect of droughts especially when occurred in successive years on tree growth of deciduous oaks. The legacy effects occurred even when climatic conditions return to normal. Given that tree growth is a central component of carbon storage often correlated with productivity and that oaks are a major (forest) component, these legacy effects have the potential to contribute to the inter-annual variability of ecosystem-level carbon cycling and long-term carbon storage.

Negative legacy effects were more pronounced in dry sites where the annual precipitation was less than 600 mm compared to mesic sites (TAP > 1000 mm) and when trees experienced repetitive droughts. Although several studies in the past identified the roles of repetitive droughts (e.g., Anderegg et al., 2020; Serra-Maluquer et al., 2018), our study is the first effort that examined the roles of repetitive droughts for determining the legacy effects of extreme droughts. We indeed found significant negative legacy effects of repetitive droughts especially in dry sites and in Mediterranean oaks. Compared to Mediterranean oaks
393 (e.g., *Q. faginea*) other species were not significantly vulnerable to repetitive droughts as
394 showed by our species-species analyses (Fig. 3). It is important to mention that
395 approximately 20% of our identified droughts were repetitive, therefore not all sites and
396 species experienced repetitive droughts (Table SM6).

397 Although the size of negative legacy effects was larger in mesic sites during the 1st
398 post-drought year a faster recovery or even positive legacies were observed in mesic sites
399 compared to dry sites. Overall, the frequencies of positive legacy effects were higher during
400 1-5 years after the extreme droughts; however, the size of the positive legacy effects varied
401 across species, drought seasonality, drought repetitiveness, and site-specific growth
402 sensitivity to climatic variables. The positive legacy effects were mostly observed in the
403 Mediterranean and temperate European oak species after non-repetitive droughts (Fig. 2). In
404 the Mediterranean, deep root systems, thicker leaves, and deciduousness enable deciduous
405 oaks to maintain relatively high pre-dawn water potentials in leaves and high water-use
406 efficiency during droughts (Abrams, 1990; Granda et al., 2018). For example, *Q. pubescens*
407 growing in Southern France under extremely dry conditions showed early leaf senescence to
408 avoid cell damage and maintained water potentials above critical thresholds (Damesin &
409 Rambal, 1995). A similar finding was observed for *Q. petraea* at a dry site where oaks
410 maintained water transport, photosynthesis and growth by using stem water reserves
411 (Neuwirth et al., 2021). The greater frequencies of positive legacy effects of extreme
412 droughts in deciduous oaks might have resulted from a release in competition among trees
413 within a forest stand, where mortality or growth decline of drought-sensitive species may
414 favor the growth of deep-rooted deciduous oak species (Ripullone et al., 2020). In addition,
415 dominant and co-dominant trees were selected for our study and these larger trees could also
416 benefit from the growth decline of the companion-suppressed neighboring trees. In the UK,
417 Cavin et al. (2013) reported that *Q. petraea* increased its growth rate by 20% in the period
418 immediately following a drought, which was due to the release of competition as a result of
419 growth decline and mortality of the competing *F. sylvatica*. The decline in competitiveness of
420 companion species over deciduous oaks during and after droughts has also been identified by
421 other studies in recent years (e.g., Dorado-Liñán *et al.*, 2017; Jourdan *et al.*, 2020; Rubio-
422 Cuadrado *et al.*, 2018; Scharnweber *et al.*, 2011; Weber *et al.*, 2008).

423 Under xeric conditions, deciduous oaks maintained higher osmolyte concentrations
424 and reduced stomatal size compared to evergreens to avoid tissue loss. While studying 12 oak
425 species, Ramírez-Valiente et al. (2020) showed that growth was not reduced even when
426 deciduous oaks lowered hydraulic conductivity and leaf size under drought. This is also in
427 line with findings of the European-wide crown conditions surveys under ICP Forests
428 revealing that temperate and Mediterranean oaks species showed the smallest (or even no
429 significant) increase in mean defoliation from 2002 to 2021 among all inspected tree species
430 (Michel et al., 2022). Ring-porous oak species transport the main share of water in the
431 outermost ring allowing them to recover quickly from drought damage by developing new
432 xylem tissues in the following year after drought (Zweifel & Sterck, 2018). In contrast, in
433 mixed species stands, diffuse-porous species, which use multiple rings for effective water
434 transport, need several years to fully replace damaged vessels (Alla & Camarero, 2012). We
435 also observed compensatory growth responses (i.e., positive legacy effects) after growth
436 decline, which is consistent with previous studies (Anderegg et al., 2015b; Bose et al., 2021b;
437 Huang et al., 2018). The transition from negative to positive legacy effects may occur from
438 drought-induced tree mortality, which relieves surviving individuals from competition (Hajek
439 et al., 2022). This particular positive growth response can, however, also be a physiological
440 reaction of trees, given the higher amount of ray parenchyma tissues (and therefore xylem
441 non-structural carbohydrates) of angiosperms as compared to gymnosperms (Morris et al.,
442

2016), to compensate for losses during periods of stress (Gessler et al., 2020; Trugman et al.,

443

2018).

444

Our analyses have shown that the legacy effects of extreme droughts observed for dry

445

and mesic sites and across regional-species groups depend on the growth-climate sensitivity.

446

This relationship is shown when we relate the legacy effects with growth-climate sensitivity

447

for previous winter temperature and previous summer water balance (Fig. 6). Anderegg et al.

448

(2015b) also observed a direct link between the magnitudes of legacy effects and growth-

449

climate sensitivity in the semi-arid regions of the northern hemisphere. In our study, higher

450

growth sensitivity to winter temperature was positively related to the size of legacy effects,

451

indicating that future warming in winter will favour the growth of trees (even with droughts).

452

In contrast, the correlation coefficient of growth with previous summer CWB was negatively

453

associated with the size of legacy effects, which may indicate that higher precipitation in

454

previous summer can predispose oak trees to more negative growth responses to extreme

455

droughts in current summer. This can be interpreted as a memory effect due to structural

456

overshoot, i.e., the development of high shoot-to-root ratios in response to favourable climate

457

conditions, which could predispose to drought damage (Jump et al., 2017; Zweifel et al.,

458

2020). Comparable structural overshoot due to high early spring water availability and

459

consequently increased drought susceptibility in the summer of the same year has been

460

observed for *F. sylvatica* (Leuschner, 2020) and for *Q. ilex* (Misson et al., 2010).

461

Seasonality of drought occurrence (D'Orangeville et al., 2018; Huang et al., 2018)

462

was also important in determining the size and temporal patterns of legacy effects in

463

deciduous oak species. The drought that occurred in previous winter induced negative legacy

464

effects in mesic sites but zero to positive legacy effects in dry sites. Oak trees growing in

465

relatively mesic sites are probably taller and associated with a faster growth tendency which

466

may require higher water availability during the winter compared to slow-growing, shorter
467 trees growing in dry sites (Harvey et al., 2020). The temperate European oak species group is
468 associated with *Q. robur* and *Q. petraea* and was sensitive to spring droughts which have also
469 been reported by Vanhellemont et al. (2019) and Bose et al. (2021b). Although
470 Mediterranean and North American oaks were not significantly different from temperate
471 European oaks in terms of their responses to spring droughts, the size of negative legacy
472 effects was larger for temperate European oaks indicating their vulnerability to drought
473 damage. In contrast to these species spring droughts increased the growth of Mongolian oaks
474 resulting in positive legacies. This difference may be related to the wetter summers
475 experienced by Mongolian oaks in eastern Asia and related to the monsoon.

476 Despite spatial variations among or even within bioclimatic regions, we found clear
477 differences in drought legacy effects among regional species groups. This indicates that
478 regional climatic conditions (Fig. SM1), species traits, and possible local adaptation of oak
479 populations (cf. Du et al., 2020) could play an important role in determining the type
480 (positive or negative) and magnitude of legacy effects. Among seven species, only *Q.*
481 *petraea*, *Q. faginea* and *Q. macrocarpa* from dry sites did not fully recover from the negative
482 legacy effects after repetitive droughts over the 1-5 years post-drought period. The higher
483 sensitivity of *Q. petraea* to extreme droughts has also been observed in earlier studies (Bose
484 et al., 2021b; Camarero et al., 2021; Cochard et al., 1992).

485 The P50 and SLA were significantly associated with the size of legacy effects,
486 although data was available for only six species. Our analyses showed that oak species
487 associated with higher P50 showed lower legacy effects of extreme droughts. These results
488 may indicate that the repair of damaged organs including the water transport system and leaf
489 area after drought occurred at the expense of radial growth in oaks from more mesic sites.
490 Greater damage to those tree organs might have stagnated the growth of deciduous oaks and
491 resulted in negative legacy effects of extreme droughts as reported by Anderegg et al.
(2015b). We also found that higher N concentration in leaf biomass was positively associated with legacy effects indicating that higher N in leaf biomass can foster the recovery process by enhancing photosynthesis rate and carbon uptake and can even result in positive legacy effects after drought. However, Ouyang et al. (2023) recently reported that higher soil N before drought could predispose *Q. petraea* seedlings to drought-induced mortality as N promotes disproportionally higher above-ground biomass (i.e., structural overshoot) making seedlings vulnerable to hydraulic failure (Gessler et al., 2017). In the present study, it is evident that larger oak trees are benefiting from higher leaf N during the post-drought recovery phases, but future research should investigate long-term relationships between post-drought resilience and nutrient use efficiency by trees (e.g., Wang et al., 2023).

Tree-ring based studies are biased by design toward trees that survived droughts (Nehrbass-Ahles et al., 2014). Stand-level analysis could provide additional insights on whether and how legacy effects vary across trees of different sizes, stand structures, and located in pure or mixed stands (Au et al., 2022; Teets et al., 2018; Weigel et al., 2023) and whether droughts disproportionately lower the growth of smaller trees while benefiting the growth of their larger counterparts or vice versa (Bose et al., 2021a; Forrester, 2019). Long-term monitored soil water information could improve the quantification of drought indices and identification of extreme drought years (Meusburger et al., 2022) and could provide additional insights into whether deciduous oaks are indeed utilizing water from deep soil layers during and after drought events (Ripullone et al., 2020). In addition, how drought-pathogen interactions influence drought impacts and recovery in deciduous oaks needs to be included as they have a considerable potential to modify legacy effects (Haavik et al., 2015).

Considering the increased frequency of extreme droughts in Europe (Spinoni et al., 2018) and elsewhere in the world (Spinoni et al., 2014), our results provide novel insights that repetitive droughts can significantly enlarge negative legacy effects (i.e., significantly
lowering the radial growth than expected) in deciduous oak species which are considered tolerant to extreme drought events. These results have important implications for future ecosystem-scale drought resilience and carbon balance as deciduous oaks are often suggested to mix with drought sensitive species for increasing the ecosystem-level resistance and resilience to droughts (Steckel et al., 2020). However, when drought has not been occurred in successive years, oak trees showed strong recovery potential especially in mesic sites. In addition, not all species of deciduous oaks were affected by repetitive droughts (Fig. 3). Therefore, site conditions and selection of oak species is crucial for mixed-species plantation. For example, *Q. robur* was not affected by repetitive droughts and even showed positive legacy effects after the droughts.

Overall, our results showed that deciduous oak species have a strong recovery potential after drought events but not when drought occurs in successive years. Recovery trajectories over time since drought plausibly vary between dry and mesic sites. The switch from negative to positive legacy effects occurred in a shorter time in mesic sites compared to dry sites and was dependent upon mean annual temperature and climate-growth sensitivity. The legacy effects lasted longer in dry and climatically non-sensitive sites. However, dry sites were not sensitive to winter droughts but to spring or summer droughts. This indicates that in addition to site aridity drought seasonality determines the size and prevalence of legacy effects of extreme droughts in deciduous oak species.

## Acknowledgements

We thank Stefan Klesse and Tanja Sanders for providing valuable comments on an earlier version of the manuscript. AKB thanks Marie Skłodowska-Curie Individual Fellowship (PROJECT ID: 749051-REFOREST) and support from the Swiss Federal Research Institute WSL. JJC thanks funding from the Spanish Ministry of Science and Innovation (PID2021-123675OB-C43 and TED2021-129770B-C21 projects). AG
542 acknowledges support from the Swiss National Science foundation SNF (310030_189109)

543 and inspiration by The Velvet Underground. EM-S was funded by the project RYC2021-

544 035078-I from the Spanish Ministry of Science and Innovation. IDL acknowledges funding

545 from Proyectos de Generación de Conocimiento, Ministerio de Ciencia e Innovación
546 (#PID2021-128759OA-I00), Fondos de Investigación Beatriz Galindo UPM-CAM

547 (#M190020074A) and Proyectos de Consolidación Investigadora 2022 (#CNS2022-135228).

548 TK, EK, MR, IS were supported by the Czech Science Foundation (23-08049S project). JA

549 and JD were supported by the Czech Science Foundation (23-05272S and 23-07533S) and
550 the long-term research development project No. RVO 67985939 of the Czech Academy of

551 Sciences. ASV was funded by the Ministry of Science and Higher Education of the Russian
552 Federation (FSRZ-2020-0014).

553

554 Conflict of Interest

555 None is declared.

556

557 Data availability statement

558 The data that support the findings of this study are available on request from the
559 corresponding author. The data are not publicly available due to privacy or ethical
560 restrictions.

561

562 References

563 Abrams, M. D. (1990). Adaptations and responses to drought in *Quercus* species of North
564 America. *Tree Physiology*, 7, 227-238.

565 Alla, A. Q., & Camarero, J. J. (2012). Contrasting responses of radial growth and wood
566 anatomy to climate in a Mediterranean ring-porous oak: implications for its future
567 persistence or why the variance matters more than the mean. *European Journal of
568 Forest Research*, 131(5), 1537-1550.
569

570

571

Allen, C. D., Breshears, D. D., & McDowell, N. G. (2015). On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene. *Ecosphere*, 6(8), 1-55.

572

573

574

Anderegg, W. R. L., Flint, A., Huang, C.-y., Flint, L., Berry, J. A., Davis, Frank W., Sperry, J. S., & Field, C. B. (2015a). Tree mortality predicted from drought-induced vascular damage. *Nature Geoscience*, 8(5), 367-371.

575

576

577

578

Anderegg, W. R. L., Schwalm, C., Biondi, F., Camarero, J. J., Koch, G., Litvak, M., Ogle, K., Shaw, J. D., Shevliakova, E., Williams, A. P., et al. (2015b). Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models. *Science*, 349(6247), 528-532.

579

580

Anderegg, W. R. L., Trugman, A. T., Badgley, G., Konings, A. G., & Shaw, J. (2020). Divergent forest sensitivity to repeated extreme droughts. *Nature Climate Change*.

581

582

583

Arend, M., Kuster, T., Günthardt-Goerg, M. S., & Dobbertin, M. (2011). Provenance-specific growth responses to drought and air warming in three European oak species (*Quercus robur*, *Q. petraea* and *Q. pubescens*). *Tree Physiology*, 31(3), 287-297.

584

585

586

587

Au, T. F., Maxwell, J. T., Robeson, S. M., Li, J., Siani, S. M. O., Novick, K. A., Dannenberg, M. P., Phillips, R. P., Li, T., Chen, Z., et al. (2022). Younger trees in the upper canopy are more sensitive but also more resilient to drought. *Nature Climate Change*, 12(12), 1168-1174.

588

589

590

591

Batllori, E., Lloret, F., Aakala, T., Anderegg, W. R. L., Aynekulu, E., Bendixsen, D. P., Bentouati, A., Bigler, C., Burk, C. J., Camarero, J. J., et al. (2020). Forest and woodland replacement patterns following drought-related mortality. *Proceedings of the National Academy of Sciences*, 117(47), 29720.

592

593

594

Begueria, S., & Vicente-Serrano, S. M. (2013). SPEI: Calculation of the Standardised Precipitation-Evapotranspiration Index. R package version 1.6. CRAN, Vienna, Austria.

595

596

597

598

Bose, A. K., Gessler, A., Bolte, A., Bottero, A., Buras, A., Cailleret, M., Camarero, J. J., Haeni, M., Hereş, A.-M., Hevia, A., et al. (2020). Growth and resilience responses of Scots pine to extreme droughts across Europe depend on predrought growth conditions. *Global Change Biology*, 26(4521-4537).

599

600

601

602

Bose, A. K., Rigling, A., Gessler, A., Hagedorn, F., Brunner, I., Feichtinger, L., Bigler, C., Egli, S., Etzold, S., Gossner, M. M., et al. (2022). Lessons learned from a long-term irrigation experiment in a dry Scots pine forest: Impacts on traits and functioning. *Ecological Monographs*, 92(2), e1507.

603

604

605

Bose, A. K., Rohner, B., Bottero, A., Ferretti, M., & Forrester, D. I. (2021a). Did the 2018 megadrought change the partitioning of growth between tree sizes and species? A Swiss case-study. *Plant Biology*, 24(7), 1146-1156.

606

607

608

609

Bose, A. K., Scherrer, D., Camarero, J. J., Ziche, D., Babst, F., Bigler, C., Bolte, A., Dorado-Liñán, I., Etzold, S., Fonti, P., et al. (2021b). Climate sensitivity and drought seasonality determine post-drought growth recovery of *Quercus petraea* and *Quercus robur* in Europe. *Science of The Total Environment*, 147222.

610

611

612

Camarero, J. J., Colangelo, M., Gazol, A., & Azorín-Molina, C. (2021). Drought and cold spells trigger dieback of temperate oak and beech forests in northern Spain. *Dendrochronologia*, 66, 125812.

613

614

615

Cavin, L., Mountford, E. P., Peterken, G. F., & Jump, A. S. (2013). Extreme drought alters competitive dominance within and between tree species in a mixed forest stand. *Functional Ecology*, 27(6), 1424-1435.

616

617

Chave, J., Coomes, D., Jansen, S., Lewis, S. L., Swenson, N. G., & Zanne, A. E. (2009). Towards a worldwide wood economics spectrum. *Ecology Letters*, 12(4), 351-366.
618 Cochard, H., Bréda, N., Granier, A., & Aussenac, G. (1992). Vulnerability to air embolism of three European oak species (*Quercus petraea* (Matt) Liebl, *Q. pubescens* Willd, *Q. robur* L). *Annals of Forest Science, 49*(3), 225-233.

619 Colangelo, M., Camarero, J. J., Battipaglia, G., Borghetti, M., De Micco, V., Gentilesca, T., & Ripullone, F. (2017). A multi-proxy assessment of dieback causes in a Mediterranean oak species. *Tree Physiology, 37*(5), 617-631.

620 Cook, E. R., and Kairiukstis, L. A. (1990). Methods of dendrochronology: Applications in the environmental sciences. Springer. ISBN-13: 978-0-7923-0586-6.

621 Corcuera, L., Camarero, J. J., & Gil-Pelegrín, E. (2004). Effects of a severe drought on growth and wood anatomical properties of *Quercus faginea* IAWA Journal, 25(2), 185-204.

622 D'Orangeville, L., Maxwell, J., Kneeshaw, D., Pederson, N., Duchesne, L., Logan, T., Houle, D., Arseneault, D., Beier, C. M., Bishop, D. A., et al. (2018). Drought timing and local climate determine the sensitivity of eastern temperate forests to drought. *Global Change Biology, 24*(6), 2339-2351.

623 Damesin, A. C., & Rambal, S. (1995). Field study of leaf photosynthetic performance by a Mediterranean deciduous oak tree (*Quercus pubescens*) during a severe summer drought. *New Phytologist, 131*(2), 159-167.

624 Dickson, R. E., & Tomlinson, P. T. (1996). Oak growth, development and carbon metabolism in response to water stress. *Annals of Forest Science, 53*(2-3), 181-196.

625 Dorado-Liñán, I., Cañellas, I., Valbuena-Carabaña, M., Gil, L., & Gea-Izquierdo, G. (2017). Coexistence in the Mediterranean-Temperate transitional border: Multi-century dynamics of a mixed old-growth forest under global change. *Dendrochronologia, 44*, 48-57.

626 Druckenbrod, D. L., Martin-Benito, D., Orwig, D. A., Pederson, N., Poulter, B., Renwick, K. M., & Shugart, H. H. (2019). Redefining temperate forest responses to climate and disturbance in the eastern United States: New insights at the mesoscale. *Global Ecology and Biogeography, 28*(5), 557-575.

627 Du, F. K., Wang, T., Wang, Y., Ueno, S., & de Lafontaine, G. (2020). Contrasted patterns of local adaptation to climate change across the range of an evergreen oak, *Quercus aquifolioides*. *Evolutionary Applications, 13*(9), 2377-2391.

628 Esper, J., Benz, M., & Pederson, N. (2012). Influence of wood harvest on tree-ring time-series of *Picea abies* in a temperate forest. *Forest Ecology and Management, 284*, 86-92.

629 Etzold, S., Ziemińska, K., Rohner, B., Bottero, A., Bose, A. K., Ruehr, N. K., Zingg, A., & Rigling, A. (2019). One century of forest monitoring data in Switzerland reveals species- and site-specific trends of climate-induced tree mortality. *Frontiers in Plant Science, 10*(307).

630 Forrester, D. I. (2019). Linking forest growth with stand structure: Tree size inequality, tree growth or resource partitioning and the asymmetry of competition. *Forest Ecology and Management, 447*, 139-157.

631 Galiano, L., Martínez-Vilalta, J., & Lloret, F. (2010). Drought-induced multifactor decline of Scots pine in the Pyrenees and potential vegetation change by the expansion of co-occurring oak species. *Ecosystems, 13*(7), 978-991.

632 Gessler, A., Bottero, A., Marshall, J., & Arend, M. (2020). The way back: recovery of trees from drought and its implication for acclimation. *New Phytologist, 228*(6), 1704-1709.

633 Gessler, A., Schaub, M., & McDowell, N. G. (2017). The role of nutrients in drought-induced tree mortality and recovery. *New Phytologist, 214*(2), 513-520.
667 González de Andrés, E., Rosas, T., Camarero, J. J., & Martínez-Vilalta, J. (2021). The intraspecific variation of functional traits modulates drought resilience of European beech and pubescent oak. *Journal of Ecology*, 109(10), 3652-3669.

670 Granda, E., Alla, A. Q., Laskurain, N. A., Loidi, J., Sánchez-Lorenzo, A., & Camarero, J. J. (2018). Coexisting oak species, including rear-edge populations, buffer climate stress through xylem adjustments. *Tree Physiology*, 38(2), 159-172.

673 Haavik, L. J., Billings, S. A., Guldin, J. M., & Stephen, F. M. (2015). Emergent insects, pathogens and drought shape changing patterns in oak decline in North America and Europe. *Forest Ecology and Management*, 354, 190-205.

676 Hajek, P., Link, R. M., Nock, C. A., Bauhus, J., Gebauer, T., Gessler, A., Kovach, K., Messier, C., Paquette, A., Saurer, M., et al. (2022). Mutually inclusive mechanisms of drought-induced tree mortality. *Global Change Biology*, 28(10), 3365-3378.

679 Hammond, W. M., Williams, A. P., Abatzoglou, J. T., Adams, H. D., Klein, T., López, R., Sáenz-Romero, C., Hartmann, H., Breshears, D. D., & Allen, C. D. (2022). Global field observations of tree die-off reveal hotter-drought fingerprint for Earth's forests. *Nature Communications*, 13(1), 1761.

683 Hartmann, H., Bastos, A., Das, A. J., Esquivel-Muelbert, A., Hammond, W. M., Martínez-Vilalta, J., McDowell, N. G., Powers, J. S., Pugh, T. A. M., Ruthrof, K. X., et al. (2022). Climate change risks to global forest health: Emergence of unexpected events of elevated tree mortality worldwide. *Annual Review of Plant Biology*, 73(1), 673-702.

688 Harvey, J. E., Smiljanić, M., Scharnweber, T., Buras, A., Cedro, A., Cruz-García, R., Drobyshev, I., Janecka, K., Jansons, Ā., Kaczka, R., et al. (2020). Tree growth influenced by warming winter climate and summer moisture availability in northern temperate forests. *Global Change Biology*, 26(4), 2505-2518.

692 Huang, M., Wang, X., Keenan, T. F., & Piao, S. (2018). Drought timing influences the legacy of tree growth recovery. *Global Change Biology*, 24(8), 3546-3559.

694 Jourdan, M., Kunstler, G., & Morin, X. (2020). How neighbourhood interactions control the temporal stability and resilience to drought of trees in mountain forests. *Journal of Ecology*, 108(2), 666-677.

697 Jump, A. S., Ruiz-Benito, P., Greenwood, S., Allen, C. D., Kitzberger, T., Fensham, R., Martínez-Vilalta, J., & Lloret, F. (2017). Structural overshoot of tree growth with climate variability and the global spectrum of drought-induced forest dieback. *Global Change Biology*, 23(9), 3742-3757.

701 Kannenberg, S. A., Maxwell, J. T., Pederson, N., D'Orangeville, L., Ficklin, D. L., & Phillips, R. P. (2019a). Drought legacies are dependent on water table depth, wood anatomy and drought timing across the eastern US. *Ecology Letters*, 22(1), 119-127.

704 Kannenberg, S. A., Novick, K. A., Alexander, M. R., Maxwell, J. T., Moore, D. J. P., Phillips, R. P., & Anderegg, W. R. L. (2019b). Linking drought legacy effects across scales: From leaves to tree rings to ecosystems. *Global Change Biology*, 25(9), 2978-2992.

708 Karger, D. N., Conrad, O., Böhner, J., Kawohl, T., Kreft, H., Soria-Auza, R. W., Zimmermann, N. E., Linder, H. P., & Kessler, M. (2017). Climatologies at high resolution for the earth's land surface areas. *Scientific Data*, 4(1), 170122.

711 Klesse, S., Wohlgemuth, T., Meusburger, K., Vitasse, Y., von Arx, G., Lévesque, M., Neycken, A., Braun, S., Dubach, V., Gessler, A., et al. (2022). Long-term soil water limitation and previous tree vigor drive local variability of drought-induced crown dieback in *Fagus sylvatica*. *Science of The Total Environment*, 851, 157926.

715 Leuschner, C. (2020). Drought response of European beech (*Fagus sylvatica* L.)—A review. *Perspectives in Plant Ecology, Evolution and Systematics*, 47, 125576.
717 Liu, H., Gleason, S. M., Hao, G., Hua, L., He, P., Goldstein, G., & Ye, Q. (2019). Hydraulic traits are coordinated with maximum plant height at the global scale. *Science Advances*, 5(2), eaav1332.

720 Madrigal-González, J., Ruiz-Benito, P., Ratcliffe, S., Rigling, A., Wirth, C., Zimmermann, N. 721 E., Zweifel, R., & Zavala, M. A. (2017). Competition drives oak species distribution 722 and functioning in Europe: Implications under global change. In E. Gil-Pelegrín, J. J. 723 Peguero-Pina & D. Sancho-Knapik (Eds.), *Oaks Physiological Ecology. Exploring the 724 Functional Diversity of Genus Quercus* L. (pp. 513-538). Cham: Springer 725 International Publishing.

726 Marqués, L., Peltier, D. M. P., Camarero, J. J., Zavala, M. A., Madrigal-González, J., 727 Sangüesa-Barreda, G., & Ogle, K. (2022). Disentangling the Legacies of Climate and 728 Management on Tree Growth. *Ecosystems*, 25(1), 215-235.

729 Martínez-Sancho, E., Dorado-Liñán, I., Hacke, U. G., Seidel, H., & Menzel, A. (2017). 730 Contrasting hydraulic architectures of Scots pine and sessile oak at their southernmost 731 distribution limits. *Frontiers in Plant Science*, 8.

732 Mazerolle, M. J. (2011). AICmodavg: model selection and multimodel inference based on 733 (Q) AIC (c). *R package version*, 1(1.15).

734 Meusburger, K., Trotsiuk, V., Schmidt-Walter, P., Baltensweiler, A., Brun, P., Bernhard, F., 735 Gharun, M., Habel, R., Hagedorn, F., Köchli, R., et al. (2022). Soil-plant interactions 736 modulated water availability of Swiss forests during the 2015 and 2018 droughts. 737 *Global Change Biology*, 28(20), 5928-5944.

738 Michel, A., Kirchner, T., Prescher, A.-K., & Schwärzel, K. (2022). Forest condition in 739 Europe: The 2022 Assessment. ICP Forests Technical Report under the UNECE 740 Convention on long-range Transboundary Air Pollution (Air Convention). 741 Eberswalde: Thünen Institute. https://doi.org/10.3220/ICPTR1656330928000.

742 Misson, L., Limousin, J.-M., Rodriguez, R., & Letts, M. G. (2010). Leaf physiological 743 responses to extreme droughts in Mediterranean *Quercus ilex* forest. *Plant, Cell & 744 Environment*, 33(11), 1898-1910.

745 Morris, H., Plavcová, L., Cvecko, P., Fichtler, E., Gillingham, M. A. F., Martínez-Cabrera, H. 746 I., McGlinn, D. J., Wheeler, E., Zheng, J., Ziemińska, K., et al. (2016). A global 747 analysis of parenchyma tissue fractions in secondary xylem of seed plants. *New 748 Phytologist*, 209(4), 1553-1565.

749 Müller, L. M., & Bahn, M. (2022). Drought legacies and ecosystem responses to subsequent 750 drought. *Global Change Biology*, 28(17), 5086-5103.

751 Nehrbass-Ahles, C., Babst, F., Klesse, S., Nötzli, M., Bouriaud, O., Neukom, R., Dobbertin, 752 M., & Frank, D. (2014). The influence of sampling design on tree-ring-based 753 quantification of forest growth. *Global Change Biology*, 20(9), 2867-2885.

754 Neuwirth, B., Rabbel, I., Bendix, J., Bogena, H. R., & Thies, B. (2021). The European Heat 755 Wave 2018: The dendroecological response of oak and spruce in western Germany. 756 *Forests*, 12(3). R

757 Niinemets, Ü., & Valladares, F. (2006). Tolerance to shade, drought, and waterlogging of 758 temperate Northern Hemisphere trees and shrubs. *Ecological Monographs*, 76(4), 759 521-547.

760 Novick, K., Jo, I., D'Orangeville, L., Benson, M., Au, T. F., Barnes, M., Denham, S., Fei, S., 761 Heilman, K., Hwang, T., et al. (2022). The drought response of eastern US oaks in the 762 context of their declining abundance. *BioScience*, 72(4), 333-346.

763 Ogle, K., Barber, J. J., Barron-Gafford, G. A., Bentley, L. P., Young, J. M., Huxman, T. E., 764 Loik, M. E., & Tissue, D. T. (2015). Quantifying ecological memory in plant and 765 ecosystem processes. *Ecology Letters*, 18(3), 221-235.
766

Ouyang, S., Tie, L., Saurer, M., Bose, A. K., Duan, H., Li, M., Xu, X., Shen, W., & Gessler, A. (2023). Divergent role of nutrient availability in determining drought responses of sessile oak and Scots pine seedlings: evidence from ¹³C and ¹⁵N dual labeling. *Tree Physiology*, *tpad105*.

767

768

769

770

771

772

773

Ovenden, T. S., Perks, M. P., Clarke, T.-K., Mencuccini, M., & Jump, A. S. (2021). Life after recovery: Increased resolution of forest resilience assessment sheds new light on post-drought compensatory growth and recovery dynamics. *Journal of Ecology*, *109*(9), 3157-3170.

774

775

Peltier, D. M. P., Fell, M., & Ogle, K. (2016). Legacy effects of drought in the southwestern United States: A multi-species synthesis. *Ecological Monographs*, *86*(3), 312-326.

776

777

Peltier, D. M. P., & Ogle, K. (2019). Legacies of more frequent drought in ponderosa pine across the western United States. *Global Change Biology*, *25*(11), 3803-3816.

778

779

780

781

Peters, R. L., Steppe, K., Pappas, C., Zweifel, R., Babst, F., Dietrich, L., von Arx, G., Poyatos, R., Fonti, M., Fonti, P., et al. (2023). Daytime stomatal regulation in mature temperate trees prioritizes stem rehydration at night. *New Phytologist*, *239*(2), 533-546.

782

783

784

Pinheiro, J., Bates, D., DebRoy, S., & Sarkar, D. (2014). nlme: Linear and nonlinear mixed effects models. R package version 3.1-117. Available at https://cran.rproject.org/web/packages/nlme/index.html.

785

786

Pinheiro, J. C., & Bates, D. M. (2000). Mixed effects models in S and S-PLUS. Springer Verlag.

787

788

789

R Development Core Team. (2022). R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. Available at www.r-project.org.

790

791

792

Ramírez-Valiente, J. A., López, R., Hipp, A. L., & Aranda, I. (2020). Correlated evolution of morphology, gas exchange, growth rates and hydraulics as a response to precipitation and temperature regimes in oaks (*Quercus*). *New Phytologist*, *227*(3), 794-809.

793

794

795

796

Rigling, A., Bigler, C., Eilmann, B., Feldmeyer-Christe, E., Gimmi, U., Ginzler, C., Graf, U., Mayer, P., Vacchiano, G., & Weber, P. (2013). Driving factors of a vegetation shift from Scots pine to pubescent oak in dry Alpine forests. *Global Change Biology*, *19*(1), 229-240.

797

798

799

Ripullone, F., Camarero, J. J., Colangelo, M., & Voltas, J. (2020). Variation in the access to deep soil water pools explains tree-to-tree differences in drought-triggered dieback of Mediterranean oaks. *Tree Physiology*, *40*(5), 591-604.

800

801

802

803

Rubio-Cuadrado, Á., Camarero, J. J., del Río, M., Sánchez-González, M., Ruiz-Peinado, R., Bravo-Oviedo, A., Gil, L., & Montes, F. (2018). Drought modifies tree competitiveness in an oak-beech temperate forest. *Forest Ecology and Management*, *429*, 7-17.

804

805

806

Ruehr, N. K., Grote, R., Mayr, S., & Arneth, A. (2019). Beyond the extreme: recovery of carbon and water relations in woody plants following heat and drought stress. *Tree Physiology*, *39*(8), 1285-1299.

807

808

Salinger, M. J. (2005). Climate variability and change: Past, present and future – An overview. *Climatic Change*, *70*(1), 9-29.

809

810

811

812

Scharnweber, T., Manthey, M., Criegee, C., Bauwe, A., Schröder, C., & Wilmking, M.

(2011). Drought matters – Declining precipitation influences growth of *Fagus sylvatica* L. and *Quercus robur* L. in north-eastern Germany. *Forest Ecology and Management*, *262*(6), 947-961.

813

814

815

Schwalm, C. R., Anderegg, W. R. L., Michalak, A. M., Fisher, J. B., Biondi, F., Koch, G., Litvak, M., Ogle, K., Shaw, J. D., Wolf, A., et al. (2017). Global patterns of drought recovery. *Nature*, *548*(7666), 202-205.
816

Serra-Maluquer, X., Mencuccini, M., & Martínez-Vilalta, J. (2018). Changes in tree

817

resistance, recovery and resilience across three successive extreme droughts in the

818

northeast Iberian Peninsula. *Oecologia*, **187**(1), 343-354.

819

Sperry, J. S., Nichols, K. L., Sullivan, J. E. M., & Eastlack, S. E. (1994). Xylem embolism in

820

ring-porous, diffuse-porous, and coniferous trees of northern Utah and interior

821

Alaska. *Ecology*, **75**(6), 1736-1752.

822

Spinoni, J., Naumann, G., Carrao, H., Barbosa, P., & Vogt, J. (2014). World drought

823

frequency, duration, and severity for 1951–2010. *International Journal of*

824

*Climatology*, **34**(8), 2792-2804.

825

Spinoni, J., Vogt, J. V., Naumann, G., Barbosa, P., & Dosio, A. (2018). Will drought events

826

become more frequent and severe in Europe? *International Journal of Climatology*,

827

38(4), 1718-1736.

828

Steckel, M., del Río, M., Heym, M., Aldea, J., Bielak, K., Brazaitis, G., Černý, J., Coll, L.,

829

Collet, C., Ehbrecht, M., et al. (2020). Species mixing reduces drought susceptibility

830

of Scots pine (*Pinus sylvestris* L.) and oak (*Quercus robur* L., *Quercus petraea*

831

(Matt.) Liebl.) – Site water supply and fertility modify the mixing effect. *Forest*

832

*Ecology and Management*, **461**, 117908.

833

Teets, A., Fraver, S., Weiskittel, A. R., & Hollinger, D. Y. (2018). Quantifying climate–

834

growth relationships at the stand level in a mature mixed-species conifer forest.

835

*Global Change Biology*, **24**(8), 3587-3602.

836

Trugman, A. T., Detto, M., Bartlett, M. K., Medvigy, D., Anderegg, W. R. L., Schwalm, C.,

837

Schaffer, B., & Pacala, S. W. (2018). Tree carbon allocation explains forest drought-

838

kill and recovery patterns. *Ecology Letters*, **21**(10), 1552-1560.

839

Vanhellemont, M., Sousa-Silva, R., Maes, S. L., Van den Bulcke, J., Hertzog, L., De Groote,

840

S. R. E., Van Acker, J., Bonte, D., Martel, A., Lens, L., et al. (2019). Distinct growth

841

responses to drought for oak and beech in temperate mixed forests. *Science of The*

842

*Total Environment*, **650**, 3017-3026.

843

Vilà-Cabrera, A., Martínez-Vilalta, J., Galiano, L., & Retana, J. (2013). Patterns of forest

844

decline and regeneration across Scots pine populations. *Ecosystems*, **16**(2), 323-335.

845

Vilonen, L., Ross, M., & Smith, M. D. (2022). What happens after drought ends: synthesizing

846

terms and definitions. *New Phytologist*, **235**(2), 420-431.

847

Wang, X., Xu, T., Xu, C., Liu, H., Chen, Z., Li, Z., Li, X., & Wu, X. (2023). Enhanced

848

growth resistance but no decline in growth resilience under long-term extreme

849

droughts. *Global Change Biology*, e17038.

850

Weber, P., Bugmann, H., Fonti, P., & Rigling, A. (2008). Using a retrospective dynamic

851

competition index to reconstruct forest succession. *Forest Ecology and Management*,

852

254(1), 96-106.

853

Weigel, R., Bat-Enerel, B., Dulamsuren, C., Muffler, L., Weithmann, G., & Leuschner, C.

854

(2023). Summer drought exposure, stand structure, and soil properties jointly control

855

the growth of European beech along a steep precipitation gradient in northern

856

Germany. *Global Change Biology*, **29**(3), 763-779.

857

Wood, J. D., Knapp, B. O., Muzika, R.-M., Stambaugh, M. C., & Gu, L. (2018). The

858

importance of drought–pathogen interactions in driving oak mortality events in the

859

Ozark Border Region. *Environmental Research Letters*, **13**(1), 015004.

860

Wright, I. J., Reich, P. B., Westoby, M., Ackerly, D. D., Baruch, Z., Bongers, F., Cavender-

861

Bares, J., Chapin, T., Cornelissen, J. H. C., Diemer, M., et al. (2004). The worldwide

862

leaf economics spectrum. *Nature*, **428**(6985), 821-827.

863

Wu, X., Liu, H., Li, X., Ciais, P., Babst, F., Guo, W., Zhang, C., Magliulo, V., Pavelka, M.,

864

Liu, S., et al. (2018). Differentiating drought legacy effects on vegetation growth over

865

the temperate Northern Hemisphere. *Global Change Biology*, **24**(1), 504-516.
866 Zang, C., Pretzsch, H., & Rothe, A. (2012). Size-dependent responses to summer drought in
867 Scots pine, Norway spruce and common oak. *Trees*, **26**(2), 557-569.

868 Zang, C. S., Buras, A., Esquivel-Muelbert, A., Jump, A. S., Rigling, A., & Rammig, A.
869 (2019). Standardized drought indices in ecological research: Why one size does not fit
870 all. *Global Change Biology*, **26**, 322-324.

871 Zuur, A. F., leno, E. N., Walker, N. J., Saveliev, A. A., & Smith, G. M. (2009). Mixed effects
872 models and extensions in ecology with R. Springer-Verlag, New York.

873 Zweifel, R., Etzold, E., Sterck, F., Gessler, A., Anfodillo, T., Mencuccini, M., von Arx, G.,
874 Lazzarin, M., Haeni, M., Feichtinger, L., et al. (2020). Determinants of legacy effects
875 in pine trees - implications from an irrigation-stop experiment. *New Phytologist*,
876 227(4), 1081-1096.

877 Zweifel, R., & Sterck, F. (2018). A Conceptual tree model explaining legacy effects on stem
growth. *Frontiers in Forests and Global Change, 1*, 9.

879
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Location of oak sites from where tree-ring width data was collected. Shaded areas represent the distribution of deciduous oak species across the Northern Hemisphere.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

**Figure 2.** Mean legacy effects of droughts for 1 to 5 years (x axes) under the presence and absence of successive drought events. (A) Legacy effects across dry (annual precipitation <600 mm), moderate (600-1000 mm), and mesic sites (>1000 mm), (B) Legacy effects observed across sensitive (tree growth significantly correlated with the climatic water balance) and non-sensitive sites, and (C) Legacy effects across four species groups based on regions of distribution. The red dashed line indicates no difference between observed and predicted growth and shaded areas in all panels represent the mean ± standard error.
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Mean legacy effects of repetitive and non-repetitive droughts for different deciduous oak species. The analyses were performed separately for each species with at least 20 observations (i.e., number of sites). All sites of *Q. alba*, *Q. rubra*, and *Q. mongolica* were from mesic regions and therefore the effect of site type was not considered in the analysis for those three species. Legacy effects were quantified as the difference between observed and predicted growth (unitless index) after extreme climatic drought. The red dashed line indicates no difference between observed and predicted growth, and shaded areas in all panels represent the mean ± standard error.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

**Figure 4.** Relationship between integrated legacy effects (integrated of legacies from 1 to 5 years after drought) and species-specific traits. In plot (A), the P50 units are MPa and the P50 data was available for *Q. alba*, *Q. frainetto*, *Q. petraea*, *Q. robur*, *Q. rubra*, and *Q. stellata*. The data for specific leaf area and N concentration were available for *Q. alba*, *Q. faginea*, *Q. petraea*, *Q. robur*, *Q. stellata*, and *Q. velutina*. Shaded areas in all panels represent the mean ± standard error.
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Mean drought legacies during 1-5-year following extreme droughts as (A) a function of site type and growth-climate sensitivity to winter temperature and as (B) a function of regional species group and growth-climate sensitivity to previous summer climatic water balance (CWB). RWI is the ring-width index.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 6.** Mean legacy effects of seasonal droughts across (A) site type and (B) regional species groups. Three seasons were considered including previous winter (previous year December – current February), current spring (March-May) and current summer (June-August). Legacy effects were quantified as the difference between observed and predicted growth (unitless index) after extreme climatic drought. Red dashed line indicates no-difference between observed and predicted growth (RWI, ring-width index) and shaded areas in all panels represent the mean ± standard error.</figcaption></figure>