The intraspecific variation of functional traits modulates drought resilience of European beech and pubescent oak

A higher frequency and intensity of droughts will impair forest productivity. Therefore, improving our understanding of which factors enhance tree growth resilience against drought has become a crucial issue, but we lack information at the intraspecific level. In this study, we investigate the role played by climatic conditions and tree characteristics in the growth response to severe droughts in two deciduous Fagaceae species near their southern distribution limit: the temperate European beech Fagus sylvatica and the Mediterranean pubescent oak Quercus pubescens. The study area is located in Catalonia, NE Spain, where 149 trees were cored covering the water availability gradient of the species in the region. In addition, tree size (diameter and height) and an ensemble of stem, leaf and hydraulic traits were collected for each tree. Growth responses to extreme droughts during the period 1980-2015 were assessed using resilience indicators based on residual basal area increment series predicted by a model including growing season water balance. Although resistance was unrelated to drought intensity, higher water availability improved recovery and reduced drought legacy effects. Taller trees showed higher resilience against drought, which could be explained by the relationship between the variation in tree height and functional traits, such as leaf nitrogen concentration, leaf turgor loss point and resistance to xylem embolism. For both species, trees with lower cavitation resistance, higher leaf nitrogen concentration and tighter stomatal regulation (suggested by less negative turgor loss point and carbon isotope composition) displayed better performance during and after severe droughts. Both species showed a progressive decrease in resilience and increase in legacy effects. Synthesis. This study highlights the relevance of analysing intraspecific variation of functional traits involved in tree water and carbon balance to improve our understanding of the resilience and variability of growth responses to drought within species.