Patterns of Adaptation to Drought in Quercus robur Populations in Central European Temperate Forests

In order to predict the future of European forests, it is crucial to assess the potential of the dominant perennial species to adapt to rapid climate change. The aim of this study was to reconstruct the pattern of distribution of drought tolerance in Quercus robur in the current center of the species' range. The distribution and plasticity of drought-related traits in German populations of Q. robur were assessed and the effects of spring phenology and species demographic history on this distribution were evaluated using a drought stress experiment in a common garden. We show that variation of drought-related functional traits, including intrinsic water use efficiency (iWUE), leaf osmotic potential (π), and rate of drought-induced defoliation, is high within Q. robur populations. However, frequency of trees with high estimated constitutive drought tolerance increases with decreasing water availability in the regions of population origin, indicating local adaptation to drought. A strong correlation between the distribution of drought-related traits and spring phenology observed in Q. robur suggests that adaptation to water deficit interacts with adaptation to the strong seasonality of the central European climate. The two processes are not influenced by the history of post-glacial recolonisation of central Europe. The results of this study provide a basis for optimistic prognoses for the future of this species in the center of its current distribution range.