Summer drought and ENSO-related cloudiness distinctly drive Fagus sylvatica growth near the species rear-edge in northern Spain

The ample distribution of common beech (Fagus sylvatica) across Europe implies that this key tree species occurs under a broad variety of climatic conditions despite its sensitivity to drought stress. Iberian beech rear-edge (southernmost) forests are located along the boundary between the Eurosiberian and Mediterranean biogeographical regions. Therefore, those forests are considered to be sensitive monitors of the effects of warming-induced drought stress on marginal tree populations. We evaluate if the radial growth of Iberian beech populations is mainly constrained by drought. Since previous findings indicated that El Niño-Southern Oscillation (ENSO) teleconnections may influence the rainfall regime in northern Spain, we also assessed if beech response to drought and water availability is modulated by this large-scale climatic pattern. We compared the recent growth patterns and responses to climate across a network of 30 tree-ring site chronologies established throughout northern Spain where beech forests are subjected to contrasting climatic conditions. Iberian beech populations located near or within the Mediterranean biogeographical region were the most sensitive to June water deficit. However, the dependency of beech growth on drought stress near the rear-edge of the species was mitigated where cloudy conditions prevail in summer, namely in mesic stands located in the Eurosiberian region. Drought stress in the latter populations was alleviated by cloudiness, which in turn depended on ENSO, and this effect on growth has been intensifying for the last decades. We prove that the sensitivity of rear-edge populations to drought, in terms of growth reduction, is greatly modulated by local or regional environmental gradients, but also by the influence of large-scale climatic variation.