Site Adaptation Prevails Over Species Determinism at Mediterranean Rear-edge Forests

Climate change may reduce forest growth and increase forest die-back events, particularly at the trailing edge, which may entail high carbon costs and alteration of the net ecosystem exchange. Yet, the spatiotemporal patterns of vulnerability across tree species' limits of tolerance to dryness are quite uncertain, although studying the response of multispecies networks is fundamental for defining ecologically relevant upper limits of forest resistance to drought, and hence to predict potentially forthcoming tipping points. We investigated here to what extent the impact of environmental changes on radial growth during the last 60 years determines future vulnerability to climate change at the regional scale for three boreal (Scots pine, silver fir, Norway spruce) and two temperate (European beech, sessile oak) tree species at their southernmost distribution limits in the Mediterranean Basin using a tree-ring network of 42 chronologies. Our analysis revealed that the estimated species-specific vulnerability did not match their a priori rank in drought-tolerance: Scots pine and beech seemed to be the most vulnerable species to forthcoming environmental change despite their contrasting physiologies. Furthermore, the response to climate observed across geographical gradients reflected the phenotypical plasticity and ecophysiological strategy of locally adapted populations, rather than the species-specific biological determinism. Additionally, regional differences in forests vulnerability to climate change across the Mediterranean Basin were linked to summer atmospheric circulation patterns, which are not correctly represented in global climate models.

According to our results, forest modelling should take into consideration the intra-species ecophysiological differences across the distribution range and likely to move towards a classification based on site-induced adaptations instead of using biological-determinism principles. In addition, regional climatology is a determinant component shaping future tree species distribution and forest growth, therefore an accurate representation of the synoptic climatology in essential in forthcoming modelling exercises.