Climate change and species range margins: predicting forest resilience in the Mediterranean region

Shifts in the geographical distributions of species are one of the major ecological impacts of global climate change. Current warming is associated with the poleward and uphill range expansions that have been observed across large numbers of species. These 'leading edge' range expansions support the hypothesis that range margins represent the limits of species' environmental tolerances, and this hypothesis underlies the relatively simple climate envelope models widely used to predict biodiversity impacts of climate change. However, this hypothesis equally predicts that range retraction should occur at the equatorial 'trailing edge' margin, as populations are pushed beyond the limits of their tolerance and yet there is much less evidence for such retractions occurring. The dynamics of trailing edge populations are not well understood, exposing a major gap in our understanding of climate change biogeography.

Beech-dominated forest at the species range edge in Vall d'Aran, Catalonia, NE Spain.

Several explanations for this disparity between theory and evidence have been proposed. There may be long time lags for climate driven extinction of trailing edge populations, masking an extinction debt from climate change. Alternatively, trailing edge populations may be locally adapted to their environment and therefore equally able to tolerate warming as populations in the range core. Alternatively, biotic interactions may be a stronger determinant of equatorial range margins than direct climatic limitation. Clearly, developing a better understanding of the dynamics of trailing edge populations will be crucial for improving predictions of the biodiversity impacts of climate change.

This project will substantially advance our understanding of how climate influences the performance of individuals in trailing edge populations and how this translates into population trends under climate change. To do this, we will build on an ongoing study at the range edge of European beech (Fagus sylvatica) in Catalonia, NE Spain. Forty study populations have been selected to span gradients of geographic marginality (isolated or connected to other populations) and ecological marginality (growing in benign or extreme climatic and ecological conditions). Using a combination of existing long-term data and new data collected through field work in Mediterranean mountain forests, you will perform advanced statistical analysis and construct population models tailored for individual populations. This will allow us to project future population trends and test whether geographical and ecological marginality increase or decrease resilience to climate change.



The boundary between temperate beech forest and Mediterranean holm oak forest in the Montseny Mountains, Catalonia.