Climate change is disrupting population performance and the distribution of tree species across the globe. Biogeographical theory suggests that rising global temperatures should drive species to move poleward and upward in elevation as they track the climates to which they are adapted. Consequently, one fundamental climate change prediction is that population loss and regional extinctions of tree species should occur in the most drought prone areas of their ranges (i.e. the rear range-edge). This prediction assumes that rear-edge populations are marginal with higher risk of extinction than range-core populations because they occur at reduced population sizes along less favorable habitats. This assumption is well supported in the literature documenting altered tree growth and mortality in response to increased drought stress. However, evidence of rear-edge populations persisting in the most drought-prone habitats is increasingly accumulating. For example, relict populations are one of the most impressive cases of persistence as they persist geographically isolated in climates significantly hotter and dryer than that tolerated by the species. The disparity of documented responses is potentially a result of complex ecological and evolutionary factors that determine population dynamics, and points out the need for a proper understanding and assessment of marginality and the identification of the key mechanisms implied in rear-edge population persistence.
Forests provide essential ecosystem functions and services, including regulation of nutrient and water cycling, atmospheric composition and climate, as well as the maintenance of biodiversity and human well-being. However, climate change is expected to alter forest functioning, with profound consequences that range from genes to ecosystems. Rear-edge populations occur along the transition between biomes and contribute to the impressive diversity of ecosystems and species found in these regions. Furthermore, rear-edge populations hold a singularity that makes them very relevant for the maintenance of ecosystem services and biodiversity, as well as very attractive for the uses and profits by human societies. In this context, the progress that this project provides is essential to refine and advance our ability to predict, monitor and manage the impacts of climate change on the function and fate of rear range-edge populations.
Here we aim to propose a framework for research design and analysis at species' rear range-edges, and we then aim to demonstrate how the framework improves our understanding of marginality and rear-edge population dynamics. In order to put into practise the framework we have established our study system at the rear range-edge of the European beech tree (Fagus sylvatica L.), a species with a high ecological importance yet highly drought sensitive. The species occurs along the transition between the Mediterranean and temperate biomes, where beech populations are distributed across a fragmented landscape and along highly heterogeneous ecological conditions. In this region, relict beech populations persist out of the species' physiological tolerances. The main objectives of this project are:
1) To propose a conceptual and methodological framework for a better understanding and assessment of marginality in order to refine predictions of rear-edge population decline.
2) To test if populations inhabiting the most drought-prone habitats show higher levels of mortality and canopy decline, and altered tree growth and limited physiological performance to increased drought stress.
3) To monitor and characterize the microclimate of rear-edge populations, and to test if microclimates contribute to the local persistence of populations under unfavourable regional climates.
4) To study the amount genetic variation of rear-edge populations across ecological and habitat fragmentation gradients, and to test if geographical isolation is associated to the loss of genetic diversity.
5) To model tree growth dynamics and predict population loss under different scenarios of climate change.
