Process-based modeling of tree species range shifts under climate change

Climate change has already caused range shifts in some tree species, and climatic predictions suggest that these shifts could increase as well as impact more species in the future. Niche-based models have been widely used to forecast species distributions under climate change, but uncertainties remain about their validation and predictions. Niche models also do not provide a clear framework to study the processes influencing species range. We use PHENOFIT, an alternative, process-based model to simulate the current distributions of 17 North American boreal and temperate tree species, and to assess potential range shifts under two scenarios of climate change taking into account colonization of suitable habitats. Using input of climate and soil properties, the model reproduced present distributions accurately and suggested that climatic constraints limit species range mainly through their impact on phenology, and secondarily through impacts on drought and frost damage of leaves and flowers. Regarding the 21^st century, our simulations show that local extinctions in the south of the species range and colonisations of new habitats north of the present distributions should occur, although the extent of the colonized areas strongly depends on the dispersal ability of the species. Our predictions also show that process-based models may lead to much lower extinction rates than niche-based models suggest. These findings provide new insights into the ecological processes shaping tree species distributions and represent an important step towards obtaining an accurate picture of the potential impact of climate on species ranges.