Evaluating the combined effects of climate and land-use change on tree species distributions

A large proportion of the world's biodiversity is reportedly threatened by habitat loss and climate change. However, there are few studies that investigate the interaction between these two threats using empirical data. Here, we investigate interactions between climate change and land use change in the future distribution of 23 dominant tree species in mainland Spain. We simulated changes up to year 2100 using a climate-dependent Stochastic Patch Occupancy Model, parameterized with colonization and extinction events recorded in 46 569 survey plots. We estimated that the distribution of 17 out of 23 tree species are expanding, and hence not at equilibrium with the climate. However, climate change will make the future occupancy of 15 species lower than expected if climate, and habitat, remained stable (baseline scenario). Climate change, when combined with 20% habitat loss, was estimated to reduce species occupancy by an average of 23% if habitat loss is spatially clumped, relative to baseline projections, and by 35% if scattered. If habitat loss occurred in areas already impacted by human activities, species occupancy would be reduced by 26%. Land-use changes leading to habitat gain (i.e. creation through e.g. reforestation), could slightly mitigate the effects of climate change; but a 20% increment in habitat would reduce climate-change-driven losses in species occupancy by only ~3%. Synthesis and applications. The distributions of the most common tree species in mainland Spain are expanding, but climate change threatens to reduce this expansion by ~18% for 15 of the 23 studied species. Moreover, if the habitat of each of these species is simultaneously lost, the occupancies of all species will be reduced further, with variation depending on the spatial pattern of the lost habitats. However, we did not detect synergies between climate change and habitat loss. The combined effect (with 20% habitat loss) was 513% less than what would be expected if the effects were additive. Importantly, reforestation could partially offset the negative effects of climate change, but complete mitigation would require an increase in forested land of ~80%, and the prioritization of territories that are less impacted by human activities.