Climate change impacts on the distribution and diversity of major tree species in the temperate forests of Northern Iran

This study aimed to assess the impacts of climate change on the distribution of major tree species in the temperate forests of Northern Iran (also known as Hyrcanian forests). We analyzed the current distributions of the eleven major tree species using an ensemble approach involving five different species distribution models (generalized linear model, generalized additive model, generalized boosting model, classification tree analysis, and random forests) and generated the ensemble maps of the current and future distribution of each species. For the future, we used five general circulation models and two representative concentration pathways (RCPs). Finally, we mapped beta-diversity and changes in alpha-diversity of the tree species under climate change. Our analyses showed that generally, the climatically suitable habitats for most of the species tend to shift and shrink in the future. A shift in major trees boundaries will be expected both along an east-west gradient and along an altitudinal gradient under climate change scenarios. The results demonstrated that climate change is likely to exert a strong influence on beta-diversity and richness of the major tree species in northern forests of Iran. In total, beta-diversity of tree species will be higher in the central and eastern parts compared with the western areas under the climate change scenarios and the mid-elevations of the western Hyrcanian forests will likely experience the lowest beta-diversity through time, meaning that the impacts of climate change on these regions are minimal. The impacts of climate change on the distribution of major tree species in the Hyrcanian forests can be considered very severe and pose a high risk of loss in forest functions and services. Forest managers will encounter several uncertainties in the wood productions, plantation, restoration, and conservation plans due to climate change in the Hyrcanian forests.