Projecting the impact of a collapsing Atlantic Meridional Overturning Circulation on European tree-species distributions 

Recently, several studies provided model-based insights on how climate change may alter European tree-species distributions. Yet, none of those have considered the implications of a collapse of the Atlantic Meriodional Overturning Circulation (AMOC), even though recent research indicates an already declining AMOC and a significant chance of a complete shutdown within the 21st century. Since an AMOC collapse results in cooler and drier climatic conditions across Europe that differ significantly from current climate model projections we need to increase our understanding of the impacts of an AMOC collapse on European forests.Under this framework, we projected future tree-species distributions across Europe for various CMIP6 scenarios, emphasizing on differences between scenarios with an active vs. an inactive AMOC. In particular, we trained climate envelope models for the 24 most abundant European tree species and performed model simulations with quantile-mapped climate projections at a very high spatial resolution of 1 km². To quantify the effects of an AMOC collapse, we compared projections based on regular CMIP6 scenarios (SSPs 1-2.6, 2-4.5, 5-8.5, 10 different models) with such resembling an AMOC collapse. In our statistical evaluation, we emphasized on relative changes in abundance probability, redistribution of species-specific core areas, and the diversity of forest’s tree-species portfolios.Our results show a stark contrast between the scenarios that account for an AMOC collapse and the control scenarios across all considered SSPs. Specifically, we observed an increase in abundance probabilities of selected tree species in Central Europe and the Mediterranean, while abundance probabilities in Scandinavia decreased substantially, indicating local extinction of the dominant tree species Norway spruce and Scots pine. Taken together, our study highlights a diverse picture of an AMOC collapse with catastrophic impacts on Europe’s boreal forests.