The Forest After Tomorrow: Projecting the Impact of a Collapsing Atlantic Meridional Overturning Circulation on European Tree-Species Distributions.

Forest tree species are expected to experience a substantial redistribution due to climate change. While previous work has emphasized the effects of a warmer and drier climate on European tree-species distributions, to date no study has investigated the potential impact of a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Here, we deploy climate-envelope models to quantile mapped, high-resolution (1km) CMIP6 climate projections and compare tree-species distributions under an active AMOC vs. an inactive AMOC scenario. Across Europe, our tree-species projections indicate contrasting impacts of the two scenarios. In Scandinavia, many of the currently abundant tree species were projected a dramatic decline and partial disappearance due to the strong cooling under an inactive AMOC. In Central and Southern Europe, however, some of the currently abundant species suffered less under an inactive AMOC compared to an active AMOC scenario while others-such as the economically important species of Norway spruce-almost went extinct. As opposed to the classic climate-change scenario supporting Mediterranean species in Central Europe, projected European tree-species portfolios consisted of a higher share of boreal, cold-tolerant species in the inactive AMOC scenario. Finally, tree-species diversity was projected to decline even stronger under an inactive vs. an active AMOC scenario. Altogether, while an AMOC collapse may locally result in more favorable conditions for specific species in comparison to a classic climate-change scenario, the dramatic economic and ecological consequences suggested by our projections indicate the urgent need for climate-change mitigation to lower the likelihood of an AMOC collapse.