Climate change impacts on long-term forest productivity might be driven by species turnover rather than by changes in tree growth

Aim Climate change impacts forest functioning and services through two inter-related effects. First, it impacts tree growth, with effects, for example, on biomass production. Second, climate change also reshuffles community composition, with further effects on forest functioning. However, the relative importance of these two effects has rarely been studied. Here, we developed a new modelling approach to investigate these relative importances for forest productivity. Location Eleven forest sites in central Europe. Time period Historical (1990) and end-of-21(st)-century climate-like conditions. We simulated 2,000 years of forest dynamics for each set of conditions. Major taxa studied Twenty-five common tree species in European temperate forests. Methods We coupled species distribution models and a forest succession model, working at complementary spatial and temporal scales, to simulate the climatic filtering that shapes potential tree species pools, the biotic filtering that shapes realized communities and the functioning of these realized communities in the long-term. Results Under an average temperature increase (relative to 1901-1990) of between 1.5 and 1.7 degrees C, changes in simulated forest productivity were caused mostly by changes in the growth of persisting tree species. With an average temperature increase of 3.6-4.0 degrees C, changes in simulated productivity at sites that currently have a mild climate were again caused predominantly by changes in tree species growth. However, at the warmest and coldest sites, changes in productivity were related mostly to shifts in species composition. In general, at the coldest sites, forest productivity is likely to be enhanced by climate change, whereas at the warmest sites the productivity might increase or decrease depending on the future precipitation regime. Main conclusions A combination of two complementary modelling approaches that address questions at the interface between biogeography, community ecology and ecosystem functioning, reveals that climate change-driven community reshuffling in the long term might be crucially important for ecosystem functioning.