Tundra conservation challenged by forest expansion in a complex mountainous treeline ecotone as revealed by spatially explicit tree aboveground biomass modeling

The subarctic forest tundra transition zone is one of the most vulnerable ecological regions worldwide and susceptible to climate change. Forest changes could lead to biodiversity losses when tundra areas become colonized. However, the impact of complex landscapes with barriers and channels for seed dispersal is highly understudied. Hence, we investigated potential tree aboveground biomass (AGB) change in mountainous central Chukotka (Siberia) with the individual-based spatially explicit vegetation model Larix vegetation simulator (LAVESI). In a climate sensitivity study, we simulate forest dynamics until 3000 CE for Representative Concentration Pathways (RCPs) with and without hypothetical cooling after 2300 CE to twentieth-century levels. The current state and spatiotemporal dynamics of tree AGB are validated against field and satellite-derived data. Our results suggest densification of existing tree stands and a lagged forest expansion depending on the distance to the current tree line (~39 percent of the total study area, RCP 8.5) under all considered climate scenarios. In scenarios with cooling after 2300 CE, forests stopped expanding and then gradually retreated to their pre-twenty-first-century position (~10 percent, RCP 8.5). However, forest remnants remain in the colonized area, leaving an imprint of forests in former tundra areas, which will likely have an adverse impact on tundra biodiversity.