A hidden mechanism of forest loss under climate change: The role of drought in eliminating forest regeneration at the edge of its distribution

The ongoing global warming and associated drying are shaping the fate of forests worldwide. While processes of tree mortality are visible and studied, a decrease in forest regeneration is mostly overlooked, although equally deleterious. Populations at the edge of tree species distribution areas are at higher risk and are hence hotspots for species extinctions. Here we use a semi-arid pine forest growing at the timberline edge of forest existence as a model for forest survival under warming and drying conditions. Seedling recruitment, including seed germination, seedling survivorship, and multiyear seedling growth, were measured along six consecutive years. To pinpoint the role of drought, we designed a field experiment, manipulating stand density at three levels and grazing regimes. Seed germination was high across all studied plots, but seedling survivorship and multiyear seedling growth were near-zero. Stand density and grazing exclusion positively affected germination. Seedling survivorship was higher in wetter years. Multiyear seedling growth was stunted by grazing, and seedling height was distributed differently across different stand densities. Our data indicate that seedling survivorship during the first dry season acts as a bottleneck for forest existence at the dry and hot edge of current forest distribution. We also quantified the roles of other stressors such as shading, and highlighted the eliminating role of grazing on multiyear seedling growth. Forest regeneration should be more closely monitored in sensitive populations, as climate change-driven forest loss can happen even without mature tree mortality.