Demographic history shapes forest tree vulnerability to climate change

Demographic history is expected to play a central role in shaping population vulnerability to climate change through its lasting effects on effective population sizes and genetic connectivity. However, existing studies report contrasting outcomes, and the consequences of alternative demographic histories have seldom been assessed concurrently across multiple taxa. Here, we analysed population genomic data from six of the major European forest tree species to address this gap. We found that, across all species, more genetically isolated populations showed reduced adaptive potential, as measured by standing levels of genetic diversity. Furthermore, populations with reduced historical gene flow and higher genetic differentiation exhibited higher genetic load and suboptimal climate adaptation, particularly in small, fragmented populations, potentially increasing their sensitivity to climate change. Finally, the beneficial effects of gene flow were evidenced by the absence of greater suboptimal climate adaptation in highly connected populations across the six species. Altogether, our results provide valuable insights into how genetic differentiation, reflecting the combined effects of genetic drift and limited historical gene flow, influences current vulnerability of forest tree populations to climate change.