Temperature-based population segregation in birch

Mean temperature of establishment years for warm‐ and cold‐year subpopulations of a naturally occurring stand of Betula pendula (birch) shows a difference equivalent to that between current temperatures and temperatures projected for 35–55 years hence, given ‘business as usual.’ The existence of ‘pre‐adapted’ individuals in standing tree populations would reduce temperature‐based advantages for invading species and, if general, bring into question assumptions currently used in models of global climate change. Our results demonstrate a methodology useful for investigating the important ecological issue of adaptation vs. range shifts as a means of response to climate change.