The interplay between demography and neutral evolution at the expansion front of a widespread conifer, Picea sitchensis

Abstract

Tree species in the northern hemisphere have advanced and retreated with interglacial and glacial periods, and are currently subject to rapid anthropogenic climate change. These observations prompt questions about the mechanisms allowing tree populations to respond quickly to selection pressures when establishing into new areas. Focusing on the northern expanding range edge of Picea sitchensis, a widespread conifer of western North America, we ask how genetic structure and diversity develop during colonization, and assess the role of demographic history in shaping the evolutionary trajectory of a colonizing population. By combining nearly 500 years of tree-ring and genetic data at the expansion front on the Kodiak Archipelago, we show that allelic richness - but not expected heterozygosity - increased rapidly during early stages of establishment in the 1600s, while genetic differentiation from populations further from the front decreased. This trend ended in the 1700s, after an increase in population growth rate. These findings highlight the major role of long-distance pollen dispersal in the recovery of genetic diversity during initial stages of colonization, and suggest that demographic dynamics including an initial lag in population growth are likely limiting factors in the adaptation of tree populations tracking their niche in a changing climate.