Latitudinal clines in bud flush phenology reflect genetic variation in chilling requirements in balsam poplar, Populus balsamifera.

We demonstrate that increased chilling lessens GDD to bud flush in a genotype-specific manner. Our results emphasize that latitudinal clines in bud flush reflect a critical genotype-by-environment interaction, whereby differences in bud flush between southern vs. northern genotypes depend on chilling. Our results suggest selection has shaped chilling requirements and depth of rest as an adaptive strategy to avoid precocious flush in climates with midwinter warming.