Climate change velocity drives rapid evolution of foliar phenology in trailing edge populations

Here we tested the overarching hypothesis that climate change velocity drives rapid evolution in bud break phenology. With field and common garden studies, we used age cohorts within 17 populations of a foundation riparian tree species distributed across multiple strong environmental gradients in the western US. We provide evidence of contemporary evolution, as young trees in trailing-edge populations have evolved to break bud approximately six days earlier than old trees in these same populations. These populations experience greater water stress than populations at the core of the species’ distribution, and the magnitude of genetic divergence in bud-break phenology is related to the velocity of change in climate water deficit over the last 100 years. This relationship to climate change velocity did not exist in core populations, suggesting that, despite similar rates of change, a threshold of water deficit stress has yet to be surpassed in those regions. Overall, the interactive effects of old trees and trailing edge populations can provide useful insight into centuries of environmental history and each independently represent important benchmarks for understanding the context for contemporary environmental change.