Adaptation of white spruce to climatic risk environments in spring: Implications for assisted migration

Abstract

The timing of spring dormancy release and bud break in trees is an adaptive trait with potentially important management implications. Depending on how spring phenology is controlled, climate warming may disrupt the synchronization of bud break with the available growing season. Further, addressing climate change through human assisted migration in reforestation programs could cause additional problems if phenology triggers of source and target locations differ. Here, we assess how phenology is controlled by heat sum and chilling requirements for a widespread and commercially important boreal tree species, white spruce (Picea glauca) in a range-wide common garden experiment. We find significant genetic population differentiation in heat sum requirements ranging from 390 to 450 degree days among regions (±12 average SE), and from 375 to 500 (±16) degree days among populations within regions. The most northern populations showed the lowest heat sum requirements, which could be explained by frost avoidance or growth optimization strategies. Our data favors a frost avoidance hypothesis, although the explanations are not mutually exclusive. Chilling requirements in white spruce were also found (approx. 15 degree days), but they showed no geographic population differentiation. Due to generally low chilling requirements, we do not anticipate de-synchronization of spring phenology with the growing season under climate warming in spruce. Synthesizing results from this study and other research on additional growth and adaptive traits, we conclude that assisted migration northward, compensating for approximately 2° C warming, is safe and yields higher growth rates for white spruce.