Predicting vegetative bud break in two arctic deciduous shrub species, Salix pulchra and Betula nana.

The factors controlling bud break in two arctic deciduous shrub species, Salix pulchra and Betula nana, were investigated using field observations and growth-chamber studies. A bud-break model was calibrated using a subset of the experimental observations and was used to predict bud break under current and potential future climate regimes. The two species responded similarly in terms of bud break timing and response to air temperature in both field and controlled environments. In the field, the timing of bud break was strongly influenced by air temperatures once snowmelt had occurred. Growth chamber studies showed that a period of chilling is required before buds break in response to warming. Model simulations indicate that under current conditions, the chilling requirement is easily met during winter and that even with substantial winter warming, chilling will be sufficient. In contrast, warm spring temperatures determine the timing of bud break. This limitation by spring temperatures means that in a warmer climate bud break will occur earlier than under current temperature regimes. Such changes in bud break timing of the deciduous shrubs will likely have important consequences for the relative abundance of shrubs in future communities and consequently ecosystem processes.