Photoperiod constraints on tree phenology, performance and migration in a warming world

Increasing temperatures should facilitate the poleward movement of species distributions through a variety of processes, including increasing the growing season length. However, in temperate and boreal latitudes, temperature is not the only cue used by trees to determine seasonality, as changes in photoperiod provide a more consistent, reliable annual signal of seasonality than temperature. Here, we discuss how day length may limit the ability of tree species to respond to climate warming in situ, focusing on the implications of photoperiodic sensing for extending the growing season and affecting plant phenology and growth, as well as the potential role of photoperiod in controlling carbon uptake and water fluxes in forests. We also review whether there are patterns across plant functional types (based on successional strategy, xylem anatomy and leaf morphology) in their sensitivity to photoperiod that we can use to predict which species or groups might be more successful in migrating as the climate warms, or may be more successfully used for forestry and agriculture through assisted migration schemes. Predictions of how temperate and boreal forests will respond to climate warming often rely on the assumption that temperature will extend the growing season and drive migration to higher latitudes. But in many cases, these responses are also affected by photoperiod, and day length cues may constrain the ability of high latitude forests to respond to climate change as currently predicted. Here we review how photoperiod signals interact with temperature to affect tree phenology, physiological processes, and the potential for species to migrate in a warmer climate.