Spatial variance of spring phenology in temperate deciduous forests is constrained by background climatic conditions

Leaf unfolding in temperate forests is driven by spring temperature, but little is known about the spatial variance of that temperature dependency. Here we use in situ leaf unfolding observations for eight deciduous tree species to show that the two factors that control chilling (number of cold days) and heat requirement (growing degree days at leaf unfolding, GDD req ) only explain 30% of the spatial variance of leaf unfolding. Radiation and aridity differences among sites together explain 10% of the spatial variance of leaf unfolding date, and 40% of the variation in GDD req . Radiation intensity is positively correlated with GDD req and aridity is negatively correlated with GDD req spatial variance. These results suggest that leaf unfolding of temperate deciduous trees is adapted to local mean climate, including water and light availability, through altered sensitivity to spring temperature. Such adaptation of heat requirement to background climate would imply that models using constant temperature response are inherently inaccurate at local scale. Drivers of spatial differences in leaf phenology are not as widely studied as temporal differences. Here the authors show that the spatial variation of leaf unfolding in 8 deciduous tree species in Europe can be explained by local adaptation to long-term mean climate conditions.