Spring predictability explains different leaf-out strategies in the woody floras of North America, Europe and East Asia.

Intuitively, interannual spring temperature variability (STV) should influence the leaf-out strategies of temperate zone woody species, with high winter chilling requirements in species from regions where spring warming varies greatly among years. We tested this hypothesis using experiments in 215 species and leaf-out monitoring in 1585 species from East Asia (EA), Europe (EU) and North America (NA). The results reveal that species from regions with high STV indeed have higher winter chilling requirements, and, when grown under the same conditions, leaf out later than related species from regions with lower STV. Since 1900, STV has been consistently higher in NA than in EU and EA, and under experimentally short winter conditions NA species required 84% more spring warming for bud break, EU ones 49% and EA ones only 1%. These previously unknown continental-scale differences in phenological strategies underscore the need for considering regional climate histories in global change models.