Testing the relative importance of the various environmental triggers of spring leaf-out

The timing of spring leaf-out in temperate and boreal trees greatly affects ecosystem functioning and global biogeochemical cycles. Yet, spring phenological responses to climate change remain uncertain due to the complex, interacting effects of environmental triggers. While spring temperature, winter chilling and day length have traditionally been accepted as the main drivers of spring phenology, recent research suggests that additional factors, such as solar radiation and air humidity in spring and leaf-out timing and temperatures in the preceding year, play an important role, too, further complicating predictions of spring phenology. Here, I test for the relative importance of each driver using ground-sourced and satellite-derived phenology observations from the Northern Hemisphere. The results show that, consistent with the ongoing advances in spring leaf-out, spring temperature remains the dominant driver of spring phenology across the vast majority of temperate and boreal forests. By contrast, winter temperatures played only a minor role, suggesting that, despite generally warmer winters, trees’ chilling requirements are mostly met. After controlling for spring climate and temporal autocorrelation in the data, spring phenology and climate of the preceding year did not affect leaf-out timing, contradicting recent observational studies. Overall, the strong and consistent effect of spring temperature predicts that the arrival of spring will continue to advance in the future, with multiple cascading effects on species interactions, forest productivity and other ecosystem functions.