Contrasting phenology responses to climate warming across the northern extra-tropics

Climate warming has substantially advanced the timing of spring leaf-out of woody species at middle and high latitudes, albeit with large differences. Insights in the spatial variation of this climate warming response may therefore help to constrain future trends in leaf-out and its impact on energy, water and carbon balances at global scales. In this study, we used in situ phenology observations of 38 species from 2067 study sites, distributed across the northern hemisphere in China, Europe and the United States, to investigate the latitudinal patterns of spring leaf-out and its sensitivity (S T, advance of leaf-out dates per degree of warming) and correlation (R T, partial correlation coefficient) to temperature during the period 1980-2016. Across all species and sites, we found that S T decreased significantly by 0.15 +/- 0.02 d degrees C-1 degrees N-1, and R T increased by 0.02 +/- 0.001 degrees N-1 (both at P < 0.001). The latitudinal patterns in R T and S T were explained by the differences in requirements of chilling and thermal forcing that evolved to maximize tree fitness under local climate, particularly climate predictability and summed precipitation during the pre-leaf-out season. Our results thus showed complicated spatial differences in leaf-out responses to ongoing climate warming and indicated that spatial differences in the interactions among environmental cues need to be embedded into large-scale phenology models to improve the simulation accuracy.