Divergent response of leaf unfolding to climate warming in subtropical and temperate zones

The conservation of subtropical forest ecosystems is critical for stabilizing global vegetation productivity and land carbon sink. However, unlike temperate trees, the effects of climatic factors on the phenology of subtropical trees remain largely unclear. In this study, we examined the response of leaf unfolding to heat and chilling accumulation and drought by ridge regression using leaf unfolding data from 154 woody species across eastern China during 1963 and 2014 and compared the differences between subtropical and temperate zones. The results showed that the mean leaf unfolding dates for species in the subtropics were 20 days earlier than those in the temperate zone; similarly, heat accumulation for leaf unfolding in the subtropics was also significantly higher than that in the temperate zone, while chilling accumulation showed the opposite trend. Forcing temperature was still the primary driver of leaf unfolding in the both zones; nevertheless, there are more species that are not significantly affected by forcing temperature in the subtropics than in the temperate zones. Furthermore, more species showed delayed leaf unfolding due to reduced chilling in the subtropics, indicating that insufficient chilling caused by climate warming may have exerted a more noticeable effect on leaf unfolding in the subtropics than in the temperate zone. In addition, the average temperature sensitivity of leaf unfolding was higher in the subtropics than the temperate zone, which may have resulted from faster heat accumulation per 1 °C temperature increase, associated with more heat-accumulating hours due to the warmer climate in the subtropics. Moreover, a larger species-specific variation in the sensitivity of leaf unfolding to forcing temperature occurred in the subtropical zone than in the temperate zone. These differences in phenological responses among species may desynchronize ecological interactions and thereby threaten ecosystem function in the subtropics under continued warming.