Delayed response of spring phenology to global warming in subtropics and tropics

Climate drivers of plant phenology in the subtropics and tropics are still unclear, which significantly hinders accurate prediction of climate change impacts on vegetation growth and carbon balance in these unique ecoregions. The basic hypothesis of process-based phenology models is that spring tree phenology is regulated by temperature and triggered by chilling temperatures during the dormancy period, followed by forcing temperatures during the growth period. That is, trees require cool temperatures during a chilling period to break endodormancy, and then enter the phase of ecodormancy during which the rate of ontogenetic development increases with increasing air temperature. Therefore, insufficient chilling requirements may slow bud growth and consequently delay budburst. Many studies have shown that chilling requirement is at present sufficient to release bud dormancy fully in temperate regions, and thus forcing temperatures play a dominant role in triggering spring tree phenology. To identify differences in mechanisms of spring tree phenology responses to air temperature between the subtropics/tropics and the temperate zone, and their possible effects on future phenological trends under global warming, we used leaf unfolding and flowering data from a tree species of tropical origin, Melia azedarach, and output daily mean air temperature data from a regional climate model (HadGEM3-RA) for the period 1981–2005 at 42 stations in southeastern China to fit unified forcing and chilling phenology models. Then, we selected optimum models for each phenophase at each station. Moreover, we predicted leafing and flowering dates across the research region over 2021–2100 under global climate warming scenarios. The results show a previously unreported phenological phenomenon: chilling will often be insufficient to break bud dormancy in the northern tropical zone and may become a crucial factor limiting leafing and flowering responses to spring warming. However, chilling will still be sufficient to break bud dormancy in the warm temperate zone, and thus may not limit leafing and flowering responses to spring warming there. Consequently, predicted leafing and flowering dates both will be delayed in the northern tropical zone but will advance in the warm temperate zone from 2021 to 2100. In the subtropical zone, the effect of chilling temperature on spring phenology will be reduced gradually from earlier to later phenophases. Thus, predicted leafing and flowering dates show decreased delaying trends and increased advancing trends from earlier to later phenophases in the subtropical zone. Our findings suggest that insufficient chilling temperature accumulation during the dormancy period may counteract the forcing temperature accumulation during the growth period in the northern tropical zone and parts of the subtropical zone, resulting in delayed leafing and flowering dates.