Divergence in the sensitivity of woody leaf unfolding phenology to daytime and nighttime temperatures

Abstract Background Plant phenology plays an important role in regulating carbon and water cycles in terrestrial ecosystems. Rising temperatures have a profound impact on vegetation phenology in the northern hemisphere, advancing spring phenology and delaying autumn phenology. However, the effects of daytime and nighttime warming on spring phenology are not well understood. Methods We investigated the response of leaf unfolding date (LUD) to daytime and nighttime temperatures over past 30 years by a total of 4,320 LUD records, including 10 deciduous tree species and 2 shrubs at 12 sites in China. We also compared the divergence of temperature sensitivity of woody LUD between early leaf unfolding species and late leaf unfolding species. Results LUD was mainly regulated by preseason minimum temperatures other than preseason maximum temperatures. Compared to maximum temperatures, minimum temperatures had more significant effects on LUD across all species during 1983–1997. LUD for early leaf unfolding species and late leaf unfolding species was sensitive to minimum temperatures and maximum temperatures during 2000–2014, respectively. Daytime and nighttime warming led to the advancement of LUD, whereas the sensitivity of leaf unfolding to nighttime temperatures decreased from the period 1983–1997 to 2000–2014. Decreased chilling requirements slowed down the advancement of LUD. The day-night-temperature GDD (DN GDD ) model had higher values of R 2 (0.93) and lower RMSE (6.33 days) compared to the threshold ( R 2 = 0.72, RMSE = 13.84 days) and GDD ( R 2 = 0.81, RMSE = 7.96 days) models. Conclusions The DN GDD model performed better on estimating woody LUD than the threshold and GDD models. This study highlights the different responses of LUD for early leaf unfolding species and late leaf unfolding species to daytime and nighttime warming, which will help us better understand plant phenological processes.