Uniforming spring phenology under non-uniform climate warming across latitude in China

Temperature is the primary factor controlling plant phenology. As temperature changes with latitude, leaf phenology in spring always shows a significant latitudinal pattern. However, under asymmetric warming at the low and high latitudes, the variability of the spring leaf phenology with latitude is becoming unclear. Based on the 23,094 observations of the leaf unfolding date (LUD) for woody species located in eastern China within latitudes 23-49°N, we analyzed the variability of LUD and its latitudinal sensitivity (S_lat, days °N^-1, expressed in delayed days per degree in latitude) during 1963-2008. The results showed an earlier LUD at the mid- (-2.2 days decade^-1) and high (-2.5 days decade^-1) latitude regions, while a stable LUD at the low-latitude regions during 1963-2008. However, the temperature sensitivity of LUD (S_T, days °C^-1, expressed in advanced days per degree in temperature) remained stable across the latitudes although a slight decreasing trend from 1963 to 2008. As a result, the non-uniform optimal preseason warming with latitude (T_lat, °C °N^-1, expressed in the increase of temperature per degree in latitude) decreased S_lat from 2.38 (days °N^-1) in 1963 to 1.55 (days °N^-1) in 2008. Further analyses indicated that the Growing Degree Hours (GDH) played a critical role in these processes, although the Chilling Hours (CH) showed significant variability after 1991. Our results provide evidence that the change in the balance of CH and GDH across latitude induced declining S_lat over the last 40 years in eastern China. Furthermore, it may continue under the future climate warming scenarios and ultimately has important consequences on the structure and function of ecosystems.