Temperature-precipitation background affects spatial heterogeneity of spring phenology responses to climate change in northern grasslands (30°N-55°N)

Understanding spatial heterogeneity of grassland phenology responses to climate change is of crucial importance for revealing regional and species differences in ecosystem processes. In this study, three spring phenology models, namely, growing-degree-day model (GDD), temperature-precipitation parallel model (TPP) and temperature-precipitation sequential model (TPS) were employed to simulate the growing season start (GSS) retrieved from remote sensing data during 1981–2014 in mid-latitude (30°N-55°N) grasslands of the Northern Hemisphere. Results show that the average accuracies of predicted GSS based on TPP (root mean square errors (RMSE) = 9.9 days) and TPS (RMSE = 9.7 days) models are slightly higher than that based on GDD model (RMSE = 10.1 days) overall. Meanwhile, TPP/TPS model also exhibits a stronger capacity to simulate interannual variation of GSS (correlation coefficient (R) = 0.38/0.41 on average) than the GDD model (R = 0.3 on average). A revised Akaike information criterion (AICc_R) by including correlation coefficient between predicted and retrieved GSS was designed for the optimal model selection. The optimal models based on AICc_R present a stronger power in capturing the temporal pattern of spring phenology. GDD, TPP, and TPS model account for 32.7%, 20.7%, and 46.6% of the whole study region, respectively. Sub-regionally, TPS model dominates the temperate grasslands (66.1%), whereas GDD model dominates the cool semidesert grasslands (58.1%). Regions occupied by GDD model as the optimal model are generally cooler and wetter during February to May than those taken up by TPP and TPS model. Further analysis indicates higher heat and water requirements are needed in the most of warmer places. Overall, this study emphasizes the important role of thermal-moisture background in controlling the spatial pattern of spring phenology of northern grasslands in response to climate change. Precipitation is very important for triggering spring phenology in the temperate grasslands but not in the cool semidesert grasslands.