Enhanced spatiotemporal heterogeneity and the climatic and biotic controls of autumn phenology in northern grasslands.

Knowledge of the response of grassland phenology towards climatic factors is essential to improve our understanding of ecological processes under global warming. To date, however, it remains unclear how climate change and associated changes in vegetation dynamics might affect autumn phenology of grasslands at the global scale. In this study, the trends in start of growing season (SOS) and end of growing season (EOS) dates were explored using remote sensing data (1981-2014). The responses of EOS to preseason temperature, rainfall, SOS, and net primary productivity (NPP) were then investigated for the mid-latitude (30°N ~ 55°N) grasslands of the Northern Hemisphere. The remotely-sensed SOS/EOS and PhenoCam-based SOS/EOS were first compared and a good correlation was observed. Trend analysis revealed that the time span of SOS/EOS (from the earliest SOS/EOS to the last SOS/EOS) and the growing season length (from SOS to EOS) have extended for the entire study region. Furthermore, a forward shift in all SOS pixels was observed in Central-West Asian grasslands, whereas no such significant trend was observed for North American grasslands and East Asian grasslands. The duration of EOS completion had shortened within North American grasslands but lengthened in Asian grasslands. Next, correlation analysis uncovered a stronger relationship between EOS and previous rainfall than between EOS and temperature, indicating the key role of water availability in controlling autumn phenology. The sensitivity of EOS to both temperature and rainfall was higher in drier and warmer locations. Moreover, a significant negative correlation between EOS and SOS was observed in part of the study region, but no significant relationship between NPP and EOS was observed. Overall, this study highlights the spatially intensified heterogeneity of spring and autumn phenology in northern grasslands and that climatic changes in precipitation might act as key drivers for modifying autumn phenology of grassland vegetation in the Northern Hemisphere.