The different impacts of the daytime and nighttime land surface temperatures on the alpine grassland phenology

Land surface temperature (LST) is often a direct control on herbaceous plants but has been underappreciated on the alpine grassland phenology in response to climate change. In the present study, we used satellite data of the Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) and LST products to study the land surface phenology (LSP) of alpine grasslands in response to LST changes in the Qinghai Lake Basin (QLB), which is on the northeastern Tibetan Plateau. Our results showed that LSP, including the start (SOS), end (EOS), and length (GSL) of the growing season, presented high spatial heterogeneity and had significant correlation with landform elevations. Both averaged SOS and EOS across QLB were advanced from 2001 to 2017, but the greater advancement of SOS compared to that of EOS led to an overall prolonged GSL. Daytime LST (LSTd) and nighttime LST (LSTn) had the contrasting effects on SOS (i.e., SOS can be delayed with the increase in LSTd, while it can be advanced with the increase in LSTn). However, increase in LSTd and LSTn in August had the same advancing effect on EOS. Moreover, LSTd played the dominant role in controlling the grassland phenology. Specifically, an 1 degrees C increase in the LSTd in the nongrowing season (i.e., from previous October to April) significantly postponed the SOS by 2.2 d and advanced the EOS by 1.1 d in August. This study highlights the utility and biological relevance of LST in research of grassland phenology and differential impacts of daytime and nighttime LST on grassland phenology.