Earlier Spring Onset Induced by Increased Autumn Precipitation Dominates Advance in Peak of Season on the Tibetan Plateau

Land surface phenology (LSP) is a critical process within terrestrial ecosystems and a key component of vegetation-environment interactions, especially in alpine grasslands that experience rapid climate change. The rapidly changing climate on the Tibetan Plateau (TP) has altered the start of the growing season (SOS) of alpine grassland, but little is known regarding the following timing and dynamics of the peak of the season (POS) grassland phenology. The framework proposed here is one of the first attempts to systematically study spring-summer phenology and biotic and abiotic drivers of the diverse LSP patterns. we used the 2000-2018 MODerate Resolution Imaging Spectroradiometer (MODIS) derived normalized difference vegetation index (NDVI) for LSP tracking and analyzed diverse phenological patterns across vegetation types and their drivers. Results show that interannual variations of SOS are driven by the previous year's water conditions while the POS is strongly correlated to the changes in the SOS. This explains the delay trend in SOS of the steppe area in western TP which experienced a drier climate in the recent 19 years. In addition, precipitation and SOS are the most important factor leading to changes in spatial variations of SOS and POS, respectively. Diverse LSP patterns suggest high water availability may hasten spring phenology and this advancing trend in spring timing could lead to an earlier peak of season in alpine grasslands on the TP, which showed the sequential control of abiotic and biotic factors from spring to summer.