Co-influence of the start of thermal growing season and precipitation on vegetation spring green-up on the Tibetan Plateau

The climate in the Tibetan Plateau (TP) has undergone significant change in recent decades, mainly in thermal and water conditions, which plays a crucial role in phenological changes in vegetation spring phenology. However, how the start of the thermal growing season (SOS-T) and the start of the rainy season (SORS) as key climatic factors affect vegetation green-up remains unclear. Given that these factors characterize thermal and water conditions required for vegetation green-up, this study investigated changes in the SOS-T and SORS from 1961 to 2022, using observation-based datasets with long time series. We found that the SOS-T and SORS have advanced across the TP in 1961–2022 and have shown a spatial pattern of advancement in the east and delay in the west in 2000–2022. Further, the co-effect of temperature and precipitation change on the start of vegetation growing season (SOS-V) in 2000–2022 was observed. Averaged across TP, the SOS-V had an early onset of 1.3 d per decade during 2000–2022, corresponding to advanced SOS-T and SORS. Regionally, the SOS-V generally occurred nearly at the same time as the SOS-T in the high-altitude meadow region. A substantial delay in the SOS-V relative to the SOS-T was observed in the desert, shrub, grassland and forest regions and generally kept pace with the SORS. Furthermore, for 50% of the vegetated regions on the TP, inter-annual variation in the delay in the SOS-V relative to the SOS-T was dominated by precipitation change, which was profound in warm-climate regions. This study highlights the co-regulation of precipitation and temperature change in the SOS-V in different vegetation cover regions in the TP, offering a scientific foundation for comprehending the impact of climate change and prospects for vegetation phenology on the TP.