Phenology of different types of vegetation and their response to climate change in the Qilian Mountains, China

The Qilian Mountains (QM) possess a delicate vegetation ecosystem, amplifying the evident response of vegetation phenology to climate change. The relationship between changes in vegetation growth and climate remains complex. To this end, we used MODIS NDVI data to extract the phenological parameters of the vegetation including meadow (MDW), grassland (GSD), and alpine vegetation (ALV)) in the QM from 2002 to 2021. Then, we employed path analysis to reveal the direct and indirect impacts of seasonal climate change on vegetation phenology. Additionally, we decomposed the vegetation phenology in a time series using the trigonometric seasonality, Box-Cox transformation, ARMA errors, and Trend Seasonal components model (TBATS). The findings showed a distinct pattern in the vegetation phenology of the QM, characterized by a progressive shift towards an earlier start of the growing season (SOS), a delayed end of the growing season (EOS), and an extended length of the growing season (LOS). The growth cycle of MDW, GSD, and ALV in the QM species is clearly defined. The SOS for MDW and GSD occurred earlier, mainly between late April and August, while the SOS for ALVs occurred between mid-May and mid-August, a one-month delay compared to the other vegetation. The EOS in MDW and GSD were concentrated between late August and April and early September and early January, respectively. Vegetation phenology exhibits distinct responses to seasonal temperature and precipitation patterns. The advancement and delay of SOS were mainly influenced by the direct effect of spring temperatures and precipitation, which affected 19.59% and 22.17% of the study area, respectively. The advancement and delay of EOS were mainly influenced by the direct effect of fall temperatures and precipitation, which affected 30.18% and 21.17% of the area, respectively. On the contrary, the direct effects of temperature and precipitation in summer and winter on vegetation phenology seem less noticeable and were mainly influenced by indirect effects. The indirect effect of winter precipitation is the main factor affecting the advance or delay of SOS, and the area proportions were 16.29% and 23.42%, respectively. The indirect effects of fall temperatures and precipitation were the main factors affecting the delay and advancement of EOS, respectively, with an area share of 15.80% and 21.60%. This study provides valuable insight into the relationship between vegetation phenology and climate change, which can be of great practical value for the ecological protection of the Qinghai-Tibetan Plateau as well as for the development of GSD ecological animal husbandry in the QM alpine pastoral area.