Abstract:

To accurately retrieve phenological information of grasslands in northern Xizang using remote sensing methods, based on MODIS 13Q1 NDVI data with 250 m spatial resolution and 16-day composite intervals from 2001 to 2023, combined with daily MODIS 250 m data received at Lhasa Station in 2024, a dataset of annual 16-day maximum NDVI was constructed after preprocessing such as calculating vegetation indices and maximum value composite (MVC). The phenological periods of grasslands in northern Xizang were extracted by combining the dynamic threshold method with elevation zonings, and the spatial differentiation pattern and inter-annual variation trend of phenology were analyzed. Correlation analysis was then conducted with annual cumulative precipitation, annual mean air temperature, and annual total sunshine hours from 7 meteorological stations in Nagqu City, northern Xizang, from 2001 to 2024. The results showed that the green-up period of grasslands in Nagqu City mostly occurred from day 171 to day 181, and the senescence period mostly occurred before day 257. The green-up period showed an advancing trend, while the senescence period showed a delaying trend. There were obvious spatial differences in the response of grassland phenology to meteorological factors. Most of the regional correlations did not reach the significant level, and only some areas had significant correlations. Among them, the correlation between the green-up period and the temperature covered a wider range, and the proportion of significant correlation was the highest; it was mainly negatively correlated with precipitation, and the overall impact of sunshine was weak, accounting for a very low proportion. The senescence period was positively correlated with both precipitation and temperature, and negatively correlated with sunshine. In summary, grassland phenology in northern Xizang exhibits obvious elevation differentiation, with an overall trend of “earlier green-up and later senescence”. Temperature is the most critical factor affecting the spatial pattern of grassland phenology in northern Xizang, and its regulatory effect is significantly stronger than precipitation and sunshine. The 7 meteorological stations are all located in the central and southern parts of Nagqu City, where a persistent “warming-drying” climate trend is observed, and the grassland ecosystem is highly dependent on water resources.

Key words: northern Xizang, grassland, elevation, green-up period, senescence period, climate response, NDVI