Evaluating of ground surface freeze–thaw and the interrelationship with vegetation cover on the Qinghai-Xizang Plateau

The annual frequency of ground surface freeze–thaw (AFGSFT) on the Qinghai-Xizang Plateau (QXP) is one of the most prominent features of the high plateau ground surface processes. Seasonal freezing and thawing of the ground surface led to changes, and sometimes anomalies, in the energy balance between the ground surface and the atmosphere, thereby impacting the ecological environment. However, the relationship between AFGSFT and normalized difference vegetation index (NDVI), as major influencing factors of near-ground surface hydrothermal processes, has not been well elucidated. Based on meteorological observation data from 1982 to 2020, National Oceanic and Atmospheric Administration Climate Data Record (NOAA CDR) NDVI data, and some auxiliary data, this study employs trend analysis, GeoDetector, and correlation analysis to explore the impact of NDVI on AFGSFT. The findings indicate that AFGSFT on the QXP has gradually decreased, while NDVI has generally shown an upward trend. NDVI exerts a strong controlling effect on AFGSFT changes. Specifically, as AFGSFT decreases, NDVI tends to increase, but the increasing NDVI gradually inhibits the downward trend of AFGSFT. Thus, the relationship between NDVI and AFGSFT trend is not merely one of amplification or inhibition but rather exhibits a more complex nonlinear relationship. Moreover, the changes in AFGSFT and NDVI in grassland areas are greater than those in other land cover types. This may suggest that grassland regions are experiencing a more rapid climate response and ground surface processes. These findings contribute to a better understanding of the ground surface characteristics of the high plateau and provide data support for formulating scientific ecological protection and climate adaptation strategies.