Effect of decreasing soil frozen depth on vegetation growth in the source region of the Yellow River for 1982–2015

Climate warming impacts on frozen ground degradation may cause large-scale consequences to terrestrial ecosystems within the source region of the Yellow River (SRYR). Spatial and temporal variations in the soil frozen depth (SFD) in SRYR were estimated using the Stefan equation for 1982–2015 and SFD effects on alpine vegetation were analyzed. Mean SFD during 1982–2015 decreased at a rate of 6.01 cm/10a, and the areal extent of the SFD greater than 160 cm decreased by 26.3%. Normalized difference vegetation index (NDVI) in the growing season (NDVIgr) discontinuously increased at a rate of 0.003/10a, increasing in 1982–2000 and declining in 2001–2015. The NDVIgr was significantly correlated to the decreasing SFD, showing a stronger relationship than that with temperature and precipitation. Comparisons before and after 2000 showed that negative relationships between SFD and NDVIgr became positive. Responses of vegetation growth to frozen ground degradation depended on different rates of decreasing SFD. Rapid decreases in SFD after 2000 led to increased evapotranspiration and reduced soil water, which negatively affected vegetation growth. Negative impacts of human activities on vegetation changed to positive contributions from the periods of 1982–2000 to 2001–2015; however, the effect was less than that of climate change and frozen ground degradation. This study provides a foundation for research on frozen ground changes and the impacts on ecological environments.