Spatial and temporal patterns and influencing factors of carbon and water cycles in different permafrost types on the Qinghai-Tibet Plateau

Understanding changes in water use efficiency (WUE) and its drivers in terrestrial ecosystems on the Qinghai-Tibet Plateau is important to reveal the response of carbon and water cycle to climate change in the area sensitive to the environment. However, the patterns of carbon and water cycles in different frozen soil zones in this area are not well understood to our knowledge. This study explores the spatial and temporal patterns of WUE, gross primary production (GPP), and evapotranspiration (ET) from 2001 to 2020 at six frozen soil zones (short-time frozen ground; thin seasonally frozen ground; middle-thick seasonally frozen ground; mountain permafrost; predominantly continuous and island permafrost; predominantly continuous permafrost) on the Qinghai-Tibet Plateau with different degrees of freezing based on remote sensing data. The climatic, edaphic, and botanic parameters influencing these patterns were then investigated. The results show that: (1) the WUE, GPP, and ET all generally increased from 2001-2020 for each type of frozen soil ecosystem although the significance and the slope of the trends differed, (2) the WUE and GPP gradually decreased as the degree of freezing increased, while ET first increased and then decreased with the freezing gradient, and (3) enhanced vegetation index was the first important variable influencing WUE for all types of frozen soil regions except for the area of short-time frozen ground. Our results highlight that the freezing degree of soil could influence the evaluation of the water-carbon cycle on the Qinghai-Tibet Plateau.