Spatio-temporal patterns and drivers of carbon–water coupling in frozen soil zones across the gradients of freezing over the Qinghai-Tibet Plateau

Abstract

The carbon and water cycle patterns of frozen soil ecosystems in regions, such as the Qinghai-Tibet Plateau (QTP), which have fragile and sensitive ecologies, are possibly affected by climate change. However, these patterns along the gradients of varying degrees of freezing remain poorly understood. In this study, we examine the spatio-temporal patterns of water use efficiency (WUE), gross primary productivity (GPP), and evapotranspiration (ET) in six frozen soil zones of the Qinghai-Tibet Plateau (QTP) with varying degrees of frozen soil. The analysis is based on the frozen soil classification criteria and remote sensing data from 2001 to 2020. The influences of six climatic, nine edaphic, and two botanic parameters on these patterns is also investigated. The results indicate that 48.05%, 24.07%, and 20% of the regions in QTP exhibit significantly upward trends of GPP, ET, and WUE, respectively. The highest increase trends were observed in GPP in the middle-thick seasonally frozen ground zone (2.36 g C m−2 yr−1), ET in the mountain permafrost zone (2.35 mm yr−1), and WUE in the predominantly continuous permafrost zone (0.003 g C kg−1 H2O yr−1). Meanwhile, GPP and WUE decrease progressively with the increase of freezing degrees, from the short-time frozen ground zone (GPP: 604.99 ± 51.99 g C m−2; WUE: 2.34 ± 0.15 g C kg−1 H2O) to the predominantly continuous permafrost zone (GPP: 114.07 ± 11.77 g C m−2; WUE: 0.33 ± 0.04 g C kg−1 H2O), while ET does not show significant differences across the three permafrost zones. The enhanced vegetation index is the primary factor affecting GPP and WUE in most frozen soil zones, whereas climate variables mainly impact ET. Our findings emphasize the importance of considering the effects of different freezing degrees of frozen soil when analyzing the response of the carbon and water cycles in the QTP in the context of climate change.