Disentangling the contributions of hydrothermal factors to fractional vegetation cover dynamics on the Qinghai-Tibet plateau

The Qinghai-Tibet Plateau (QTP) is a cornerstone of global climate and terrestrial ecosystems, playing a pivotal role in carbon cycling, water balance, and energy exchange. While research on QTP vegetation dynamics has traditionally focused on temperature (Ta) and precipitation (PRE), the independent effects of vapor pressure deficit (VPD) and soil moisture (SM) remain largely obscured, primarily due to strong hydrothermal coupling and pervasive multicollinearity among these variables. Therefore, we employed ridge regression to quantify the independent contributions of key hydrothermal factors to fractional vegetation cover (FVC) dynamics across the QTP from 1982 to 2020. The principal results are summarized as follows: (1) Spatiotemporally, FVC exhibited a distinct southeast-to-northwest declining gradient. Over the period 1982–2020, a consistent greening trend was observed at a rate of 0.0006 yr −1 . Specifically, vegetation greening occurred across 66.07% of the plateau, primarily in the arid and transitional zones, contrasting with localized vegetation degradation in the relatively humid zone. (2) Among the hydrothermal factors examined, VPD surpassed temperature to emerge as the dominant driver, explaining 31.69% of the FVC variation and controlling 44.31% of the total area. Ta followed, with a relative contribution of 24.15% and a dominant area of 27.40%. Deep soil moisture (SM 3 ) accounted for 14.75% of the FVC trend, with a dominant area of 13.76%, exerting a critical influence in the arid zone. (3) FVC responses to VPD were highly heterogeneous across hydrothermal regimes: increasing soil moisture mitigated atmospheric drought stress to sustain slow greening in the arid zone; moderate increases in VPD promoted significant greening under adequate moisture in the transitional zone; whereas the synergistic interplay of high VPD and reduced soil moisture intensified vegetation decreasing in the relatively humid zone on the QTP. By disentangling the complex interplay between atmospheric and soil moisture, this study highlights the spatially heterogeneous sensitivity of alpine vegetation to hydroclimatic shifts, offering essential scientific evidence for ecosystem-specific conservation and adaptive management under global warming. • FVC on the QTP increased by 0.0006 yr −1 during 1982–2020, mostly in arid and transitional zones. • VPD explained 31.69% to the FVC trend and dominated 44.31% of the QTP. • The response of FVC to VPD varied in different hydrothermal regimes on QTP.