Divergent vegetation greening's direct impacts on land-atmosphere water and carbon exchanges in the northeastern Tibetan Plateau

Abstract

Changes in vegetation have pronounced effects on water and carbon cycles. In the past few decades with significant warming, vegetation in the Tibetan Plateau (TP) has become overall greening, particularly in its northeastern part. However, the effects of these changes on land-atmosphere water and carbon exchanges in the TP remain insufficiently understood. Here, we use a water‑carbon coupled model, Penman-Monteith-Leuning Version 2, to quantify the direct impacts of vegetation change on evapotranspiration (ET) and gross primary production (GPP) in the Yellow River Source (YRS) region, a greening hotspot in the northeastern TP. We show that ET and GPP in the YRS increased significantly from 1982 to 2018, with trends of 1.72 ± 0.21 mm yr⁻² and 3.96 ± 0.55 gC m⁻² yr⁻² (both p < 0.001), respectively. The change in leaf area index (LAI) was the dominant driver of GPP's increase, contributing 79 %, followed by atmospheric CO₂ concentration and the climatic factors. However, vegetation greening had a limited impact (11 %) on ET since the increases in plant transpiration (E_c) and canopy evaporation (E_i) were largely offset by the decline in soil evaporation (E_s). Instead, the climatic factors contributed most (72 %) to ET change over the past 37 years. Nevertheless, vegetation changes played a key role in altering ET components, with LAI contributing nearly 40 % to trends in E_s and E_c, and over 70 % to the E_i trend over the past 37 years. Our results highlight the distinct roles that vegetation plays in regulating land-atmosphere water and carbon exchanges at high altitudes.