Unstable carbon sink growth in alpine grasslands of the Qinghai–Tibet Plateau

Introduction Alpine grasslands on the Qinghai–Tibet Plateau (QTP) are central to regional carbon cycling and climate regulation. Although warming and increasing precipitation have driven widespread greening and greater carbon uptake, it remains uncertain whether these gains are accompanied by greater ecosystem stability. Methods We examined carbon sink capacity and temporal variability using net ecosystem productivity (NEP) and proportional variability (PV), and assessed recovery resilience indirectly using lag-one autocorrelation (TAC) as a conditional proxy. Grasslands were classified into eight types based on the trend directions of NEP, PV, and TAC. Multivariate spatial regressions were used to examine hydroclimatic associations with these trends. Results Annual NEP increased across 81.7% of the grassland area between 2000 and 2024, with significant increases (p < 0.05) across 31.7%. Plateau-scale mean annual NEP increased significantly at 0.553 g C m ⁻² yr ⁻² (p = 0.006). Regional mean PV and TAC also increased, indicating greater temporal fluctuations and potentially slower recovery under the assumptions underlying TAC. Robust sinks (T1), with increasing NEP and declining PV and TAC, covered 20.2% of grasslands, whereas 61.6% belonged to T2–T4, where increasing NEP co-occurred with rising PV, TAC, or both. Vulnerable sinks were the largest category (32.6%) and were concentrated in semi-arid alpine steppes and alpine deserts, whereas robust sinks occurred mainly in eastern alpine meadows. Mean annual precipitation was the strongest predictor associated with NEP trends, and vapor pressure deficit was the strongest predictor associated with PV and TAC trends. Discussion Stronger carbon uptake does not necessarily translate into a more stable ecosystem. Evaluating carbon uptake together with temporal variability and conditional resilience indicators can inform assessments of QTP alpine grasslands under climate stress.