Optimizing restoration duration to maximize CO2 uptake on the Tibetan Plateau

Grasslands cover one-third of the terrestrial area, though half of them have been degraded and primarily due to overgrazing. The Tibetan Plateau (TP) is home to the largest area of alpine grassland on Earth, experiencing a typical degradation-restoration story over the past three decades. With the large-scale implementation and longterm duration of ecological projects aimed at restoring degraded grasslands by fencing across the TP, there is considerable uncertainty regarding the direction and magnitude of changes in the carbon (C) sink function. To address this, we combined ten pairs of systematic field observations at the site scale with literature compilation, aiming to clarify whether restoration still contributes to C uptake and its driving factors. Our results show that after a decade of restoration, living biomass was enhanced by over 80 %, and litter accumulation increased by 3.68 +/- 1.25 times across all ecosystem types, although these changes were barely reflected in soil C. Litter caused consistent negative impacts on CO2 uptake across all vegetation types, despite restoration having increased CO2 uptake by half in alpine steppes and alpine meadows. This negative impact was more evident in alpine wetlands, where the quantity of litter was double the living biomass. The thick layer of litter produced a shading effect that further limited the photosynthetic yield of living plants, regulating the restoration-induced CO2 gain or loss. Combining 246 cases across the TP further highlights the need to shorten the duration of restoration to maximize CO2 uptake while also benefitting sustainability, which is deserving of attention from both the scientific community and policy-makers.