Upslope and downslope vegetation expansions on the warming and wetting Qinghai–Tibet Plateau

Abstract Vegetation lateral expansion into barren land is an understudied process of global greening, yet it is crucial for ecological safety and draws widely public attention. Using remote sensing data, we present for the first time a comprehensive assessment of vegetation expansion across the ecologically fragile Qinghai–Tibet Plateau. During 2002–2023, vegetation expanded across nearly 1/5 of the barren land, with a change rate (–3.89 to −4.95 × 10 4 km 2 /10a) comparable in magnitude to the regional glacier retreat and permafrost thaw. The upper limit shifted upslope (∼35 m), while the lower limit shifted downslope (∼150 m). This bidirectional expansion mode is associated with the alleviation of water limitation at lower elevations (due to wetting) and energy limitation at higher elevations (due to warming). Nevertheless, projections suggest this rapid downslope expansion may decelerate. The identified bidirectional pattern challenges the prevailing assumption of a uniform upslope shift in alpine vegetation under climate change.