Plant patch size, not number, determines vegetation recovery in mildly degraded Bangor alpine steppes under grazing exclusion

The alpine grassland vegetation on the Qinghai-Tibet Plateau is composed of plant patches in varied sizes. It remains uncertain whether vegetation recovery following grazing exclusion (GE) in degraded grasslands is driven by increases in patches number (NP), patch size (PS), or both. We based our predictions on two hypotheses: GE intensifies plant competition, and facilitation prevails near patches while competition prevails in interpatch spaces. We predicted that the NP would remain stable or decrease and PS would increase under GE treatment. To evaluate these predictions, we conducted a study in six lightly degraded alpine grasslands under free grazing (FG) conditions in Bangor County, Tibet Autonomous Region, China, with corresponding GE treatments using transects in 2017 and 2018. Results revealed that four sites in 2017 and five sites in 2018 had reduced NP and increased PS, with probabilities of 0.033 (2017) and 0.004 (2018), respectively, and a joint probability of 0.0001 under the null hypothesis that GE does not affect NP or PS. The NP reduction was solely due to the decrease in small patch sizes. An increase in PS was common across species, and a predominant tendency for NP reduction was observed among species across the sites. The overall changes in NP and PS were primarily driven by the three most abundant species (contributing more than 60% in both years), rather than by shifts in floristic composition. Our findings highlight that vegetation recovery in Bangor alpine steppes following GE relies solely on the expansion of existing patches rather than the recruitment of new ones in interpatch gaps. We recommend prioritizing growth-promoting measures, such as nutrient or water management, over seed addition when assisting with GE for restoring lightly degraded grasslands.