Impact of functional groups on aboveground biomass in alpine grassland communities

Alpine grasslands on the Tibetan Plateau (TP) are highly vulnerable to various treatments and face significant degradation risks due to global environmental changes. However, the response of these grasslands to different external treatments remains uncertain, and the patterns behind functional group responses are unclear, impeding our ability to restore alpine grasslands under changing climate. To address this gap, we compiled a comprehensive database of 797 experimental observations of alpine grasslands in the TP, classified these communities into four common functional groups (e.g., grass and sedge), and conducted a meta-analysis to evaluate the response of aboveground biomass (AGB) to nine different treatments (e.g., grazing and nitrogen addition). Meta-regression was used to analyze the changes in AGB with treatment intensity and duration. We also used functional group asynchrony and unevenness to describe the complementary effects (CEs) and selection effects (SEs) of communities under these treatments. We found that among all the nine treatments, grazing had the biggest negative impact on the alpine grassland community by reducing AGB by 40.3%, mainly through decreasing the biomass of grass and sedge. Conversely, nitrogen and phosphorus addition had the largest positive effect, increasing AGB by 39.4% mainly by promoting the growth of grass. However, the increase in AGB was not significant in the interactive experiments involving grazing and nitrogen addition. The unevenness of the community decreased as grazing time prolongs, simplifying the structure of the community, with severe nutrient losses and weakened CE, while nitrogen addition could rapidly increase the AGB of grass and enhance the SE. CE or SE of communities can be enhanced through the restoration of specific functional groups, favoring the recovery of vegetation. Our study also revealed the potential of nitrogen compensation for repairing the damage caused by overgrazing on alpine grasslands.