Data collection
Over the period of the three summers (late July to early August) of 2013-2015, in total 716 field sites were surveyed across the transect (Fig. S1). The spatial geographical coordinates of each site were recorded by GPS (62 s, Garmin, USA), and the land-use type of each site was also recorded: exclosure (51 sites), grazing (461 sites), or haying (212 sites). Most of the grazing sites were under heavy grazing (which is why we are referring to ‘overgrazing’ in the following), but we lack the data on accurate grazing intensity. For the haying sites, the aboveground biomass is harvested once every year.
At each site, vegetation was sampled with five to eight 0.5 m × 0.5 m plots (4748 plots in total), and all vascular plant species and their cover and density were recorded. The aboveground biomass was then harvested to ground level, oven-dried at 65°C for 48 h, and then weighed when returned to the laboratory. Species richness was defined as the number of species in each plot, based on cover data from field investigation. Soil samples were collected from three randomly assigned quadrats at each site, and a composite sample consisting of three soil cores (3-cm diameter, 10-cm depth) was collected from each quadrat (carefully removing the surface organic materials). After field collection, soil samples were taken to the laboratory, where they were sieved, air-dried for one month, and stored for laboratory analyses. Soil organic carbon (SOC) was determined with a total organic C analyzer (vario Toc, Elementar, Germany).
The aridity index (AI, the ratio of precipitation to potential evapotranspiration) of each site was calculated using data from WorldClim, including annual mean precipitation and temperature. To facilitate the interpretation of our results, we used 1-AI as our surrogate of aridity, and this index increases with decreasing annual mean precipitation (Pearson’s r = -0.87, P < 0.001) and increasing annual mean temperature (Pearson’s r = 0.28, P < 0.001) in our database. To examine how the relationships between species richness and ecosystem functioning change with environments, the sites were divided into four aridity gradients: < 0.5 (231 sites), 0.5-0.6 (250 sites), 0.6-0.7 (161 sites), and > 0.7 (82 sites). These classes cover major global aridity-related ecosystem thresholds (Berdugo et al. 2020).