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).