Introduction
Earth’s ecosystems have been severely affected by anthropogenically
driven environmental change (Barnosky et al. 2012), causing
dramatic biodiversity loss (Díaz et al. 2019) and, consequently,
threatening their capacity to provide ecosystem functioning and services
(Cardinale et al. 2012; Brose & Hillebrand 2016; Díaz et
al. 2019). Grasslands are a globally important biome that comprises
approximately 22% of the Earth’s ice-free land (White et al.2000), and provides an important contribution to global food supply and
security (O’Mara 2012). Grasslands are the most endangered terrestrial
ecosystem type due to overexploitation by rapidly increasing human
population and consumption (Millenium Ecosystem Assessment 2005), and
are more vulnerable to environmental change than some other ecosystems
(Hoekstra et al. 2005). Therefore, understanding how grasslands
respond to environmental change and land-use is extremely important for
global sustainability. However, the combined effects of climate change
and land-use on biodiversity and ecosystem functioning are still poorly
understood (Siebert et al. 2019).
Biodiversity has been increasingly recognized as a major determinant of
ecosystem dynamics and functioning (Tilman et al. 2014), partly
because species vary in their contributions to different functions
(Isbell et al. 2011; Meyer et al. 2018). Empirical
evidence comes from both experimental and observational studies (Duffyet al. 2017), and indicates that complementarity effects mainly
explain the positive effects of biodiversity on ecosystem functions
(Loreau & Hector 2001; Reich et al. 2012). For example, plant
communities with higher diversity are able to capture more resources due
to dissimilar resource use strategies, convert them into new biomass,
and store more carbon (Hooper et al. 2005; Cardinale et
al. 2011; Carol et al. 2018). Indeed, recent studies indicate
that effects of environmental change and human disturbances are mediated
by changes in biodiversity (Isbell et al. 2013; Allan et
al. 2015; Dainese et al. 2019; Beaumelle et al. 2020).
However, the relative importance of different, co-occurring
environmental change drivers in driving biodiversity-mediated changes in
ecosystem functioning and their potential context-dependencies have
rarely been studied (Eisenhauer et al. 2019; Giling et al.2019). Biodiversity effects may depend on the environmental context and
stress (Guerrero-Ramírez et al. 2017; Ratcliffe et al.2017), driving the balance between competitive and facilitative
interactions (Wright et al. 2017).
Aridity is the most important abiotic factor influencing many grassland
ecosystems, because most grasslands are water limited (Knapp & Smith
2001; Merbold et al. 2009; Harpole et al. 2011). The
projected increases in aridity will diminish the ability of global
grasslands to provide life-supporting ecosystem services (Dai 2013;
Sherwood & Fu 2014; Trenberth et al. 2014; Berdugo et al.2020). Aridity has been reported to reduce plant species richness and
productivity, and alter the structure of above- and belowground
communities from local to global scales (Tilman & EI Haddi 1992; Zhao
& Running 2010; Maestre et al. 2015; Berdugo et al.2020). Because ecosystem functions, such as nitrogen cycling, carbon
sequestration, and litter decomposition, are regulated by these biotic
attributes, ecosystem functioning is likely to decline with increasing
aridity (Maestre et al. 2012; Delgado-Baquerizo et al.2013; Berdugo et al. 2017; Durán et al. 2018; Beaumelleet al. 2020). Furthermore, land-use strategies have been found to
modulate the response of grasslands to climate change (Jonas et
al. 2015; Karlowsky et al. 2018; Stampfli et al. 2018).
Yet, how land-use and aridity interactively affect grassland
biodiversity and ecosystem functioning has been rarely explored,
particularly so at large spatial scales.
Livestock grazing and haying are prevalent land-use types in global
grasslands, and occur across virtually all biomes from arid to wet
climates (Asner et al. 2004; Steinfeld et al. 2006; Erbet al. 2017). The growing human population and mounting food
demands require an increase of livestock production (Bouwman et
al. 2005; Tilman et al. 2011), which will likely lead to higher
grazing and haying intensity in natural grasslands. Grazing, especially
by native ungulates, and haying have been reported to facilitate plant
diversity and productivity and increase soil carbon (C) and nitrogen (N)
stocks (Sirotnak & Huntly 2000; Socher et al. 2012; Conget al. 2014; Wang et al. 2019). However, overgrazing and
intensive haying can reduce plant diversity and biomass as well as soil
functions, and exacerbate grassland degradation (Eldridge et al.2016; Eldridge et al. 2017). The stocking rates and grazing
intensities that plants experience under managed grazing by livestock
can be extremely high relative to the levels of grazing by native
ungulates that plants experienced throughout their evolutionary
histories (Milchunas & Lauenroth 1993). For example, herbivore biomass
per unit primary productivity can be an order of magnitude greater in
managed livestock grazing systems than in natural grazing systems
(Oesterheld et al. 1992). Further, the influence of grazing on
grasslands will be altered with environmental change. For instance, in
arid lands, grazing is a pervasive driver of loss of soil functions
(Bridges & Oldeman 1999), resulting in reductions in soil organic C and
aboveground biomass (Gallardo & Schlesinger 1992; Asner et al.2004). In semiarid regions, grazing could lead to woody encroachment and
thus to an increase in both above- and belowground carbon stocks
(Eldridge et al. 2011; Anadón et al. 2014). A global
meta-analysis revealed large differences in the response of
C3- and C4-dominated grasslands to
grazing under different rainfall regimes (McSherry & Ritchie 2013).
Therefore, grazing and haying may have context-dependent effects, which
is why the ecosystem consequences of land-use on grasslands need to be
examined in a wide range of environments.
Here we examine how plant species richness, plant biomass, and soil C
concentrations respond to a spatial gradient of aridity (ranging from
0.25 to 0.89, covering major global ecosystem thresholds; Berdugoet al . 2020) and land-use (grazing and haying) at 716 sites along
an extensive environmental gradient of 2,200 km across China (Fig. S1).
This region is one of the major sources of animal products in China and
acts as a natural green barrier towards the vast cropland regions, thus
having crucial ecological and economic importance (Kang et al.2007). We hypothesized that (1) aridity and land use will decrease plant
diversity, (2) ecosystem functioning would increase with increasing
plant species richness, (3) effects of aridity, grazing, and haying on
ecosystem functioning would be mediated via variation in plant species
richness.