Introduction
Grasslands are one of the largest terrestrial ecosystems on Earth
(Suttie et al. 2005). Aboveground biomass production is one of the
important functions in grasslands, providing many essential services to
humanity such as feed for livestock, carbon storage, and climate
mitigation (Bengtsson et al. 2019). However, increasing nitrogen
deposition (Galloway et al. 2004) and accelerating herbivore
extirpation (Ripple et al. 2015; Atwood et al. 2020) are
altering aboveground biomass and stable provision (i.e. stability) of
aboveground biomass in grasslands (Hautier et al. 2015, 2020;
Blüthgen et al. 2016; Qin et al. 2019; Zhang et al.2019; Borer et al. 2020). Stability is a multi-dimensional
concept (Donohue et al. 2013; Arnoldi et al. 2019); here
we examine temporal invariability, calculated as the mean of aboveground
biomass through time divided by its standard deviation in local
communities (alpha stability) and larger spatial scales (aggregated
local communities; gamma stability). Metacommunity theory clarifies that
alpha stability and asynchronous dynamics among local communities
(spatial asynchrony) determine gamma stability (Wang & Loreau 2016;
Wang et al. 2019).
In grassland ecosystems, nutrient addition often decreases alpha
stability (Hautier et al. 2015; Koerner et al. 2016; Zhanget al. 2016; Liu et al. 2019), and these effects may
propagate to gamma stability (Zhang et al. 2019; Hautier et
al. 2020). In contrast, herbivore
exclusion has been found to have positive, neutral, or negative effects
on grassland stability, depending on the herbivore species excluded and
spatial scales studied (Halpern et al. 2005; Hautier et
al. 2015; Blüthgen et al. 2016; Ren et al. 2018;
Ganjurjav et al. 2019; Qin et al. 2019; Saruul et
al. 2019; Liu et al. 2021). Effects of herbivores on stability
may be more apparent at the larger spatial scale because herbivores
usually promote vegetation heterogeneity in the landscape due to
selective grazing, trampling, and localized deposition of urine and dung
(Glenn et al. 1992; Howison et al. 2017). However, to our
knowledge, no study has investigated how nutrients and herbivores
jointly regulate grassland stability across multiple spatial scales.
In contrast to stability, a range of studies have examined the joint
effects of nutrients and herbivores on grassland species richness
(Proulx & Mazumder 1998; Worm et al. 2002; Bakker et al.2006; Hillebrand et al. 2007; Alberti et al. 2010, 2011;
Yang et al. 2013; Borer et al. 2014b; Beck et al.2015; Koerner et al. 2018), community evenness (Hillebrandet al. 2007), community composition (Milchunas & Lauenroth 1993;
Chase et al. 2000; Grellmann 2002; Hartley & Mitchell 2005;
Alberti et al. 2017; Hodapp et al. 2018), and aboveground
biomass (Milchunas & Lauenroth 1993; Chase et al. 2000; Moran &
Scheidler 2002; Alberti et al. 2010, 2011; Borer et al.2020). The majority of these studies found strong interactive effects
between nutrients and herbivores. For instance, herbivores typically
decrease species richness at low nutrients or productivity, while
increasing it at higher nutrients or productivity (Proulx & Mazumder
1998; Bakker et al. 2006; Hillebrand et al. 2007; Boreret al. 2014b). Herbivores also can consume extra aboveground
biomass stimulated by nutrient addition particularly when their
abundance is high (Borer et al. 2020).
As plant diversity and biomass can impact stability indirectly and
directly, it is likely that nutrients and herbivores also jointly impact
grassland stability. For instance, nutrients or herbivores can
indirectly impact gamma stability by regulating species richness at the
local and larger spatial scales (alpha and beta diversity, respectively)
(Hautier et al. 2015; Zhang et al. 2019; Liang et al. 2020). Although
less often tested, nutrients or herbivores can also impact alpha and
gamma stability through community evenness (Grman et al. 2010;
Liang et al. 2020) and community dissimilarity across time and space
(Koerner et al. 2016; Zhang et al. 2019). Our
understanding of plant diversity (including alpha and beta diversity,
evenness, community dissimilarity across time and space) on stability
are limited because different studies usually focus on different plant
diversity metrics (Grime 1998; Tilman et al. 2006; Polleyet al. 2007; Grman et al. 2010; Hautier et al.2015; Koerner et al. 2016). Therefore, it remains unclear which
plant diversity metrics are the major factors mediating the effects of
nutrient and herbivores on alpha and gamma stability. Assessing the
relative contribution of different facets of plant diversity to alpha
and gamma stability can deepen our understanding of the underlying
mechanisms for stability across spatial scales and help prioritize
conservation efforts.
Here, we used a globally coordinated grassland experiment, Nutrient
Network (NutNet) (Borer et al. 2014a) to assess (1) the joint
effects of nutrient addition and herbivore exclusion on the temporal
stability of aboveground biomass at the local and larger spatial scales
(i.e. alpha and gamma stability); (2) the relative contribution of
different facets of plant diversity in mediating the responses of alpha
and gamma stability to nutrients and herbivores. We hypothesized that
(1) nutrient addition decreases alpha and gamma stability, while
herbivore exclusion worsens these decreases particularly at sites with
high grazing intensity; (2) Nutrient addition decreases gamma stability
via reducing alpha stability, which is regulated by all facets of plant
diversity. Herbivore exclusion decreases gamma stability not only via
alpha stability but also via spatial asynchrony, because herbivores
often increase vegetation heterogeneity in the landscape (i.e. spatial
community dissimilarity).