INTRODUCTION
Human activities have resulted in substantial increases in the frequency
and intensity of extreme climate events worldwide (Meehl et al., 2000;
IPCC, 2014). These climate
extremes, which are projected to increase in the next decades, may exert
large impact on the structure and functioning of plant communities, as
observed for various regions following naturally occurring precipitation
extremes (Buckland et al., 1997; Breshears et al., 2005; Ciais et al.,
2005; PeƱuelas et al., 2007; Knapp et al., 2015) and in studies
experimentally imposing precipitation deficits (Heitschmidt et al.,
2005; da Costa et al., 2010; De Boeck et al., 2011; Dreesen et al.,
2014). Considerable efforts have been directed towards assessing the
stability of ecological communities experiencing extreme climate events
(e.g., Tilman & Downing, 1994; Wardle et al., 2000; Jasper & Frank,
2010; Hoover et al., 2014). However, until recently research on this
topic has mainly focused on the resistance and/or recovery of ecosystem
functions (particularly biomass), paying far less attention to the
resistance and recovery of the structure of ecological communities. This
one-dimensional approach, however, precludes a comprehensive
understanding of ecological stability across levels of ecological
organization. On the one hand, community functional and structural
stability may be positively associated with each other, a scenario that
may arise when functional resistance/recovery of an ecosystem is closely
linked to the resistance/recovery of community structure (Allison &
Martiny, 2008; Baert et al., 2016; Guelzow et al., 2017; Hillebrand et
al., 2018; Polazzo & Rico, 2021). On the other hand, functional
response of a community to disturbance may not necessarily parallel its
structural response. In particular, ecosystem functions are expected to
be generally less responsive to disturbance than community structure, as
functional redundancy among species may allow communities to mitigate
their functional changes despite potentially substantial structural
deviation from their pre-disturbance states (Yachi & Loreau, 1999;
Allison & Martiny, 2008). Within this context, accumulating theoretical
and empirical evidence suggests asynchronous population dynamics among
species as a potentially important mechanism stabilizing ecosystem
functions (Ives et al., 1999; Yachi & Loreau, 1999; Loreau & de
Mazancourt, 2008; Hector et al., 2010; Hautier et al., 2014; Xu et al.,
2021). However, greater asynchrony also indicates lower community
structural stability, as it corresponds to greater change in species
composition (Allan et al., 2011; Hillebrand et al., 2018). Thus it is
possible for structural and functional stability to be decoupled or even
negatively associated with each other.
Species diversity is often thought as an important factor influencing
plant community resistance (Caldeira et al., 2005; Anja et al., 2012;
Baert et al., 2016) and recovery (Jasper & Frank, 2010; Kreyling et
al., 2017; Wagg et al., 2017). More diverse communities may better
withstand disturbance, and recover more quickly from disturbance, due to
the greater probability of containing species resistant to disturbance
and species resilient after disturbance in more diverse communities, as
well as increased asynchronous species responses to environmental
changes in more diverse communities (Yachi & Loreau, 1999). On the
other hand, theory suggests that more diverse communities may take more
time to return to their steady states after disturbance (May, 1973; Ives
& Carpenter, 2007), potentially resulting in lower structural and
functional recovery with increasing diversity. Moreover, more diverse
communities may be more likely to contain species vulnerable to
disturbance, making it more difficult to maintain stable structure and
function after disturbance. In apparent accordance with these opposing
predictions, empirical studies of the diversity-resistance/recovery
relationships have reported mixed results. For instance,
ecosystem
functional resistance has been found to increase (Tilman & Downing,
1994; Kahmen et al., 2005; Isbell et al., 2015), decrease (Pfisterer &
Schmid, 2002; Allison, 2004; De Boeck et al., 2008), or remain unchanged
(DeClerck et al., 2006; Wang et al., 2007; Van Ruijven & Berendse,
2010; Carter & Blair, 2012) with species diversity. Likewise, positive
(Tilman & Downing, 1994; Van Ruijven & Berendse, 2010; Vogel et al.,
2012), negative (Pfisterer & Schmid, 2002) and neutral (Carter &
Blair, 2012; Xu et al., 2014; Isbell et al., 2015) diversity-ecosystem
functional recovery relationships have been
reported. By comparison, few
studies have addressed the diversity-structural stability relationships,
showing that species diversity may increase (Frank & McNaughton, 1991;
Baert et al., 2016) or decrease (Van Peer et al., 2004) structural
resistance, or shows varied relationship with structural resistance
(Shurin et al., 2007); we know of only one study that examined diversity
effect on community structural recovery (Baert et al., 2016). Therefore,
future studies should consider examining how species diversity relates
to both functional and structural resistance/recovery for a more
comprehensive understanding of ecological consequences of ongoing
biodiversity loss.
Rather than species diversity, the mass ratio hypothesis suggests that
the properties of an ecosystem are largely determined by its dominant
species (Grime, 1998). Consistent with this hypothesis, functional
resistance and/or recovery of grassland communities in response to
drought were found to be closely related to the resource-use strategies
(Mackie et al., 2019), life history (Ruppert et al., 2015), traits and
abundance (Macgillivray et al., 1995; Florence et al., 2014; Stampfli et
al., 2018) of dominant plant species. Functional resistance of forest
communities to drought has also been reported to depend upon dominant
tree species (DeClerck et al., 2006). By comparison, the role of
dominant species in modulating community structural resistance and
recovery has rarely been assessed. Elucidating the importance of
dominant species, relative to species richness and asynchrony, for
resistance and recovery at functional and structural levels would
provide a more mechanistic understanding of ecosystem dynamics under
ongoing anthropogenic changes.
Increase in atmospheric nitrogen deposition has been reported to alter
species richness (Vitousek et al., 1997; Suding et al., 2005), species
asynchrony (Isbell et al., 2009; Hector et al., 2010), and the abundance
of dominant species (Xu et al., 2015a; Bowman et al., 2018), with
potential consequences for community resistance and recovery. However,
nitrogen enrichment effects on community resistance and recovery remain
understudied. The few empirical studies on this topic reported that
increased nitrogen availability could increase grassland functional
recovery after drought (Kinugasa et al., 2012; Xu et al., 2014), and
increase (Hofer et al., 2017) or have little effect (Xu et al., 2014) on
grassland functional resistance to drought. No studies, to our
knowledge, have explored the linkage between nitrogen input and
community structural resistance/recovery. As a common land-use practice
in grasslands, mowing may also result in changes in species diversity
(Maron & Jefferies, 2001; Williams et al., 2010; Socher et al., 2013)
and the abundance of dominant species (Clark & Wilson, 2001; Galvanek
et al., 2015). Nevertheless, we also know little about how mowing
affects plant community resistance and recovery (but see Vogel et al.,
2012), especially from the structural perspective.
In this study, we experimentally simulated nitrogen deposition and
mowing in a temperate semiarid grassland in Erguna, Northeast China to
assess their effects on grassland resistance and resilience from both
functional and structural perspectives. The temperate semiarid grassland
constitutes an important component of the Eurasian grassland biome,
providing a range of necessary products and services for local human
populations. Atmospheric nitrogen deposition is projected to increase in
the next decades in this region (Liu et al., 2011), which, combined with
mowing as a common grassland management practice (Wang et al., 2011),
may impose significant impact on this important ecosystem that is also
projected to experience more frequent and intense drought (Lu et al.,
2021). During the experiment, our study grassland experienced natural
drought in three consecutive growing seasons (2015 to 2017, Fig. S1),
reducing aboveground plant biomass by 73.3%, 58.4% and 45.3%,
respectively. This natural drought event provides us an excellent
opportunity to assess the response of our study grassland to drought
under nitrogen deposition and mowing scenarios. We aimed to 1) explore
how nitrogen enrichment and mowing influence functional and structural
stability (resistance and recovery) of the temperate grassland in
response to drought, and 2) elucidate the pathways through which
nitrogen enrichment and mowing influence functional and structural
stability.