DISCUSSION
Many terrestrial ecosystems are
increasingly experiencing drought events. However, despite much
research, a general understanding of mechanisms underlying ecosystem
resistance to drought and recovery after drought remains elusive. This
is at least, in part, due to the focus on the functional resistance and
recovery in previous studies of ecological stability, which have paid
relatively little attention to structural resistance and recovery.
Elucidating how ecosystems respond to drought is becoming particularly
pressing when these ecosystems are simultaneously undergoing chronic
environmental changes (e.g., increased N deposition), which may have the
potential to alter ecosystem responses to drought (Xu et al., 2014; Shi
et al., 2018). Using a manipulative field experiment conducted in a
temperate semiarid grassland, our study showed that N enrichment and
mowing had strikingly different effects on grassland biomass and
structural resistance/recovery. Specifically, nitrogen addition resulted
in reduced biomass resistance and increased biomass recovery, whereas
annual mowing resulted in reduced structural resistance and increased
structural recovery. Importantly, our study demonstrated that dominant
species resistance/recovery, rather than species diversity, largely
modulated the effects of nitrogen enrichment and mowing on both
functional and structural resistance/recovery of our study grassland.
An appreciable number of studies have shown that nitrogen enrichment
often reduces grassland biomass temporal stability (Yang et al., 2012;
Hautier et al., 2014; Hautier et al., 2015; Song & Yu, 2015; Xu et al.,
2015b; Zhang et al., 2016; Zhang et al., 2019; Hautier et al., 2020; but
see Grman et al., 2010). Much less, however, is known about how nitrogen
enrichment influences grassland biomass resistance to disturbance (Xu et
al., 2014; Shi et al., 2018) and recovery after disturbance (Kinugasa et
al., 2012; Xu et al., 2014). Our study showed that nitrogen addition
decreased plant community biomass resistance to drought, but increased
biomass recovery after drought. As nitrogen addition resulted in
increased plant community biomass, indicating nitrogen limitation in our
study grassland, this opposite effect of nutrient addition on biomass
resistance and recovery also means that more productive communities lost
more biomass during drought but recovered more biomass after drought
(Fig. S7). The strong negative relationship between biomass resistance
and pre-draught community biomass (r 2 = 0.918,P < 0.001) lends strong support to the hypothesis that
drought-induced plant biomass reduction is sensitive to pre-drought
community biomass (the biomass-dependent resistance hypothesis; Wang et
al., 2007). This result is also consistent with the idea that more
fertile grasslands should be more responsive to climate change (Grime et
al., 2000). This result arose presumably because of increasing water
shortage associated with communities that attained greater biomass, and
therefore, increased evapotranspiration, following N addition (Friedrich
et al., 2012; Valliere et al., 2017; Shi et al., 2018). More
mechanistically, we found that nitrogen enrichment reduced community
biomass resistance mainly by exacerbating the biomass loss of L.
chinensis during drought. L. chinensis is a perennial
rhizomatous grass with relatively high foliar nitrogen content (Cui et
al., 2010), and nitrogen enrichment tends to increase its abundance,
stature and root biomass (Pan et al., 2005; Bai et al., 2009). As the
most dominant species in our study grassland, L. chinensis plants
are known to show reduced stomatal conductance and photosynthesis, and
in turn, reduced biomass production when experiencing drought (Xu &
Zhou, 2006), especially under nitrogen enrichment (Shi et al., 2018).L. chinensis plants are also known to allocate more of their
biomass into their belowground components when experiencing drought (Xu
& Zhou, 2006), particularly under nitrogen enrichment (Shi et al.,
2018), presumably as an adaptive strategy to maximize water intake. This
strategy may have allowed L. chinensis to quickly expand its
clonal reproduction when more water becomes available, contributing to
increased community biomass recovery in the nitrogen addition plots
after drought.
Compared with nitrogen deposition,
we know relatively little about how mowing influences grassland
stability. The few studies that examined the effect of mowing on
grassland biomass temporal stability have produced mixed results (Yang
et al., 2012; Zhang et al., 2017).
Even less is known about how
mowing influences grassland resistance and recovery. The only study on
this topic, to our knowledge, examined the response of experimental
grasslands to experimentally induced drought under mowing of different
frequency, reporting that grassland biomass resistance declined with
increasing mowing frequency (Vogel et al., 2012). However, our study
found that mowing promoted community biomass resistance, primarily via
enhancing the biomass resistance of L. chinensis (Fig. 3a). The
fact that mowing reduced community and dominant species biomass and made
our study grassland less vulnerable to drought is again consistent with
the biomass-dependent resistance hypothesis (Wang et al., 2007). The
apparent discrepancy between the results of our study and Vogel et al.
(2012) may be explained by the difference in mowing frequency between
the two studies. Whereas mowing was implemented once per year in our
experiment, it was implemented two or four times a year in the
experiment of Vogel et al. (2012), with only the more frequent mowing
treatment diminishing biomass accumulation and reducing community
biomass resistance to drought in the latter study. Note that increased
species asynchrony under mowing (manifested in the SEM as an indirect
effect of mowing via L. chinensis biomass resistance) also
contributed to community biomass resistance (Fig. 3a). Mechanistically,
mowing reduced the abundance of dominant, drought-sensitive species
(i.e., L. chinensis ), which offered greater opportunities for
species that were less abundant but more tolerant of drought (e.g.,C. duriuscula, S. baicalensis , Thermoposis lanceolata ) to
compensate for its biomass loss under drought, leading to greater
asynchrony.
One of our most important results is that only mowing altered grassland
structural stability, resulting in reduced grassland structural
resistance and increased grassland structural recovery. Notably, these
effects of mowing on community structural stability were largely
mediated via dominant species structural stability, which was strongly
associated with community structural stability (Fig. 3). Importantly,
mowing had opposite effects on L. chinensis biomass stability and
dominant species structural stability. Mowing reduced dominant
structural resistance despite its positive effect on L. chinensisbiomass resistance, as three of the other four dominant species
exhibited the trend of declining resistance to drought under mowing
(Fig. S8a-d). On the other hand, mowing increased dominant structural
recovery despite its negative effect on L. chinensis biomass
recovery, as all the other four dominant species exhibited the trend of
increased recovery from drought under mowing (all P <
0.001; Fig. S8e-h). These results correspond to the fact that L.
chinensis , as the most dominant species, imposed negative effects on
the other dominant species (Fig. S9) but suffered substantial biomass
loss from mowing (Fig. S5), with ensuing consequences for the abundance
of other species and the structural stability of our study communities.
Previous work has shown that mowing reduces the dominance of L.
chinensi s by hampering the formation of regeneration buds and the
propagation of rhizomes (Yang et al., 1995). Our results are consistent
with previous studies reporting that mowing tended to reduce the
abundance of dominant species, resulting in increased light availability
(Molina et al., 2021) and, in turn, increases in grassland plant
diversity (Leps, 2014; Yang et al., 2019; Molina et al., 2021). Our
study, however, focused on examining the effects of mowing on community
resistance and recovery, demonstrating the importance of dominant
species for driving the reassembly of communities experiencing drought.
Overall, our results point to the importance of dominant species for
determining both biomass and structural stability of our study grassland
experiencing drought. Species richness was not identified as a
significant predictor of grassland biomass or structural stability (Fig.
3). We attributed these results to the predominant role of dominant
species in contributing to plant biomass production and driving plant
community assembly in our study grassland. In our experiment, the most
dominant species, L. chinensis, accounted for almost half of
grassland aboveground biomass. Its response to drought not only largely
drove grassland biomass resistance and recovery, but also imposed
substantial influences on the responses of other species (including
other dominant species) to drought, with subsequent influences on
community structural resistance and recovery. Our results are thus
consistent with the findings of several previous studies that plant
community functional resistance and recovery are modulated by the traits
of dominant species (Macgillivray et al., 1995; DeClerck et al., 2006;
Hoover et al., 2014). Together, these results provide strong support for
the mass ratio hypothesis that properties of an ecosystem are largely
determined by its dominant species, rather than species diversity
(Grime, 1998). Moreover, our finding that the response of dominant
species to disturbance also strongly influenced the reassembly of
communities undergoing disturbance suggests that the mass ratio
hypothesis may also be applied at the community level. We suggest that
elucidating traits that regulate dominant species resistance to
disturbance and recovery from disturbance, such as the pre-draught
biomass as identified here, are particularly important for predicting
both community and ecosystem responses to increasingly frequent
disturbance events.
A recent meta-analysis (Hillebrand & Kunze, 2020) reported that
functional and structural stability of ecological communities tended to
be positively correlated, such that functional recovery of communities
following disturbance is more complete with greater structural recovery.
In line with this trend, we found that the recovery of the structure of
our study grassland promoted its biomass recovery (Fig. 3b; Fig. S6b).
This pattern emerged as the recovery in the biomass of other species
complemented that of L. chinensis in contributing to community
biomass recovery, particularly under mowing that inhibited L.
chinensis biomass accumulation. The meta-analysis of Hillebrand and
Kunze (2020) also found that for some communities, functional recovery
was possible in the absence of structural recovery, a pattern
attributable to the functional redundancy among species (Yachi &
Loreau, 1999; Allison & Martiny, 2008). Likewise, we found that the
biomass resistance of our study communities was decoupled from their
structural resistance (Fig. 3a). This pattern arose as drought
substantially altered grassland community structure, but L.
chinensis remained a predominant contributor to community biomass and
drought-tolerant species partially compensated for the loss of biomass
of drought-intolerant species.