DISCUSSION
Climate change, including increased temperature and variability in
precipitation as well as changed distribution patterns of temperature or
precipitation, have had an important impact on community composition and
species dynamics of the Inner Mongolia temperate grassland over the past
several decades (Ma et al., 2010; Zhang et al., 2020), which may affect
the stability of community biomass. In our study, based on 29 years of
field observation data, we found that the functional group perennial
rhizome grass biomass intra-annual stability, dominant species
intra-annual biomass stability and intra-annual species asynchrony
contributed to stabilize community intra-annual biomass. In addition,
decreased accumulated precipitation from June to September
(P6-9) decreased community intra-annual stability by
reducing intra-annual species asynchrony.
Our findings are consistent with previous studies that have reported
that temperate grasslands in Inner Mongolia have experienced warming and
drying over the past few decades (Ma et al., 2010; Li et al., 2015).
Notably, we found that seasonal temperature and precipitation changes
were more pronounced than inter-annual changes. However, we found no
relationships between seasonal warming and community biomass
intra-annual stability, which is consistent with results of climate
control experiments on temperate grassland and alpine meadow (Ma et al.,
2017; Yang et al., 2017). In contrast, we found evidence that decreased
seasonal precipitation affects the stability of community biomass. This
is consistent with previous studies showing that seasonal precipitation
affects the community structure and productivity of grassland ecosystem
(Bai et al., 2004; Guo et al., 2012; Robinson et al., 2013; Chen et al.,
2020; Zhang et al., 2020). Consistent with previous reports, our results
indicate that seasonal precipitation reduced community stability by
reducing species asynchrony (Xu et al., 2015; Ma et al., 2017).
Several field observations and theoretical models have suggested that
community stability may increase with increasing species richness (Jiang
et al., 2009; Gross et al., 2014; Mougi et al., 2012). Our results are
consistent with the positive diversity-stability relationship often
reported in experimental studies. In the present study, although linear
regression did not show a relationship between species richness and
community intra-annual stability (Fig. 6g), SEM showed that species
richness indirectly contributed to community intra-annual stability
through a positive relationship between dominant species stability (Fig.
7). None of the diversity-dependent stabilizing mechanisms, including
overyielding (Fig. S4a), diversity-dependent species asynchrony (Fig.
S4b) and the portfolio effect (Fig. S4c), operated. In experiments
manipulating species diversity, overyielding (i.e., positive
diversity-productivity relationships) often contributes to stabilize
community biomass through time (Cardinale et al., 2006; Gross et al.,
2014). But productivity does not necessarily increase with diversity in
natural communities, especially in long-term observations like ours
(Adler et al., 2011; Grace et al., 2007). SEM results showed that
precipitation in July (P7) could promote community
species richness, which is consistent with previous studies (Adler and
Levine 2007; Cleland et al., 2013). Increased precipitation benefits
shallow-rooted species that are disadvantaged in dry soils, thereby
promoting species richness, especially in July when plant growth was
strongest (Yang et al., 2011a). The species richness of functional
groups PB, PF and AB was also positively correlated with precipitation
in July (Table 1). In addition,
seasonal warming (T8 and T6-8)
negatively affected the species richness of community and functional
group PF (Table 1). We found a decreasing trend in community species
richness during the 30-year fenced period (Fig. 4a), which may be
attributed to the moderate disturbance hypothesis (i.e., under fenced
conditions, competition between species for resources increased, and
rare species richness decreased, leading to a decrease in community
species richness) (Herrero and Oesterheld 2018;
Wang et al., 2020a). In addition,
the seasonal decrease in precipitation may be another reason for the
decrease of community species richness (Fig. 1) (Robertson et al., 2010;
Adler and Levine 2007).
Ecosystems are largely controlled by the characteristics of the dominant
species, i.e., the mass ratio hypothesis (Grime, 1998), which may even
constrain the effect of species diversity on biomass stability (Polley
et al., 2007). Many experimental studies have shown that when dominant
species regulate community stability, the diversity-stability
relationship is weak or insignificant (Grime 1998; Grman et al., 2010;
Wang et al., 2020b). In our study, we found support that dominant
species stability largely contributes to community intra-annual
stability (Sasaki et al., 2011; Wilsey
et al., 2014;
Xu
et al., 2015). It is noteworthy that seasonal climate warming and
precipitation changes did not affect the dominant species stability
(Table 1). In our study area, the dominant species, two perennial
rhizome grasses, Leymus
chinensis and Agropyron cristatum , and two perennial bunchgrass,Stipa grandis and Achnatherum sibiricum , account for
62.9% of community above-ground biomass. Compared with the functional
groups SS and AB with lower relative abundance, the dominant species had
greater biomass stability (Fig. 5). These species are relatively
insensitive to environmental changes, which may be related to their
ability to obtain nutrients through a well-developed root system, and
they have higher plant height and larger specific leaf area, which
allows them to get more light (Yang et al., 2011b; Zhang et al., 2020).
Although functional group PR and functional group PB plants have greater
access to resources, their seasonality in resource use is also evident
(Table 1). The relative biomass of functional group PR was positively
correlated with cumulative precipitation through June, while the
opposite was true for functional group PB. This may be related to their
life history, with functional group PR tending to emerge earlier, having
a larger leaf area and better access to water in the early part of the
growing season (Yang et al., 2011b; Zhang et al., 2018; Zhang et al.,
2020).
Species asynchrony is a common feature of ecological communities
(Gonzalez et al., 2009; Bluthgen et al., 2016), and could be dependent
on asynchronous species responses to environmental fluctuations (Ives et
al., 2007; Loreau et al., 2008). The biomass of each species changes
through time, but the total biomass of the community is retained due to
the increase in biomass of some species compensating for the decrease in
biomass of other species (Ma et al., 2010; Douda et al., 2018). In our
study, the community biomass did not significantly vary over the 29
years investigated (Fig. 2), and species asynchrony determined the
community biomass stability to a great extent (Fig. 7). SEM analysis
showed that precipitation from June to September (P6-9)
weakened species asynchrony (Fig. 7), which is different from previous
studies (Xu et al., 2015; Ma et al., 2017; Chi et al., 2019). This may
be related to the grassland’s seasonal water use strategy. Water is the
main limiting factor of productivity in arid and semi-arid grassland
(Sala et al. 2012; Yiruhan et al., 2014), it creates intense competition
among species for water sources (Yang et al., 2011b). The non-dominant
and rare species in this study area are mainly perennial hybrid grasses
and biennial plants, which mainly take advantage of precipitation in the
late growing season (July to September) (Table 1) (Bai et al., 2004; Li
et al., 2015; Zhang et al., 2020). Adequate moisture may reduce
competition among species for water sources, with both deep-rooted and
shallow-rooted species preferring to utilize shallow soil water,
especially in the late growing season, when moisture promotes the
productivity of flowering forbs and shallow-rooted annuals, such as,Iris tenuifolia , Thalictrum petaloideum , Allium
tenuissimum , Chenopodium aristatum , and Artemisia
sieversiana (Loreau and de Mazancourt 2013; Zhang et al., 2020). When
there is more water in the growing season from June to September,
competition for water between species decreases and plants tend to grow
in sync, resulting in a decrease in asynchrony between species (Zhang et
al., 2018; Wang et al., 2020b), as shown by our SEM results (Fig. 7). In
addition, there was a decreasing trend in accumulated precipitation from
June to September (P6-9) during the observation period
1981-2011 (Fig. 1), which may have contributed to the negative
relationship between the precipitation pattern and species asynchrony.
Seasonal distribution of precipitation may directly provide new evidence
for precipitation and asynchronous dynamics.
An important finding of our study is that functional group biomass
stability is also one of the important contributors to community
intra-annual stability. In simple linear regression, functional group PR
biomass stability was significantly positively correlated with community
stability (Fig. 6a), while functional group PF biomass stability was
significantly negatively correlated with community stability (Fig. 6c).
However, in SEM, the negative effect of PF was excluded and only the
positive effect of PR was retained (Fig. 7). This may be due to the fact
that PR had the highest biomass in the community (34.5%), while PF only
accounted for 14.9% of the community biomass. In addition, there were
two PR species among the four dominant species. PR was the dominant
functional group in this study area, which stabilized community
productivity to a large extent. In contrast, functional group PF was
mainly composed of secondary dominant or rare species, which contributed
relatively little to the stability of community biomass, so it could not
be considered as one of the main factors affecting the stability of
community biomass.
Based on a long-term study spanning 29 years of observation, our study
provides new empirical evidence for ongoing reduced seasonal
precipitation leading to reduced biomass intra-annual stability in the
temperate grassland in North China, which has important theoretical
significance for us to take active measures to deal with climate change.