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.