The Impact Of Long-term Grazing Intensity On Functional Groups Richness, Biomass, And Species Diversity In an Inner Mongolian Steppe Grassland

Livestock grazing is one of the major land uses, causing changes in the plant community's structure and grasslands composition. We assessed the effect of grazing intensity on aboveground biomass, species richness, and plant functional group (PFG) diversity in a temperature meadow steppe in Hulunbuir in northern China, involving 78 plant species from eight functional groups. Four grazing intensity classes were characterized, including light, moderate, heavy, and no grazing, based on stocking rates of 0.23, 0.46, 0.92, and 0.00 animal units per hectare. Our results show that the richness of short species, including perennial short grass, perennial short grass, and legume increased under light to moderate grazing, while no effect of grazing was observed on the richness of shrubs. With increasing grazing intensity, the aboveground biomass of perennial tall grasses and perennial tall forbs decreased significantly, while that of annual/biennial plant functional groups increased. The community diversity and evenness of annual/biennial plants increased significantly with grazing intensity. We concluded that heavy grazing has negative impacts on plant functional group richness and aboveground biomass.


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Livestock grazing is considered one of the primary biotic factors that affect the natural grassland 30 ecosystem functions (Olff and Ritchie 1998). Specifically, species diversity response to grazing intensity 31 varies from location to location (De Bello, et al. 2006). This suggests that the magnitude of grazing intensity The mean maximum temperature in Hulunbuir is measured at 36.17 °C in July, and a minimum 98 temperature of -48.5 °C in January. The annual frost-free period ranges from 85 to 155 days. Sunshine 99 duration accounts for 2650-3000 hours per year. The area receives an average annual precipitation of 350 100 to 400 mm, 80% of which falls between July and September. Chernozem soils are commonly found here.

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The

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Experimental Layout 107 108 The experiment was carried out on 600 hectares of natural grassland, with three grazing intensities and 109 a no-grazing control (NG). The grazing intensities were 0, 0.23, 0.46, and 0.92 animal unit (AU) ha -1 for 110 NG, light grazing (LG), moderate grazing (MG), and heavy grazing (HG), respectively. A 500 kg adult 111 cattle is defined as 1 AU. The NG plot has been fenced and excluded from grazing since 2014. Each 112 treatment plot occupies 5 ha of grassland with three replicates. In total, 12 plots were randomly distributed.

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The experiments were conducted during the grazing seasons (June to October) in 2014-2017. Drinking

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In each treatment plot, five sampling quadrats of 1 m 2 were randomly located. Plants in each quadrat 118 were trimmed to 2.5 cm in height. We divided the harvested species parts trimmed off into eight PFGs: 119 perennial tall grass, perennial short grass, shrubs, legumes, Liliaceae herbs, annual/biennial plants, perennial tall forbs, and perennial short forbs (Ma, et al. 2010). Based on our sampling records, we estimated 121 the species richness. To explore potential compositional changes related to grazing intensity, we also 122 estimated the diversity index of each PFG, defined as the ratio of the rate of the abundance of species 123 belonging to a particular functional group to the rate of occurrence of all the species captured in each quadrat.

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The following formulas were used to calculate alpha diversity (Shannon-Wiener diversity index (H)),

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where RC is the relative coverage of the total species, RD is the relative density and RB is the 133 relative biomass. Gamma diversity, also known as site diversity, is the total species richness of a 134 site (Zhang, et al. 2014).

Data analysis
136 We used a one-way analysis of variance (ANOVA) to examine the changes in species richness 137 and diversity of each PFGs (all plant species) across the grazing intensity treatments using the SAS

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With grazing intensity, the decrease of dominance species was higher in PTG and PTF (3.85%), while 184 the PSG and PSF dominant species increased with grazing intensity 3.85% and 8.97% respectively (Table3).

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Most of the C4 photosynthetic pathway dominant species appeared in PTF (5.13%) and PSF (2.56%). About 186 17.94% of the total grassland species showed a positive reaction to grazing intensity by increasing their 187 aboveground biomass (Table 3), while 16.66% showed a negative response to grazing intensity by 188 decreasing their aboveground biomass.  PTF Schizonepeta multifida 0.17±0.12 b 0.22± 0.07b 1.3± 0.5a 0.97±0.43 a in Hulunbuir grassland is slightly greater than that of α diversity (Fig. 2b and c). The contribution of α and 235 β diversity of the eight PFGs across the grazing intensities to γ diversity was different ( Fig. 2a-2c). Species 236 diversity was higher in PTF and PSF than other functional groups. The high diversity in PTF and PSF was 237 mainly due to the higher number of species richness in these functional groups. We also found a significant 238 difference (p < 0.05) in Pielou evenness of the eight PFGs across the grazing intensities (Fig. 2d). The  richness support this theory. In sum, species richness increased in both LG and MG plots.

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We found that the effect of LG and MG on species richness did not differ and this may be related to  production than other species. This strategy allows for rapid growth and early establishment after the first 274 rainfall and improves grazing tolerance.

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Our results showed that the total aboveground biomass of the PFGs decreased with increasing grazing 276 intensity, and this is consistent with earlier reports (Wang, et al. 2016;Wu, et al. 2014a). Moreover, the 277 plant composition changed due to the differential response of the PFGs to grazing intensity, which is weaker 278 at the community level than for individual species based on aboveground biomass. The change in plant community composition is broadly driven by the decline in the biomass of the palatable species (especially 280 PTG functional groups) and the increase in the aboveground biomass of ABP plants. These results agree 281 with the previous study by (Sternberg, et al. 2015) that an increase in grazing intensity increases the 282 dominant species of Gramineae which comprises of the most palatable species of grasses and forbs.

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Notably, some functional groups increased (e.g., ABP and SHR) while others decreased (e.g., PTG, PTF, Grassland management decisions must, therefore, take seasonal development patterns and grassland 295 succession into account, both of which are closely related to photosynthetic pathways. In Hulunbuir 296 grassland, grazing induces the abundance of C4 species at the early succession stage. This is because the 297 C4 species has the potential to produce a sufficient quantity of seeds that match their high seed dispersal 298 capacity.

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In this study, we found only nine species with the C4 photosynthetic pathway, while the majority of 300 others (67 in total) have C3 pathways. The relatively low quantity of C4 species could be premised on the 301 relative abundance of species within the eight PFGs. Our results suggested that C4 species are common in 302 forbs than other PFGs and this corroborates the earlier report by (Wang 2002) that grazing intensity 303 significantly influenced the abundance of C3 plants in a 4-year trial. Whereas C3 plants decreased with 304 increasing grazing intensity, we found that the C4 plant increased as grazing intensifies. Simultaneously, 305 summer temperatures also increased, providing an alternative explanation for the observed increase in C4 306 plants.

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In Hulunbuir grassland, the structure and composition of the species change over time and space. Our

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The patterns of species grown under different water ecotype can help us to understand the community 315 structure, diversity and productivity of temperate grassland (Fay, et al. 2002;Fowler 1986 (Hickman, et al. 2004).
The contribution of α and β diversity to γ diversity is the basis of understanding the composition of 347 biodiversity (Jost 2007;Zhang, et al. 2014). There are different opinions on the relative importance of α 348 and β diversity as a function of γ diversity. Whereas some researchers hold the view that α diversity is more 349 important, others attached a higher value to β diversity while some groups of scholars believe that the two 350 (i.e., α and β) are of equal importance (Jost 2007;Meynard, et al. 2011). In this study, we found that β 351 diversity contributes to gamma diversity more than α diversity for all PFGs across the grazing intensity and 352 that total diversity is greater in species-rich PFGs (e.g., PTG and PSF) than species-poor PFGs (e.g., SHR

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In this study, various sensitive indices of long-term grazing were set and evaluated. Grazing intensity 368 had a different effect on (change vegetation composition) depends on the productivity and plant functional 369 groups of grassland communities. There was a significant negative effect of grazing intensity on the 370 aboveground biomass (AGB). Therefore, the grazing intensity levels are inverted the grazing pressure from 371 non-grazing to heavy grazing intensity. AGB can be used as a potential sensitive index for meadow 372 productivity. In this study, LG and HG represent the best and worst responses of aboveground biomass to 373 grazing, respectively. We found that the richness of the PFGs only differed for PSG and LILY across the 374 grazing intensities. Perennial short grass species were higher in the LG and MG plots. The PTG showed 375 significantly decreased with grazing intensity, while PSG increased with grazing intensity. Also, this study 376 showed that the effect of grazing density on grassland diversity was consistently higher in tall herbaceous
The results of this study considerably contribute to the sustainable management of grassland resources 379 in the study area. In addition, the results provide a perspective for evaluating current grazing management 380 scenarios and carrying out timely adaptive practices, so as to maintain the ability of the grassland system 381 to perform its ecological functions in the long term.

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Acknowledgments 391 We thank the staff of Hulunbuir Grassland Ecosystem Research Station for their assistance during field 392 sampling, and our colleagues from the Agricultural Resources and Regional Planning Institute for helping 393 us to make these figures.

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The authors declare no conflict of interest.