Dual effects of climatic factors and grazing on vegetation diversity
This study analyzed the interactive effects of grazing and climate on grassland vegetation diversity in arid and semi-arid areas through vegetation survey. The results (Figure 3, Figure 4, Figure 5, and Figure 6) supported that grazing had an impact on seasonal differences in vegetation diversity in arid areas. In general, there was no significant difference in vegetation diversity between rainy season and dry season (P >0.05), which indicated that the effect of seasonal change on vegetation diversity was seasonal but had no significant effect. Survey studies have shown that there is no significant seasonal difference in annual plant growth in arid and semi-arid areas, and some studies have shown that grazing intensity has a significant effect on annual plant germination and growth. Comprehensive analysis of seasonal differences of different grazing intensities and vegetation showed (Figure 5, Figure 6) that seasonal differences of vegetation diversity were the largest in moderate grazing intensities (2 and 3), but smaller in low grazing intensities (0 and 1) and high grazing intensities (4). In addition, NMDS analysis results (Figure 3 and Figure 4) also show seasonal differences.
Grazing activity may change the effect of precipitation on vegetation diversity. Precipitation in rainy season may be the key climatic factor affecting vegetation diversity in arid and semi-arid regions. In arid and semi-arid regions, precipitation, especially during the growing season, is considered a greater limiting factor for vegetation growth than temperature (Bao et al., 2014; Rishmawi et al., 2016). In addition, there were seasonal differences in the impact of precipitation on vegetation diversity in the five regions investigated. The variation characteristics of vegetation diversity on the precipitation gradient in the rainy season were significantly different from those in the dry season, and this difference was particularly prominent at low grazing intensity (0, 1 and 2) (Figure 1 and Figure 6). Low grazing intensity in grassland area means low intervention state, representing the original state of vegetation. The variation of vegetation diversity with precipitation in the rainy season is larger, which means that the difference of vegetation diversity between arid and semi-arid regions may be mainly caused by the difference of precipitation in the rainy season. Vegetation diversity was positively correlated with RP at low grazing intensity (0 and 1), but negatively correlated with RP at medium and high grazing intensity (2 and 3). In arid areas, the relationship between vegetation and water resources will be disturbed by high-intensity grazing activities, and the opposite rule will appear.
The effects of grazing on vegetation diversity were also different under different climate models. Except for SP, the variation trend and amplitude of vegetation diversity under the grazing gradient did not show significant differences between the dry season and the rainy season, indicating that there may be no obvious seasonal difference in the impact of grazing on vegetation diversity (Figure 2). The vegetation diversity of the four regions (HL, DL, EE, WA) decreased with the increase of grazing intensity in both dry and rainy seasons, but the vegetation diversity of SP (desert steppe, warm and dry climate) increased significantly with the increase of grazing intensity in rainy season. At medium and high grazing intensity, the three diversity indexes in the five regions increased with the increase of grazing intensity, and the trend of increasing was most obvious in SP. However, increased diversity does not necessarily mean increased stability of plant communities. The coverage of low-height plants was also proportional to grazing intensity in warm and dry climates and was higher than that in humid climates (Figure 7b). These results indicated that high-intensity grazing in warm and dry climate would significantly weaken the dominance of plants with high plant height and enhance the competitiveness of plants with low plant height, thus increasing the diversity, but the aboveground biomass might decrease accordingly. Our results showed that grazing had a greater impact on the dominant patterns of plant communities under drier and warmer climate patterns.