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.