The Effects of Grazing Systems on Plant Communities in Steppe Lands—A Case Study From Mongolia’s Pastoralists and Inner Mongolian Settlement Areas

This study selected two adjacent soums (one on each side of the border) with basically the same natural conditions in the border region of typical steppe of Mongolia and China as the study area. We employed quadrats sampling method and remote sensing to set three perpendicular lines that dissect the two countries’ boundary and seven lines parallel to the boundary to form a rectangle shape as a means to compare plant community response to different grazing systems under natural conditions. Then, we discussed the reasons for the degradation of Mongolian grassland together with the five periods of remote sensing NDVI data. The results of the quadrat conditions survey and dynamic NDVI survey are as follows: (1) The basic characteristics of plant communities: the values of average height, total coverage and total aboveground biomass reduce from forbidden grazing > rotational grazing > continuous grazing. There are significant differences in average height and total aboveground biomass among the three grazing systems (p<0.05). The total coverage for forbidden grazing and rotational grazing are significantly greater than continuous grazing (p<0.05) but no significant difference between rotational grazing and forbidden grazing. (2) The responses of 10 species with dominance greater than 3% in the entire study area to different grazing systems are analyzed and found that the effects of different grazing systems on different species are significantly different. Though the grassland degradation and adverse succession are very obvious in the study area, the typical steppe is still maintaining the perennial grass-dominated plant communities that are resistant to grazing and drought. S. grandis and L. chinensis are at absolute advantage in forbidden grazing area but their dominance reduces in rotational grazing area where A. frigida and C. duriuscula are at an advantage. After S. grandis degenerated in continuous grazing area, S. krylovii is at advantage. Besides, the dominance of other species that are resistant to grazing and drought such as C. acuminatum, A. polyrhizum, A. tenuissimum and C. squarrosa are obviously increasing in continuous grazing is the highest. (3) There are no significant differences in species richness R, Shannon-Wiener index, Pielou evenness index among the three different grazing systems. This results explains that different grazing systems across the border between China and Mongolia have not had a significant impact on the species diversity of the community. From Simpson indexes of continuous grazing and forbidden grazing which are significantly greater than that of rotational grazing, it can be seen that the diversity is developing towards the direction of significant difference. (4) In areas with different grazing system, the dominance of water-based functional groups and life-form functional groups are significantly different. The aridification phenomenon of continuous grazing and forbidden grazing is significantly greater than rotational grazing; the dominance of xerophytes in continuous grazing and forbidden grazing are significantly greater than rotational grazing (p<0.05); and the dominance of intermediate xerophytes in rotational grazing is significantly greater than that of forbidden grazing and continuous grazing (p<0.05). The dominance of perennial grass in forbidden grazing and continuous grazing are significantly greater than that of rotational grazing (p<0.05) but no significant difference between forbidden grazing and continuous grazing. The dominance of perennial weed in rotational grazing is significantly greater than both forbidden grazing and continuous grazing (p<0.05) but no significant difference between forbidden grazing and continuous grazing. (5) Among the five periods NDVI data, the NDVI of rotational grazing in 1989, 2005, 2011 and 2016 are higher than those of continuous grazing, among which the NDVI of rotational grazing in 2011 and 2016 are significantly higher than those of continuous grazing (p<0.05). The results of forbidden grazing and grazing NDVI dynamics show that forbidden grazing plays a protective role on aboveground biomass and coverage. (6) The calculated result of frequency of the M. Godron’s community stability test shows that the stability reduces from continuous grazing > rotational grazing > forbidden grazing. The calculated result of coverage stability shows that the stability reduces from rotational grazing > continuous grazing > forbidden grazing. Comprehensive analyses show that under the same natural conditions and grazing intensity, the effects of different grazing systems for typical steppe plant communities are significant and to a certain extent, rotational grazing system is better than continuous grazing system.