Impacts of Livestock Grazing on the Steppe Plant Community of Eastern Shore Valleys of Lake Hovsgol, Northern Mongolia
Steppe plant communities were investigated along a livestock grazing gradient in six eastern shore valleys of Lake Hovsgol, Northern Mongolia. The goal of this study is to describe the steppe vegetation in four strata of a semiarid mountain landscape: south-facing upper and lower steppe, riparian zone and north-facing lower steppe with the goal of examining how livestock grazing, slope, and aspect influence plant community structure and composition. This region has a long history of grazing.
Not only the six study valleys along the livestock grazing differed in plant community structure, composition and biomass but also plant communities along the landscape strata within one valley were different. There were two different trends of responses of plant species richness and dominance as apparent responses to livestock grazing. Plant species richness decreased in both south-facing upper and lower steppe, but increased in north-facing lower steppe and riparian zones of the heavily grazed valleys. The opposite pattern occurred in species dominance. The dominance of grazing tolerant and unpalatable species increased in south-facing steppe.
Both the north facing steppe and riparian zone of ungrazed valleys are narrow ecological habitats for plants because they have a high soil moisture content, low soil temperature due to the wide-spread distribution of permafrost and receiving less direct solar radiation than the south facing steppe. Soil moisture is 60-70% and soil temperature is 5-6°C in the north facing lower steppe, 40-50% and 7-8°C in riparian zone of ungrazed valleys; in contrast, 10-20% and 15-20°C in north facing lower steppe, 30-40% and 10-12°C in the riparian zone of heavily grazed valleys. This narrow ecological condition contributes to the dominance of moss and cool season species of sedges that are characteristic of vegetation in permafrost distributed regions with no large effect of herbivores. Heavy grazing by livestock in the riparian zone and north facing steppe appears low soil moisture and high soil temperature, this condition prohibits the high dominance in growth of the moss and cool season sedges and supports the growth of the other forbs.
Intensive livestock grazing can graze unselectively and create widespread erosive, detrimental soil disturbances, leaving only a few tolerant plant species. In both south facing upper and lower steppe of the heavily grazed valleys abundance of unpalatable species Thymus gobicus, Potentilla acaulis and Artemisia frigida were increased and constituted more than half percent of the total green plant biomass of the communities. All the dominant species are high resistance to grazing. In riparian zone Carex duriuscula was dominant and Leymus chinensis and Potentilla anserina were codominants where as Festuca lenensis, Koeleria macrantha, Artemisia commutata, Hierochloe sp. and Helictrotrichon schellianum were dominated in north facing lower steppe of the heavily grazed valleys. All the species are xerophytic and highly palatable grasses to sheep and goat.
In general canopy cover of green plants of south facing steppe is high in moderately grazed, medium in ungrazed and low in heavily grazed valleys. The largest differences in canopy cover were observed for litter and bare ground. Bare ground was much greater on south facing upper and lower steppe of the heavily grazed valleys compared to ungrazed valleys. We expected that lower soil moisture and higher soil temperature in south facing upper and lower steppe of the heavily grazed valleys compared with the ungrazed valleys relating with the increasing of bare ground cover because major losses of soil water occur via bare soil evaporation in arid and semiarid region. Also the increasing bare soil cover may effect on this ecosystem nutrient cycling; soils under plants had consistently higher total and microbial C and N levels than soils without plant cover and it leads plant-induced soil heterogeneity in arid and semiarid areas. Plant-induced soil heterogeneity, initiated by grazing appears ecosystem degradation and desertification. The opposite pattern of the increasing of bare ground cover occurred in litter cover. Litter cover was 40-45% of total cover in south facing lower steppe of ungrazed valley whereas it was lower by 3 times in the lower steppe of Turag. The decreasing of litter cover results in decreasing of soil moisture, and carbon limitation to soil microbial communities, this limitation results in lower N immobilization and higher plant available nitrogen pools, there is also greater potential for nitrogen loss from these pools by leaching and gaseous loss pathways.
In the ungrazed valleys riparian zone and north facing lower steppe are more productive due to moss biomass than other parts of the valleys. Livestock grazing was resulted in decreasing of total aboveground live plant biomass of these landscape positions; it was lower by 10 times. Also there was very clear negative effect of livestock grazing on litter buildup in all landscape positions.
All of our indicators demonstrate that with heavy grazing, steppe soils have less soil moisture to support plant growth; this is also associated with a shift to xerophytic plants. Areas with reduced grazing levels have a greater dominance of grasses and sedges, much greater dead plant tissue or necromass covering the soil surface and this may be protecting soil moisture, reducing losses to evaporation. For the north facing slope and riparian zone where is a shallow active layer above permafrost, livestock grazing may appear to cause the loss of moss cover, a critically important insulation blanket for permafrost. The loss of the main insulators of soils due to heavy grazing may appear to also be associated with permafrost degradation.