Mechanisms Of Grazing Livestock Species And Intensity Affecting Plant Community Structure And Grass And Livestock Production In A Typical Steppe Of Inner Mongolia

Grassland is one of the largest terrestrial ecosystems in the world,and livestock grazing is the most important land use in the grassland region.In recent decades,the livestock overgrazing and climate change have induced increasingly severe grassland degradation,especially in semiarid grassland regions.Maintaining the grassland ecosystems health,and enhancing their ecological regulation and livestock production capacity,are the major issues in grassland science and management for the sustainability of grassland ecosystems.To investigate the response of grassland plant community structure and productivity,as well as livestock production performance to various grazing management under the background of global climate change is the prerequisites for understanding the impact of future climate and land use changes on grassland ecology and production,and develop the adaptive management strategies for controlling grassland degradation,improving grassland ecological environment and grassland livestock production.In the typical steppe region in the Xilingol league of Inner Mongolia,we established a grazing experimental to investigate the effects of sheep grazing at four intensities(no grazing control,light grazing,moderate grazing,and heavy grazing)(Experiment 1)and the effects of three livestock species(sheep,goats,cattle,no grazing control)at a moderate grazing intensity(Experiment 2)on grassland plant community structure and grassland-livestock production capacity.The typical steppe grassland is dominated by Leymus chinensis and Stipa grandis.We aimed to reveal the effects and mechanisms of grazing livestock species and intensity and their interaction with climate fluctuations on grassland community structure and productivity,as well as livestock performance.We obtained following main results based on this grazing experiment over six years.1.Grazing at all intensities significantly reduced the height and biomass of grassland plant communities,and their proportion in total biomass,but increased plant density;and the reduction in community height and residual biomass and the increase in plant density significantly increased with grazing intensity.2.Grazing altered the species composition of the communities.The grazing intensity experiment showed that nil or light grazing by sheep increased the relative biomass of L.chinensis,while moderate grazing increased the relative biomass of S.grandis and Cleistogenes squarrosa,and heavy grazing increased the relative biomass of C.squarrosa and subordinate species(including small amounts of forb and annual and biennial species).The livestock grazing experiment showed that sheep grazing increased the relative biomass of S.grandis and C.squarrosa,while goat grazing increased the relative biomass of L.chinensis and S.grandis,and cattle grazing increased the relative biomass of C.squarrosa.Sheep heavy grazing and moderate grazing by three livestock species all increased the species richness of plant communities.3.Within the range of grazing intensity studied,grazing significantly improved the aboveground net primary productivity(ANPP)of grassland communities by promoting plant over-compensatory growth,with L.chinensis being the plant with the strongest over-compensatory growth after defoliation;the mixed subordinate species in the community also showed an over-compensatory growth under heavy grazing.Grazing at all studied intensities significantly increased the ANPP of grassland communities,but moderate and heavy grazing significantly reduced the belowground biomass of plants(BGB)and its proportion in the total biomass,indicating that the aboveground over-compensatory growth of plants caused by moderate and heavy grazing might be at least partly at the expense of sacrificing the BGB,so the compensatory growth mechanism and sustainability after grazing still require longer-term research.Due to difference in the feeding habits of different livestock species,the compensatory growth of grassland plants differed among livestock species.4.Grazing intensity and precipitation interactively determined grassland productivity.In dry years,grazing at light and moderate intensity improved the grassland productivity;while in wet years,grazing at all the three intensities promoted grassland productivity.The inter-annual precipitation fluctuation was the major factor driving the grassland productivity dynamics,and an increase in precipitation could weaken the impact of grazing intensity on grassland productivity.Plant compensatory growth after grazing increased first and then decreased with the increase in grazing intensity.The grazing intensity for achieving the maximum compensatory growth is high in the year with high precipitation and plant productivity.5.The seed bank density in the 0-10 cm soil layer of the typical steppe soil increased significantly with increasing grazing intensity.Moderate and heavy grazing significantly reduced the seed bank density in the surface 0-5 cm layer of soil,but significantly increased the seed bank density in the 5-10 cm layer of soil.The moderate and heavy grazing significantly increased the proportion of annual and biennial plant seed bank density,while reduced that of perennial grasses.Within the range of grazing intensity studied,the similarity between aboveground vegetation and soil seed bank increased with increasing grazing intensity.The total density of the plant community and the density of subordinate species increased with increasing grazing intensity,along with the increase of the seed bank density in the soil.In addition,grazing increased species richness and Shannon-Wiener index of the soil seed bank,but reduced evenness index.6.Changes in plant community composition due to grazing intensity and livestock species were mainly a consequence of the forage selection of different livestock species under different grazing intensities.Cattle preferred to eat S.grandis and mixed subordinate species,goats preferred to eat C.korshinskyi and C.squarrosa,and sheep preferred to eat L.chinensis and mixed subordinate species.Under different grazing intensities,the feeding habits of sheep were affected by both the quantity and nutritional quality of vegetation,depending on the availability of vegetation.Under light grazing,sheep preferred to eat L.chinensis and C.korshinskyi,under moderate grazing they preferred to eat L.chinensis and mixed subordinate species;and under heavy grazing they preferred to eat L.chinensis,S.grandis,C.squarrosa,and mixed subordinate species.Feeding preferences were the main cause of changes in plant community structure,and the selection indices of sheep to various plant species was negatively correlated with the relative biomass of these species.7.No significant difference were detected in daily feed intake per sheep,but feed intake per hectare increased,with increasing grazing intensity.Live-weight gain(LWG)per livestock and grass-livestock conversion efficiency decreased,and LWG per hectare first increases and then decreased on grazing gradients,with the optimal LWG per livestock and per hectare achieved under moderate grazing.In conclusion,with the adaptive grazing management in our experiment,moderate grazing achieved the best grassland plant production and livestock productivity.However,the moderate grazing intensity varies,in terms of stocking rate,across the years in our adaptive management,depending on the grassland conditions that are mainly driven by precipitation.In summary,this six-year grazing experiment comprehensively revealed the effects of grazing intensity and livestock species on the community structure and productivity,and livestock performance in typical steppe grasslands in Inner Mongolia.The results provide insights into the interaction processes and mechanisms between herbivores and vegetation,and the interactive mechanisms of precipitation and grazing intensity affecting grassland community structure and productivity.These findings provide an important bas for the development of sustainable grassland management systems to adapt the future climate and land-use change.