Effects Of Grazing On Plant Community Diversity And Productivity In An Alpine Grassland On The Tibetan Plateau

Herbivores alter plant community diversity and productivity in many of the world's ecosystems,but the magnitude and the direction of herbivore effects on plant community diversity and productivity vary widely within and among ecosystems.Livestock grazing is the most intensive land-use activity in grasslands,which presents a major driver of shaping the biodiversity,functioning and stability of grasslands.China has made excessive use of the production functions of grassland and ignored its ecological functions in the production of animal husbandry for a long time,resulting in the increasingly serious degradation of grassland and the continuous decline of grassland function.Therefore,how to improve and maintain grassland biodiversity and productivity has become an important research topic.The Qinghai-Tibetan plateau is one of the most important natural pastures in China.In recent decades,the increased intensity of human activities(e.g.,overgrazing)has dramatically caused the loss of biodiversity and the decline of productivity in the alpine ecosystem on the Qinghai-Tibetan plateau.Although several new management strategies have been formulated to address the resulting changes in pasture conditions,there is a lack of demonstration of control experiments on the ecological mechanism behind these management principles,which restricts our scientific management of the alpine grassland grazing ecosystem to a great extent.Therefore,we established a largescale grazing experimental platform in the northeast of Qinghai-Tibetan Plateau and obtained plant coverage and biomass data at the species level by continuously monitoring for over 3 years since 2019.We explore the effects of different grazing intensities on plant community diversity and productivity under four yaks grazing intensities(control,light grazing,moderate grazing and heavy grazing).In addition,a simulated moderate grazing experiment is established to examine changes in plant community diversity and productivity response to mowing,simulated trampling,dung and urine addition and their combinations over 2-years.This paper attempts to explain the potential mechanism of grazing on plant community diversity and productivity in alpine grassland.We found some novel results and got the following conclusions:(1)Compared with the control and heavy grazing,we found that light and moderate grazing intensity significantly increased species richness and Shannon diversity,which was consistent with the prediction of the “moderate interference hypothesis”.On the contrary,the beta diversity was significantly reduced in light grazing and moderate grazing,which was mainly due to the increase of species richness and the enhancement of species dispersal ability among communities under these grazing intensities.In addition,heavy grazing significantly changed the composition of the plant community.Therefore,modest(light and moderate)grazing used in our study area will help to maintain higher species diversity.This finding can provide theoretical support for formulating relevant biodiversity conservation policies in this area under the grazing.(2)In our study period,the total aboveground biomass of the plant community decreased significantly with the increase in grazing intensity.From the perspective of functional groups,the aboveground biomass of grasses and forbs decreased significantly with the increase in grazing intensity,but the aboveground biomass of sedges and legumes did not change in grazing treatment.This is mainly because the sedges have a tolerance to herbivory,while legumes plants(such as Oxytropis and Thermopsis lanceolata)are toxic in the growing season and have not been eaten by yaks.Therefore,when building grazing artificial grassland in this area in the future,sedges and legumes plants should be properly introduced,which will contribute to the sustainable utilization of artificial grassland.(3)In the third year of the experiment,we studied the aboveground and underground net primary productivity of plant communities.we found that the aboveground part of plant community showed overcompensation in light grazing and moderate grazing,but the underground part of plant community showed undercompensation.In addition,the aboveground and underground parts of plant communities under heavy grazing showed exact compensation.However,the total net primary productivity above-and below-ground showed exact compensation.It showed that the growth of aboveground part of plant community caused by grazing may be at the expense of underground part.Over time,grazing may lead to carbon loss in the region.(4)The effects of grazing on root dynamic showed that root standing crop,root production and root mortality decreased significantly in light grazing and heavy grazing,but there was no significant change in moderate grazing.In addition,there was no significant difference in root turnover under different grazing intensities.Interestingly,we found that heavy grazing changed the vertical distribution of plant community roots and tended to allocate more root biomass to the shallow soil layer.This is mainly because grazing changed the composition of plant community,resulting in the decrease of deep root species and the increase of shallow root species.Therefore,this shift in root toward the top soil layers under grazing may affect the stability of soil organic carbon in the subsoil.(5)Simulated moderate grazing had no significant effect on alpha diversity,beta diversity and underground net primary productivity of plant community in the two-year simulation period,but the aboveground net primary production increased significantly by mowing treatment in the second year.The results showed that the effect of grazing on plant diversity was likely to depend on grazing history,and mowing is the main factor controlling the growth of aboveground parts.In conclusion,the modest grazing(light and moderate grazing)plays a positive role in maintaining plant diversity and productivity in alpine grassland during our experimental period.These findings may provide new insights into the biodiversity protection and ecosystem management of an alpine grassland on the Qinghai-Tibetan Plateau.Unfortunately,this study is only the result of three years,and the prediction of the long-term effect of grazing is relatively weak.Therefore,it is still necessary to continuously observe the long-term changes of plant community diversity and productivity in the future,which will help to provide stronger theoretical support for the ecological protection and management of alpine grassland.