The Effects And Regulation Mechanisms Of Grassland Patchy Degradation And Herbivore Grazing On Plant Diversity

Grasslands are the most widespread terrestrial ecosystem on earth,supporting high biodiversity and providing important ecosystem services.Due to the influence of human activities such as long-term livestock grazing and increasing climate change,roughly 70% of grasslands in China have been degraded to some extent,which has brought serious threats to grassland biodiversity and ecosystem services.Therefore,understanding the mechanism of plant diversity changes during grassland degradation and the effects of livestock grazing are of great scientific significance for protecting grassland biodiversity and restoring degraded grasslands.Grassland degradation commonly exhibits obvious features of patchy degradation,which divides the intact grassland into many small original vegetation patches isolated by degraded patches,resulting in habitat loss and fragmentation.Although numerous studies have been carried out at home and abroad on the impact of grassland degradation on plant diversity,the effects of this typical feature of grassland degradation(patchy degradation)on plant community assembly processes and diversity have been ignored,limiting our understanding of the mechanism of biodiversity changes in degraded grasslands.Grazing is one of the main land uses for grasslands and a key determinant of grassland plant species composition and diversity.However,it is unclear whether the effects of grazing on plant communities depend on grassland patchy degradation.Therefore,this study took the spatial characteristics of grassland patchy degradation as the entry point.Based on community assembly theory,metacommunity theory,and island biogeography theory,We conducted the study on the landscape scale of patchy degraded grassland and the patch scale of fragmented original vegetation,and integrated plant diversity characteristics of different dimensions(species diversity,functional diversity)and scales(α,β,γ).This study,thus,comprehensively and deeply analyzed the mechanism of influence of grassland patchy degradation on plant diversity,as well as the further regulation mechanisms on livestock grazing effect,hoping to offer new insights into changes in plant diversity in degraded grasslands.In this study,we selected typical non-degraded Leymus chinensis grasslands and patchy degraded grasslands in the Songnen Plain in northeast China.We examined the effects of grassland patchy degradation and different types of livestock grazing(cattle,sheep)on plant diversity through in situ and long-term controlled grazing experiments,and further analyzed community assembly mechanism by null model.We obtained the main findings as follows:(1)Based on the study about the impact of grassland patchy degradation on landscape-scale plant species diversity,we found that grassland patchy degradation had no significant effect on plant α diversity,but increased the spatial heterogeneity of plant communities(β diversity),and thus increased plant γ diversity.Based on null model analysis,we found that grassland patchy degradation affected the intensity of the influence of species interaction and environmental filtering(two main deterministic processes of community assembly)on plant communities.Specifically,patchy degradation reduced the dominance of L.chinensis,the dominant species of grasslands,and relaxed competition pressures on other non-dominant species,thus weakening the role of species interaction over plant communities.Instead,patchy degradation added the role of environmental filtering over plant communities via increasing soil environmental heterogeneity.The changes in community assembly processes induced by patchy degradation improved the spatial heterogeneity of plant communities and thus increased plant γ diversity.(2)Based on the study about the effects of grassland patchy degradation on landscape-scale plant functional diversity,we found that grassland patchy degradation significantly increased the community-weighted mean of plant height and specific leaf area,but decreased the community-weighted mean of leaf dry matter content and leaf carbon content.The changes in plant community functional traits were mainly due to species turnover caused by patchy degradation.During the process of patchy degradation,the relative abundance of dominant species L.chinensis with more resource-conservative strategies decreased,while that of annual and biennial plants and perennial forbs with more resource-acquisitive strategies increased,which shifted the functional composition of the community toward acquisitive strategies.Consistent with the results of species diversity,patchy degradation had no effect on functional α diversity but significantly increased the plant functional β diversity and the interpretation of environmental variables for its variation.These results indicate that patchy degradation did not change the niche differentiation of coexisting species within local communities but significantly improved the spatial niche differentiation of coexisting species between local communities,mainly through enhancing the role of environmental filtering.(3)Based on the study about the effect of fragmented original vegetation patch size on plant diversity,we found that the total species diversity(γ diversity)in the entire patch decreased with decreasing patch size,which showed a positive island species-area relationship.According to the island biogeography theory,this change may be due to sampling effects.This positive relationship remained true after controlling for sampling effects by rarefying all patches to an equal number of plant individuals or quadrats.This suggests that smaller patches contained fewer plant species not only because of sampling effects but also because of other mechanisms.By dissecting the distribution pattern of within-patch diversity,we found that plant α and β diversity within patches decreased with decreasing patch size.The lower α diversity on smaller patches was driven primarily by the decrease in soil quality within patches,while the lower β diversity was mainly due to the decrease in soil heterogeneity within patches.These results indicate that with the intensification of fragmentation caused by grassland patchy degradation,original vegetation will face a greater risk of species loss.(4)Based on the study about the effects of different types of herbivores on plant species diversity during grassland patchy degradation,we found that patchy degradation changed the effects of herbivore grazing on plant diversity,which depended on the types of grazing animals and the scale of plant diversity.Specifically,patchy degradation did not change the effect of grazing on plant α diversity but significantly changed the effect of grazing on plant βand γ diversity.Moderate grazing of livestock(cattle,sheep)maintained plant γ diversity and increased plant spatial heterogeneity in non-degraded grasslands.However,in patchy degraded grasslands,both cattle and sheep grazing decreased γ diversity,and sheep grazing significantly reduced plant spatial heterogeneity(cattle grazing had no effect).These results suggest that patchy degradation induces negative effects of livestock grazing,especially sheep grazing,on grassland plant diversity.Further analysis of community assembly mechanisms showed that in patchy degraded grasslands,sheep grazing promoted more stochastic plant community assembly and thus decreased β diversity,while cattle grazing maintained deterministic community assembly and thus had no significant effect on β diversity.(5)Based on the study about the effects of different types of herbivores on plant functional diversity during grassland patchy degradation,we found that grassland patchy degradation changed the impact of grazing on plant functional diversity,which depended on the species of grazing animals and the functional diversity indicators considered.Results of the community-weighted mean calculated by a single trait showed that grazing caused a shift of plant communities towards more acquisitive strategies,especially sheep grazing.This may be because grazing not only promoted the growth of acquisitive plant species by changing species turnover but also promoted a more resource-acquisitive strategy of plant individuals by changing intraspecific trait variability.In addition,grazing plays a stronger role in regulating plant community functional traits than patchy degradation.The results of functional diversity calculated by multi-traits showed that in non-degraded grasslands,sheep grazing significantly increased functional α diversity,while cattle grazing did not affect functional β diversity.Grazing significantly increased functional β diversity.In the patchy degraded grasslands,grazing had no significant effect on functional α diversity.Sheep grazing significantly decreased functional β diversity,while cattle grazing significantly increased functional β diversity.Plant functional diversity,compared with species diversity,was more sensitive to grazing,and grazing had a stronger effect on the spatial niche differentiation of coexisting species between local communities.In conclusion,habitat fragmentation caused by grassland patchy degradation at the landscape scale enhanced the plant spatial heterogeneity(β diversity)by increasing the role of heterogeneous environmental filtering thus higher plant γ diversity,and also promoted a shift of plant communities towards more acquisitive strategies.On the patch scale of fragmented original vegetation,the decrease in patch area reduced plant α and β diversity within patches by decreasing habitat quality and heterogeneity and thus reduced the total plant diversity(γdiversity).Patchy degradation caused negative effects of livestock grazing,especially sheep grazing,on grassland plant diversity.Herbivore grazing promoted more stochastic plant community assembly,as well as plant communities with more acquisitive strategies.This study indicates that in addition to focusing on the characteristics of degradation gradient,the characteristics of patchy degradation are also particularly important for the study of grassland degradation,and research should be carried out from multiple spatial scales(landscape scale,patch scale).Maintaining habitat diversity of patchy degraded grasslands at landscape scale,as well as preventing habitat degradation and maintaining large patches of original vegetation at patch scale would be effective strategies for reducing the extinction risk of plants.This study also suggests that,from the perspective of biodiversity protection,grassland grazing management needs to distinguish grazing animal species according to grassland health status.In non-degraded grasslands,moderate grazing of livestock(cattle,sheep)is conducive to maintaining and improving plant diversity.However,with the aggravation of grassland patchy degradation,livestock grazing,especially sheep grazing with small body sizes,should be managed more carefully.