Effects Of Grazing On Plant Functional Diversity In Northern China

Human-induced global changes have largely threatened the earth ecosystem,causing significant decreases in biodiversity worldwide,which would further affect ecosystem processes and services,and ultimately harm we human beings.Therefore,knowledge on biodiversity changes under global changes is crucial for predicting the structural and functional alterations of ecosystems.Traditional biodiversity research only considers changes in species diversity,ignoring that diverse species can have different functions in the ecosystem.Recently,functional diversity,which focuses on species traits,has gradually flourished and been shown to be able to reflect the response of ecosystems to environmental changes.Grasslands are the largest managed terrestrial ecosystems in China,and livestock grazing is one of the most common land uses across these landscapes.However,livestock overgrazing,one of the most important global changes,has strongly impacted grasslands causing ecosystem degradation.Under such circumstances,it is vital to predict the changes of functional diversity of plant communities in grasslands,for it provides us guidance for management and conservation of grasslands and maintenance of grassland biodiversity.The study focused on two main grassland types — meadow steppe,typical steppe and seven vegetation types in northern China.Combining long-term grazing exclusion experiments,indoor cafeteria trials with analyses of experimental data,we tested the differences in vegetation composition,above-ground leaf traits(specific leaf area,leaf dry matter content,leaf nitrogen content,leaf phosphorus content),below-ground root traits(specific root length,root tissue density,root nitrogen content,root phosphorus content)and soil properties between grazed and non-grazed grasslands.Our study aimed at revealing the effects of grazing on functional traits of dominant species,species diversity,functional trait diversity and functional redundancy in different types of grasslands,and clarifying the potential mechanisms of grazing effects on plant functional diversity in different types of grasslands.The important results and conclusions are as follows:(1)Grazing not only significantly reduced the relative abundance of dominant species,but also changed the functional traits of dominant species.Grazing effects were dependent on types of dominant species and selected functional traits.The responses of functional traits to grazing were similar between S.baicalensis and S.grandis.However,for L.chinensis,the responses of functional traits to grazing were different across sites.The sensitivity of functional traits to grazing was also different.In general,among leaf traits,species leaf area and leaf nitrogen content were sensitive to grazing.Grazing decreased specific leaf area largely because dominant species here are perennial plants,which would adopt grazing avoidance strategies under grazing disturbance.Leaf nitrogen content was increased by grazing,making plants more acquisitive.Different from above-ground traits,all below-ground traits were sensitive to grazing,but grazing effects differed between grassland types.(2)The effects of grazing on species diversity of plant communities were dependent on grassland types.In meadow steppes where species richness was high,species evenness was low,grazing significantly increased plant Shannon-Wiener diversity,Simpson diversity,species richness and evenness.However,in typical steppes where species richness was low,species evenness was high,grazing significantly decreased species evenness,but had no effect on Shannon-Wiener diversity,Simpson diversity and species richness.Furthermore,we found that the paths of grazing effects on species diversity of plant communities were different in different types of grasslands.(3)The effects of grazing on functional trait diversity of plant communities were dependent on grassland types and functional indices.For community weighted mean,the effects of grazing were dependent on functional traits and grassland types.In meadow steppes where above-ground traits were acquisitive,below-ground traits were conservative,grazing made below-ground traits of plant communities more acquisitive;In typical steppes where above-ground traits were conservative,below-ground traits were acquisitive,grazing also made below-ground traits more acquisitive.We also found that the paths of grazing effects on community weighted mean of distinct traits were different.For Rao’s quadratic entropy,grazing effects were also dependent on grassland types.In meadow steppes where Rao’s quadratic entropy was high,grazing had no significant effects.However,in typical steppes where Rao’s quadratic entropy was low,grazing significantly increased Rao’s quadratic entropy.For functional richness,evenness and divergence,grazing only significantly decreased functional divergence in both meadow and typical steppes.(4)For functional redundancy,our studies found that grazing effects were also grassland type-dependent.In meadow steppes where functional redundancy was low,grazing significantly increased plant functional redundancy;In typical steppes where functional redundancy was high,however,grazing significantly decreased plant functional redundancy.As a result,there was higher plant FR in meadow steppes than that in typical steppes across grazed grasslands.We also found that grazing increased plant functional redundancy mainly by improving plant species diversity in meadow steppes,while grazing decreased plant functional redundancy mainly by improving plant functional trait diversity in typical steppes.It is true that grazing decreased soil resources both in meadow and typical steppes,but there were no casual relationships between soil resources and functional redundancy.In conclusion,the effects of grazing on plant functional diversity were dependent on grassland types.In meadow steppes with higher productivity,grazing significantly increased species diversity and had no effects on functional trait diversity of plant communities;In typical steppes located in low precipitation region,grazing had no effects on species diversity but significantly increased functional trait diversity of plant communities.Furthermore,livestock grazing reversed plant functional redundancy of two grassland types.In meadow steppes with lower functional redundancy,grazing largely increased functional redundancy;in typical steppes with higher functional redundancy,grazing decreased functional redundancy.As a result,there was higher plant functional redundancy in meadow steppes than that in typical steppes across grazed grasslands.Our results suggest that,facing the current widely grazing situation for grasslands in China,possible species loss occurred in typical steppes would pose a greater threat on its ecological function than that in meadow steppe.We highly recommend that diverse grassland ecosystems require different management practices,and more attention should be paid to the conservation of typical steppe.