Relationships Between Plant Functional Traits And Functional Diversity In A Stipa Breviflora Desert Steppe Under Long-Term Grazing With Different Stocking Rates

The Stipa breviflora desert steppe is an important component of Eurasian steppes,and its ecosystem functions are mainly determined by the plant functional traits.This study was conducted at Stipabreviflora desert steppe in Siziwang Banner,Inner Mongolia,northern China,based on a continuous 12-year sheep grazing plots with four stocking rates(0(Control,CK),0.93(Light Grazing,LG),1.82(Moderate Grazing,MG),2.71(Heavy Grazing,HG)sheep hm-2 half year-1).We analyzed the responses of stocking rate on soil properties,plant community,plant species,species population and also on species diversity,functional diversity and functional group diversity,as well as the interactions among those diversities in 2016(wet year)and 2017(drought year).The function mechanisms and responses of plant species and ecosystem functions to long-term grazing were discussed.The results are summarized as follows:(1)The stocking rate had significant effects on individual species and their functional traits in the studied desert steppe.The leaf area,leaf number,stem biomass,leaf biomass and the seed biomass of dominant species were reduced with increasing stocking rate.The moderate grazing promoted the accumulation of individual plant biomass.However,heavy grazing resulted in the prominent position of Stipa breviflora in the community.The leaf area,average plant height,individual leaf mass and leaf biomass of Stipa breviflora were reduced but the number of root tips,branches and crossing of root system were increased with the increasing stocking rate.Overall,the aboveground biomass of Stipa breviflora was decreased but the belowground biomass of that was increased with the increasing of stocking rate.(2)The stocking rate had significant effects on the species diversity,functional diversity and functional group diversity in the studied desert steppe.Species diversity,functional group diversity and Rao's quadratic entropy indexes were gradually decreased with the increasing of the stocking rate.(3)Grazing and climatic factors co-affected ecosystem functioning.Grazing year and stocking rate had great impact on degree of species dominance.Grazing(year and stocking rate)and climatic factors(precipitation and air temperature)co-affected the quantitative characteristics of the plant community(dry weight,height,coverage and density).The increase of precipitation and air temperature increased species diversity.Species diversity was the highest under light stocking rate in 2017,and species diversity and functional group diversity were greater in 2016 than in 2017.The functional richness in CK and LG were greater than in MG and HG,and the functional evenness was greater in HG than in CK,LG,MG in 2016;the functional richness was greater in HG than in CK and LG in 2017.(4)The ecological functions of the plant species,population and community are significantly related to each other.The species diversity increased with the increasing of quantitative characteristics of plant community.The species diversity was positively related to functional diversity and functional groups diversity,while the community characteristics were negatively related to functional diversity but were positively related to functional groups diversity.Positive relationships were also showed among Rao's quadratic entropy index,species diversity and functional group diversity,and a linear negative correlation was showed between functional richness and functional group diversity.However,functional richness and species diversity had negative exponential relationship,and a monotonic linear relationship were observed between functional evenness and evenness of species diversity.(5)Soil physical and chemical properties and plant chemical properties were changed under four stocking rates and different soil depth.Total carbon,total phosphorus and carbon to nitrogen ratio in the whole plant were higher under lower stocking rate,and the total nitrogen and nitrogen to phosphorus ratio were higher under higher stocking rate.The total nitrogen,total phosphorus and nitrogen to phosphorus ratio in leaves were higher than those in stems and roots,but the carbon to nitrogen ratio in leaves was lower than those in stems and roots.