Community Assembly Mechanisms Of Local Stipa Breviflora Community In Desert Steppe

In the 21st century,China has transformed from an ecologically degraded nation to a marvel of ecological restoration.A variety of ecological engineering techniques have made this restoration possible in the desert steppe.After the restoration efforts,vegetation coverage and biomass increased significantly over the landscape.A few lessons can be carried forth from this restoration in future efforts to maintain and develop the desert grassland ecosystem.The first is that long-term grazing exclusion ought to be utilized to serve an ecological function,and the second is that moderate grazing can promote both ecological functions and productivity.How to develop particular desert grassland depends on the development of vegetation and environment in tandem.Vegetation assembly is key because the maintenance of grassland ecosystem productivity and stability depends greatly on the biodiversity of grassland communities.Uncovering the mechanisms behind vegetation assembly will advance knowledge used in sustaining and restoring diversity,especially in the degraded grasslands of the desert steppe.Soil habitat condition,or the degree of desertification,is a key factor in determining community composition in the desert steppe.Widening the scope one step further,grazing disturbance is the key driving force for soil desertification.In this study,we selected local Stipa breviflora communities to undergo one of three treatments;long-term grazing exclusion,moderate utilization and heavy utilization.The scalar characteristics of spatial organization in the actual communities can be described as follows:Many mini-patches(about 0.5m2)exist in plant communities affected by long-term grazing exclusion and moderate grassland utilization.However,S.breviflora communities exist in heavily-utilized grasslands in the form of larger patches(100-400m2).We aimed to study assembly mechanisms of local S.breviflora communities by considering scalar characteristics of spatial organization,as well as the differences in soil habitat types.In the above study,the principal coordinates of neighbor matrices(PCNM),variability decomposition,null-model simulation,community ordination,and multivariate regression tree analyses were performed based on species composition,the plant functional traits,functional diversity,and phylogenetic community structure,respectively.The main results from the study are outlined below.1?S.breviflora communities were competitively structured and regulated by environmental filtering and limiting similarity in the long-term grazing exclusion treatment.(1)S.breviflora was dominant,with Lespedeza potaninii and Convolvulus ammannii as the main accessory species.The average values of species richness and biomass per community were 10.73 and 106.1 g/m2,respectively.(2)The soil clay content explained 80%of the total variation in community composition.(3)Environmental processes mainly regulated the leaf dry matter content and leaf specific area,while limiting similarity mainly regulated stoichiometric lability for traits in an individual.However,the functional structure was negative for traits in aggregate.(4)Multiple trait-based assembly processes operate simultaneously in species composition and differ in degree depending on the type of mini-patch in community.2?S.breviflorra communities were non-competitively structured and mainly regulated by environmental filtering in the moderate utilization treatment.(1)S.breviflora was dominant,with L.potaninii and Cleistogenes squarrosa as the main accessory species.The average values of species richness and biomass per community were 13.73 and 124.44 g/m2,respectively.(2)The coarse sand soil content explained 63.6%of the total variation of in community composition.(3)The environmental processes mirrored those of the community undergoing the long-term grazing exclusion treatment,but the functional structure was positive for traits in aggregate.3?5.breviflora communities were non-competitively structured and regulated partially by limiting similarity and dispersal limitation,but mostly by environmental filtering in the heavy utilization treatment.(1)The average values of species richness and biomass were 7.5 and 82.96 g/m2,respectively.Community composition varied with habitat;the "sierozem(H1),sierozem-to-sandy-soil transition area(H2),and sandy soil(H3)" communities transitioned from S.breviflora + L.potaninii + C.squarrosa communities to Artemisia scoparia + L.potaninii communities.This change in community structure can be attributed to the presence of different soils,with the coarse sand present at 20-40 cm,and the clay at 10-20 cm.(2)Among all physical and chemical environmental factors in the soil,the content and spatial distribution of different soil particle sizes accounted for 6.7-45.9%of the total variation in community composition.Dispersal limitation only accounted for 5%of the total variation.(3)Environmental filtering mainly occurred in sandy soil(H3),regulating the leaf specific area,leaf carbon and phosphorus concentration.Limiting similarity mainly occurred in the sierozem habitat,regulating the leaf dry matter content and leaf nitrogen concentration.From the sierozem(H1)to the sandy soil habitat(H3),the functional structure was positive for traits in aggregate,with the degree of aggregation decreasing as coarse sand soil content increased.(4)The phylogenetic community structure had a divergent structure with no obvious trend.(5)The sierozem habitat(H1)plants had significantly higher leaf dry matter content and lower leaf nitrogen content(P<0.05)than the other habitats,while the sandy soil habitat(H3)plants had a higher specific leaf area and leaf phosphorus content(P<0.05).The functional diversity of a plant community was highest(P<0.05)in the transitional area habitat(H2),where the functional diversity of leaf dry matter content was negatively correlated with leaf phosphorus content and specific leaf area.The functional dispersion in plant community was positively correlated with biomass.In summary:1.Over a gradient ranging from long-term grazing exclusion to heavy utilization,the degree of soil desertification was found to increase with increasing intensity of utilization.(1)Species diversity and biomass showed an increasing trend,before decreasing.Moderate grazing can promote both ecological and production functions.(2)Patterns of competitive species coexistence disappeared.The regulatory intensity of limiting similarity decreased,and then increased for community composition and structure.The regulatory intensity of environmental filtering in communities decreased.2.Environmental processes were important drivers at the local-scale in S.breviflora communities in the desert steppe under the three grassland utilization treatments.The difference in soil particle content and distribution created a resource gradient through influencing soil nutrients,moisture content and particle distribution.As soil coarsening increased,the plant life strategy shifted from a "conservative resource use strategy" to a "rapid growth strategy," which in turn determined the community composition.3.The effect of random processes on community composition was evident in both the mini-patch(about 0.5m2)and the S.breviflora community patch(100?400m2).The decrease in regulatory intensity of environmental filtering indicated that grazing factors are among the most influential of habitat factors on community composition.