Seasonal Changes of the Relationship between Species Richness and Community Biomass in Grassland under Grazing and Exclosure in Horqin Sandy Land,Northern China

This study carried out an analysis of seasonal changes in relationships between vegetation cover,plant density,species richness and above-ground biomass in grassland under grazing and exclosure in Horqin Sandy Land.Results showed that in grassland being grazed and fenced,vegetation cover,richness and biomass were lower in April than those in August,while plant density showed a reverse trend.Vegetation cover over the season was higher in fenced grassland than that in grazed grassland;biomass in June and August was higher in fenced grassland than that in grazed grassland,while plant density in April and June and was lower in fenced grassland than that in grazed grassland.In fenced grassland,the relationship between species richness and biomass was significantly negative in August(p0.01);in grazed grassland,the relationship between plant density and biomass was significantly positive in the April(p0.01) and was significantly negative in August(p0.01).The relationship between plant density and total biomass for dominant species varied with seasonal changes under grazing and exclosure.In grassland under grazing,the relationship between plant density of Cleistogenes squarrosa and biomass was significantly positive in April(p0.01) and was significantly negative in August(p0.01);the relationship between plant density of Artemisia scoparia or Setaria viridis and biomass was significantly negative in August(p0.01);the relationship between plant density of Salsola collina and biomass was significantly positive in April and June(p0.01).In fenced grassland,the relationship between plant density of Lesredeza davurica or Salsola collina and biomass was significantly positive in August(p0.01).These results suggest that long-term grazing,seasonal changes and their interaction significantly influence vegetation cover,plant density and biomass in grassland(p0.05).Long-term grazing also affects the seasonal changes of plant density and biomass of dominant species,which further affects the seasonal relationship between plant density and biomass in grassland.Species competition in fenced grassland results in the seasonal changes of relationship between species richness and biomass.