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Effects of different disturbances on diversity and biomass of communities in the typical steppe of loess region

Typical steppe is a type of grassland which forms in inland temperate semi-arid climatic conditions,where xerophytic perennial bush grasses,especially Stipa spp.,dominate.Grassland ecosystems are one of the largest terrestrial ecosystems affected by human activities.It is important to study the impact of different types of disturbance on the biodiversity and biomass of grasslands ecosystems during restoration and reconstruction work.Exclosures,grazing,fertilization and burning are four major types of anthropogenic disturbance occurring in grassland ecosystems,and have very important impacts on them.To provide a scientific basis for the protection of grassland biodiversity and the sustainable use of grasslands,the influences of different disturbances on the characteristics of above-ground vegetation were studied in a typical steppe in the loess region of China based on the field investigation and monitoring.We established plots representing four different types of treatments: exclosure(E),exclosure plus fertilization(EF),exclosure plus burning(EB) and grazing(G).All plots had the same soil type,topography and hydrological conditions.During the entire experiment,we determined total coverage,vegetation height,individual density,aboveground biomass and diversity of the plant community in each plot.Our results show the dominant vegetation community type of G plots is Agropyron cristatum+Potentilla acaulis+Artemisia sacrorum,a community which is the result of long-term grazing.The dominant vegetation community type of the E plots is Stipa grandis+Poa sphondylodes+Medicago ruthenica,a result of fencing.The dominant vegetation community type of the EF plots is Carex duriuscula+Stipa bungeana+Stipa grandis.The dominant vegetation community type of the EB plots is Artemisia sacrorum+Stipa bungeana+Stipa grandis.The different disturbance regimes strongly influenced community diversity and biomass.To be more specific,total coverage,above-ground biomass and the dominance indices were all at the highest level in the EF treatments in four types of treatments.This may attributed to grass species which play a dominant role in this community.Also,with low evenness and diversity indices,this result was consistent with niche theory which states differences among species in their environmental tolerances,niche requirements,and competitive abilities determine coexistence.Total coverage,height and aboveground biomass were all lowest for the G treatment,while community richness and diversity indices were higher for G than other three treatments.This result supports the intermediate disturbance hypothesis which states that biodiversity will be greatest in communities subjected to moderate levels of disturbance.Only the individual density and evenness index was higher with the E treatment than with the other treatments.Thus we can document the trend for biomass as EF EB E G.These demonstrate long-term exclosure is a severe type of disturbance in grassland ecosystems.Also,the community richness indices(R and Ma) showed a certain tendency within these four treatments: G EF EB E and community evenness indices(Jsw and Ea) could be sequenced in the order of E G EB EF.Meanwhile,the order of diversity indices(H′ and D) was as follows: G E EB EF,while the dominance index was opposite the order of diversity index.Finally,there was negative correlation between community productivity and Shannon-Wiener and Simpson indices,which can be explained by interactions between root and shoot competition.