Effects Of Grazing And Precipitation On Plant Community Stability In Stipa Breviflora Desert Steppe

Stipa breviflora desert steppe is an important natural steppe in Inner Mongolia.It is crucial to maintain local animal husbandry and ecological security.Overgrazing and drought stress has caused the decline of productivity and plant diversity,the degradation and the reducing of ecosystem stability.In this paper,firstly,the study of community organization and interspecific relationship was used to clarify the effects of grazing and precipitation on the stability of organizational structure and ecological niche of plant population.Then,the study of aboveground biomass and diversity was used to clarify the effects of grazing and precipitation on the stability of productivity,plant diversity and the relationship of them.At the last,the study of time stability and species asynchrony was used to clarify the effects of grazing and precipitation on these indexes and the relationship between diversity and stability.The aim was to evaluate and analyze the response of plant community stability to grazing and precipitation objectively.At the same time,it was to provide theoretical support for the diversity protection and stability maintenance of desert steppe.The main conclusions are summarized as follows:1.Precipitation had no significant effect on the organization of main species,functional groups and community.Precipitation and grazing had no significant interactive effect too.However,the effects of grazing on the organization of each scale were severe.With grazing,the organization of plant community dropped to 69%of it with no grazing.The rank of most species' organization declined.The organization of dominant species(Stipa breviflora,Artemisia frigida and Cleistogenes songorica)increased or had no change.These species still kept dominantly in community with grazing.The organization of subordinate species(Kochia prostrata,Allium tenuissimum and Caragana stenophylla)decreased significantly.The organization of perennial grasses increased significantly.On the contrast,the functional group which has most abundant species(perennial forbs),its organization decreased significantly.The results showed that grazing caused the plant community structure worse.It became a simple structure which dominated by very few plants.2.With no grazing,the overall association was non-significant negative association among the main species.The proportion of negative association pairs was 57.1%.However,with grazing,the overall association was significant positive association.The proportion of positive association pairs was 85.7%.The result showed that grazing changed the relationship of main species from competition to coexistence.Percentage of co-occurrence(PC),Ochiai index(OI),Dice index(DI)were all decreased with grazing.It indicated that the relationship between the main species was looser with grazing than with on grazing.Ecological niche differentiation happened.The construction and simulation of the Lotka-Volterra model can clearly show the dynamic process of the interspecific relationship.Grazing changed the process between dominant species.But long-term grazing of this intensity will not result in the disappearance of these species.About precipitation,the overall association of major species was significantly positive association in rainny years,non-significant positive association in normal years and non-significant negative association in dry years.The result showed that the decrease of annual precipitation changed the relationship of main species from coexistence to competition.3.Grazing and precipitation had significant interactive effects on aboveground biomass of the community.The sequence of biomass from high to low was:rainy years,normal years,dry years.The biomass with no grazing was larger than with grazing.Grazing caused the difference between normal years and dry years significant.It means that the effects of drought on the stability of community productivity were intensified.At the scale of functional groups,grazing and precipitation had significant interactive effects to aboveground biomass of perennial grasses,shrubs,semi-shrubs and perennial forbs.In rainy years,the aboveground biomass of perennial grasses was larger with grazing than with no grazing.But in other precipitation years it was smaller.The aboveground biomass of other functional groups was smaller too.For perennial grasses,shrubs,semi-shrubs and perennial forbs,grazing caused the decline of biomass under drought stress more significant.4.Grazing and precipitation had significant interactive effects on dominance index of community,and had non-significant interactive effects on richness,diversity and evenness index.The sequence of all indexes from high to low was:rainy years,normal years,dry years.The indexes with no grazing were larger than with grazing.The diversity of perennial grasses decreased when precipitation decreasing.However,grazing had no significant effect on it.Precipitation had no significant effect on the diversity of shrubs and semi-shrubs.But grazing decreased it significantly.The decrease of precipitation led to a significant decrease of diversity of perennial grasses.Grazing decreased its richness,diversity and evenness index,but increased its dominance index.Both with no grazing and with grazing,no matter what type of precipitation year,there was a positive linear correlation between diversity and biomass.5.Grazing and precipitation had non-significant interactive effects on time stability and species asynchrony.For single factor,precipitation had no significant effect on them,but grazing reduced them significantly.Both with no grazing and with grazing,there was linear positive correlation between species asynchrony and time stability,richness and time stability,richness and species asynchrony.For time stability and species asynchrony of functional groups,grazing and precipitation only had non-significant interactive effects on shrubs and semi-shrubs.Grazing increased the time stability of perennial grasses and perennial forbs,and decreased the time stability of shrubs,semi-shrubs and annuals and biennials.Except perennial grasses,the species asynchrony of other functional groups was significantly decreased with heaving grazing.Precipitation only significantly affected the time stability of shrubs and semi-shrubs,and did not affect that of other functional groups.Precipitation had significantly effects on the species asynchrony of perennial grass and perennial forbs.