Study On Characteristics And Mechanisms Of Degradation And Enclosure Restoration About Artemisia Ordosica Community In Mu Us Desert

Artemisia ordosica community,which plays a key role in biodiversity protection,desertification combating,ecosystem stability maintenance,and reconstruction of degraded grassland ecosystems,is the most widely distributed and representative community in Mu Us desert.However,due to multi-factors,the community has been seriously degraded in recent decades.Enclosure,as one of the main restoration measures to restore the degraded grasslands in arid and semi-arid region,is widely used in this area.However,there is a lack of comprehensive comparative study on the degradation and restoration process of A.ordosica community.So the plots with different degradation degrees and enclosure times,which located in typical distribution area of A.ordosica community at Yanchi County,were selected to simultaneously investigate population,community and soil.We analyzed the quantity and population structure characteristics,spatial distribution parttem,community characteristics,and soil physical-chemical properties in order to discuss the process and mechanism of degradation and restoration.Our results could provide theoretical guidance for vegetation restoration and management of degraded A.ordosica communities in this area.The key findings of the research were as follows.(1)The grasslands degradation could affect regeneration and growth of A.ordosica population.It not only decreased the density,coverage and total aboveground biomass of population,but also reduced the height,crown and aboveground biomass of individual plants.The age composition of population would change after degradation,combining with the decreasing of seedlings and adults,and the increasing of aging and dead individuals.This affected the population structure in different degradation and restoration stage.The trend of population structure in degraded plots would change as follows:involution-form in lighter degraded plot?growth-form in light degraded plot?stable-form in moderate degraded plot?involution-form in severely degraded plot.Accompany with the growing development of seedlings after enclosing,the crown,height and aboveground biomass of individual plants significantly decreased which resulted in the stuntedness of population.The population coverage,density and total aboveground biomass would reach maximum value after enclosed for 10 years,and then would decrease as the increasing of fencing time.The trend of population structure in fencing grasslands would change as follows:growth-form(enclosed for 5 years)?involution-form(enclosed for 10 years)?stable-form(enclosed for 15 years)?involution-form(enclosed for 25 years).(2)The A.ordosica population,which showed patchy and zonal distribution characteristics,had strong spatial dependence.The spatial heterogeneity of population would significantly change during the degradation and restoration of grasslands.The overall performance of spatial heterogeneity was as follows:degraded plots>enclosed plots;heavily degraded plots>lightly degraded plots;long-term enclosed plots>short-term enclosed plots.(3)The total population,seedlings and adults of A.ordosica displayed aggregate distribution at small scale(0-2m),and randomly or evenly distributed at large scale(20-25m),while the aging and dead populations mostly distributed randomly or evenly at different scales.Enclosure measure resulted in the increasing aggregation scale of seedlings.The seedlings were positively correlated with adults,while were independent with agings and deads at small scales.Four dominant companion populations,Sophora alopecuroides,Astragalus melilotoides,Cynanchum komarovii and Stipa capillata,showed different degrees of aggregated distribution at small scales.Moreover,the aggregation scales of different populations were unlikely in the same plots.The aggregation degree in enclosed plots with relatively higher vegetation coverage was greater than that in degraded plots with lower vegetation coverage.The four dominant companion populations were competitive related with A.ordosica population at small scale,while were independent at large scale.(4)The proportion of Compositae,Gramineae,Chenopodiaceae and Leguminous plants accounted for more than 70%of the total species in the study area.Perennial herbs dominated in study area,followed by annual herbs,and shrubs were the least species.With the degradation and restoration of vegetation,the species composition significantly changed.The coverage,height,density,aboveground and underground biomass,root-shoot ratio,litter,Richness index,SW and SP diversity and Evenness index were significantly affected by different treatments(P<0.05).With the increase of degradation degree,the litter gradually decreased,root-shoot ratio gradually increased,and Evenness index did not change significantly,while the other community indicators and community stability increased first and then decreased.The vegetation gradually recovered,characterized by significantly(P<0.05)increasing of various vegetation characteristics after enclosing.The community stability also increased in this progress.However,those vegetation characteristics would decrease after enclosed for 10 years.The community stability was strongest after enclosed for 15 years.(5)Different treatments(P<0.01)and soil layers(P<0.05)significantly affected various soil physiochemical properties.There had some correlation between various soil physical and chemical properties with different degrees.With the increase of degradation degree,soil water content increased first and then decreased,soil bulk density and sand content gradually increased,while soil clay content,silt content and various soil chemical factors(total nitrogen,total phosphorus,available nitrogen,available phosphorus,and available potassium)gradually decreased.The soil clay,silt,soil moisture content,and virious soil chemical properties would gradually increase after fencing,however the soil moisture would redecrease after enclosed for 25 years.The comprehensive soil fertility index of different soil layers would change in the process of vegetation degradation and restoration.The general trend of comprehensive soil fertility was as follows:long-term enclosed plots>lightly degraded plots>short-term enclosed plots>moderate degraded plots>severely degraded plots.In the process of vegetation degradation and restoration,the fluctuation of soil crusts would significantly affect the content of virious soil physichemical properties,and different soil crust types had unlikely effects on virious soil physichemical properties.(6)The results of RDA analysis showed that the cumulative contribution rate of the first two axes to the relationship between community and soil properties was 78.95%,which indicated that the first two axes could well explain the relationship between soil properties and community.Different soil properties had different effects on community in this area.Soil water content,soil organic matter,bulk density and soil clay content all had great influence on plant communities and plots,while the soil available nitrogen and pH value had less influence.