Spatial Distribution Of Vegetation And Soil In Temperate Savanna Ecosystem,Inner Mongolia

Ulmus pumila-dominated savanna is an important savanna type where U.pumila is a dominant species,and is the vegetation succession climax community adapting to the semi-arid climate of Otindag Sandy Land.Ulmus pumila-dominated savanna,playing a great role in maintaining the region's ecological stability and ecological security,has higher habitat diversity,biological diversity and structure diversity.In late 20 th century,under the influence of climate change and human activities(farming and grazing),Ulmus pumila-dominated savanna was destroyed,the regeneration of Ulmus pumila trees was limited,along with them,the vegetation coverage and species diversity of the system also decreased.The problem of low ecological support level has affected the whole area of ecological and economic security.Therefore,the comprehensive research work about spatial distribution of Ulmus pumila-dominated savanna is pressing subject.On this basis,the theoretical support will be provided to the preservation and restoration work.The research object was by investigating Ulmus pumila-dominanted savanna in the Otindag Sandy Land,by studying the spatial distribution characteristic of natural Ulmus pumila-dominanted savanna,which formation was discussesed,and the potential ecological process and interference factors were analyzed.The main results were as follows:(1)The ages structure of population were shown two peaks,with a characteristics of sharp-dropping in the seedling,stability in juveniles,and slow-declining in the adults,suggesting an obvious declining of population.In the eastern area,the number of age class?(DBH<5 cm),?(10 cm?DBH<15 cm),?(15 cm?DBH<20 cm)was maximum,in the northern,southern and western regions,the number of age class?(DBH<5 cm),?(20 cm?DBH<25 cm),?(25 cm?DBH<30 cm)was maximum.The problem of population decline was more serious in the northern,southern and western regions,relative to that in the eastern region.The key period of Ulmus pumila population to update is age class?(5 cm?DBH<10 cm).On the basis of avoiding interference for this age class,and strongly protecting is the key to benignly develop the population.(2)These woody species(Ulmus pumila trees,Caragana microphylla,Spiraea aquilegifolia)exhibited significant aggregations at small scales.However,the shrubs had a larger aggregation scale than the trees.Seed diffusion-limited,sprout tillers and habitat heterogeneity play important roles in the spatial patterns of woody species.Significant positive associations between the different stages of Ulmus pumila trees were found at small scales.The association of adult trees and seedling is described as "nurse-protégé interaction".Caragana microphylla and Spiraea aquilegifolia were no significant associations with Ulmus pumila trees at all scales.Ulmus pumila trees and two shrubs have different ecological niche,not compete with each other.(3)The population density of elm was highest in the eastern region,lowest in the south,and centered in the northern and western regions.There were an uniform spatial distribution between Ulmus pumila trees and the average temperature,the temperature may be an important environmental factor affecting the distribution of Ulmus pumila trees.The spatial distribution of morphological indicators(DBH,height,crown breadth)was on the contrary with population density,which showed that Ulmus pumila population had "hydrophobic" phenomenon,along with the individual growing,population density was gradually reduced.There was a significant negative correlation between the density of Ulmus pumila population and the density of shrub community,indicating that Ulmus pumila trees and shrubs had opposite spatial distribution pattern.The spatial distribution between herbaceous community and soil physical and chemical properties had a good consistency.The physical and chemical properties of soil may be the main factors influencing the distribution of the herbaceous community.(4)The sand dune could significantly affect the spatial pattern of vegetation and soil physical and chemical properties.The vegetation of trees,shrubs,and herbaceous communities was significant different of micro terrains.The vegetation in leeward slope was the most abundant,while the vegetation in windward slope was the most sparse.The soil physical and chemical properties decrease successively from interdune,leeward slope,slope top to windward slope.Under the action of wind erosion,The soil physical and chemical properties in windward slope were worst,interdune could promote the soil-forming process,having a relatively good soil structure and nutrient conditions.(5)Grazing is an important disturbance factor leading to degradation of forest stand structure.With the increase of grazing intensity,the number of woody plants was gradually reduced.Effect of grazing on the population structures of Ulmus pumila trees mainly reflected in the damage of Juvenile trees and seedling.Grazing reduced the scale of spatial aggregation,and accelerated the self-thinning process.The direct factor to trigger shrub encroachment is the shift of grazing pressure after long-term overgrazing.(6)Ulmus pumila trees significantly affected the spatial distribution characteristics of herbs and soil physical and chemical properties under the crown.Ulmus pumila trees and herbaceous community presented a good symbiosis.Ulmus pumila trees might process on the growth of herbs and the improvement of soil physical and chemical properties by adjusting the redistribution of soil moisture and light environment under crown.The results well confirmed the fertile island effects with Ulmus pumila tree as the center.Under the background of the current climate,the existence of shrubs may not have direct effect on the growth of Ulmus pumila trees.But this study found that as the arid climate is aggravating,the number of Ulmus pumila trees are likely to continuously decrease,the number of shrub(such as Caragana microphylla,one drought-tolerant plant.)may continue to increase.The Ulmus pumila-dominated savanna ecosystem may degenerate into the shrub-dominated ecosystem if no effective protection and restoration measures will be performed.To protect the landscape of Ulmus pumila-dominated savanna,we propose to develop some nature reserves to protect this specific ecosystem in the Otindag Sandy Land.