The Dynamics Of Vegetation And Ecological Characteristics Of Dominant Populations For Desert Riparian Forest In The Upper Reaches Of Tarim River

Water is an important factor limiting the speciation, evolution, succession and distribution in arid areas.The plant community of desert riparian forest in the upper reaches of Tarim River as the research object,3sections,4sample strips and permanent large-scale plots were be established with field investigation, long-term monitoring, and soil chemical analysis. The flora, community structure and spatial pattern, interspecific association, biodiversity, ecological niche, gradient alalysis, ecological process and successive mechanism of desert riparian forest were studied and maintenance mechanism of community structure stability and protection measures of dominant population were discussed. The research was based on the theory and methods in floristic geography, quantitative ecology, mathematics statistics, geostatistics and ecological niche theory.The main obtained results are as follows:1The plant community structure and species composition were simple and stratification was obvious. The flora had an overwhelming number of temperate elements and life form was dominated by dwarf, small phaeneraphyte. P.euphratica, P.pruinosa and Tamarix ramosissima were constructive species, the species composition and distribution difference were the evolution result of plants adapting to the dry desert environment for a long time. Community distribution formed different forest stand and community types along the river center to the outside with different canopy density and age structures. Species distribution randomness and high spatial heterogeneity showed community was not stable and vegetation happened succession.2Community quantitative feature decreased obviously along the gradients of groundwater levels and dominant populations declined, life form functional group composition and plant water ecological function changed. The decrease of vegetation coverage was due to decline of herbaceous coverage. Dominant species retained its dominant position in the process of degradation. Degradation order was herbâ†'shrubâ†'woody and surviving species with strongly drough-enduring ability and spatseness was obvious feature. Vegetation density, coverage and crown width of dominant species could be as indicator of groundwater change.3The main species had higher niche overlaps and significant negative association, interspecific competition and repulsion leaded to the low level of organization structure and limited the species coexistence. Dominant species had wide ecological amplitude and competition ability for resources utilization to low water and nutrient soil, this was the cause for wide distribution. Community dissimilarity and habitat heterogeneity expanded, the decrease of species diversity was due to decline of herbaceous diversity. Degradation of species diversity happened at the4.0m and threshold at5.5m of groundwater depths. Groundwater level decrease was the main force to drive community succession, population decline and restrict population regeneration. 4Optimal competition range of objective tree was six meters in woody and pattern scale was5mx5m in desert plant community, and artificial thinning should be applied DBH<20cm of dominant population. Interspecific competition was the one of force to drive dynamic of spatial pattern and formation of differentiation pattern of dominant population.5The soil water mainly came from runoff infiltration and the rise of shallow groundwater, and its variation was mainly affected by structural factors. Spatial distribution of soil water content was banded structure and decreased gradually from north to south direct. Soil organic matter and total amount of nutrient were low and soil texture turned worse and soil coarsen and ecological function of soil decreased. Soil salt accumulated on surface soil and hydrochemistry type of groundwater were dominated by Ca2+-Mg2+-Cl-.The soil water content and organic matter affect obviously vegetation coverage, species diversity. Soil water content decreased the species abundance of desert plant, further decreased population size, eventually led to decrease of vegetation coverage.6The structure of woody, shrub and herb was the initial stage of community succession. The desert community degraded with the order:herb stage, shrub stage and woody stage.The groundwater level was the key factor of all environmental factor to affect the distribution pattern of plant community, secondy was soil water content and organic matter.7Structure of P.euphratica population were expanding type in middle and low sections and P.pruinosa were declining types in the upper reaches of Tarim River, it tended to decline and die out gradually along the river. The value of self-thinning of populations were close to-3/2and they were endangered plant. P.pruinosa had higher degradation rate than P.euphratica, and analysis of time sequence prediction indicated dominant population declined fastly in the future. The distribution patterns of dominant population changed from clumped to random pattern during development stage, appearing diffusion trend. This result was the interaction between competition and habitat. Protection of existing living trees and their habitats are important to the sustainable development of dominant population in the upper reaches of the Tarim River.8The half lethal underground water level (GDW50) was as a community degradation index.The rational ecological groundwater level for vegeation normal growth and maintenance high biodiversity was<4m and the threshold of groundwater depth for communit degradation was about5.5m.This could be used as a basin theory for vegetation restoration and ecological water transportation. In brief, keeping the rational ecological groundwater level was the effective path to sustain desert ecosystem stability.