Analysis And Reconfiguration Of Degraded Desert-oasis Ecotone In The Lower Reaches Of Tarim River

Ecological restoration and reconstruction of damaged ecosystem becomes more and more concerned issue of the society on continental river basin in arid zone. The results of analysis and reconfiguration of degraded ecosystem provide scientific basis for planning and design of regional ecological restoration and reconstruction project and has important directive significance in desert-oasis ecotone in the lower reaches of Tarim River. On the basis of the theory of restoration ecology, this study took "history-of- degradation→analysis-of-status-quo→reconfiguration-of-vegetation-community" as the main line, explored the regional water environmental change and history of degradation of the lower reaches of Tarim river about 50-year, the change of shallow water table depths about 50-Year and history of degradation in desert-oasis ecotone, described the characteristics of habitat and vegetation community and the relationships between habitat and vegetation community in desert-oasis ecotone, and put forward the strategy of vegetation community reconfiguration for desert-oasis ecotone on the basis of status quo. The methods and technologies of this study included dendroecology, geostatistics, quantitative ecology, remote sensing and geographic information system. The main results showed:(1) The regional water environmental change were various and complicated about 50-year. The water environmental condition did not display completely that the nearer to the upper reaches(Daxihaizi reservoir) the site was, the better its water environmental condition was, reflecting degree of ecology degeneration of different regions in the downstream were not same; The worst periods of water environmental conditions occurred mainly in the 1990s. This phenomenon was attributed to the breaking flow of river downstream for more than 30 years. The best periods of water environmental conditions appeared in the 1960s and at the beginning of this century, and this phenomenon was attributed to perennial water period and eco-water period. The abrupt change examination indicated that the regional water environmental change had the obvious sudden change effect. The regional water environmental change in the lower reaches of Tarim River was determined by quantity of surface flow, distance to the upper reaches (Daxihaizi reservoir), distance to ancient watercourses, distance to the waterring watercourse, micro-topography and pattern of regional shallow water table determined by regional geological structure et al. jointly.(2) The analysis of regional shallow water table depths about 50-year reconstructed by Tamarix hispida Willd. Tree-ring width data and NDVI change classification between 1972 and 2005 both showed that degree of ecology degeneration become gradually serious from 1950 to 1990s, but regional ecology is on the way to preliminary restoration after 2000 because of eco-water transfer in desert-oasis ecotone. There was an obvious relationship between tree-ring widths of Tamarix hispida Willd. and regional shallow water table depths. The analysis result showed that there was the evident negative correlation(-0.917) between the STD tree-ring index and the 3-6 Month shallow water table depths in desert-oasis ecotone in the lower reaches of Tarim River, indicating obvious tree physiology significance. Using the stepwise linear regression models, the 3-6 Month shallow water table depths about 50-year in desert-oasis ecotone was reconstructed. Method of"Leave-one-out"was used to estimate stability of regression function based on statistics between the observed and estimated series, the result of reconstruction lengthened hydrology records of regional shallow water table to reach 42a. For the series of the construction, there was the rise trend in regional shallow water table depths from 1958 to 2000. Furthermore, the shallowest periods of shallow water table depths appeared in the 1950s, and that the deepest periods occurred mainly in the 1990s. The abrupt change examination indicated the reconstruction series had obvious response to the breaking flow of river downstream and eco-water transfer. The results showed the course of ecology degeneration had obvious synchrony with regional water environmental change determinated by shallow water table depths and the course of vegetation degeneration reflected by change of NDVI index in desert-oasis ecotone. (3) Environmental factors had obvious spatial heterogeneity; spatial autocorrelation was distinct both in vertical gradients and in level gradient in desert-oasis ecotone. Along and across the direction of the river water flowing and along the direction of the oasis to desert, all the soil elements were characterized by the partial variation on the basis of the whole regularity variation.And this partial variation was clearer across the direction of the river water flowing.The analysis of habitat heterogeneity showed that soil salinity and soil nutrient in 0～200cm soil depths in ecotone had decreasing trend with the increasing of the soil depth, while the change trend of pH, CO32- and soil moisture content were opposite. There is distinctly different between 0-60cm soil depths and 60-200cm soil depths as for soil factors, 40-60cm soil depths was the abrupt layer of soil physical and chemical properties in ecotone. Soil physical and chemical properties of 0-60cm soil depths in level gradient had obvious spatial heterogeneity, spatial autocorrelation was distinct. The max range of soil salinity, soil nutrient and soil moisture content were 2390m, 4906m and 11430m. The changing range of all indexes of the soil offers a reference for future study how to locate the points for collecting the soil samples.The soil salinity was higher and the soil nutrient was lower in ecotone, but shallow water table depths was lower and was in favor of vegetation restoration and reconstruction; the simulation of the soil elements spatial distribution can offer the information of habitat pattern in every point of study area for restoring regional vegetation.(4) The analysis about diversity of vegetation community showed that plant species was seldom. Eighteen species belong to nine family and seventeen groups existed in the study area. Type of vegetation community was unitary, classified into four types; Based on the result of object-based classification, distribution of vegetation community was broken, significantly, strong spatial heterogeneity, the size of Tamarix spp. Community was the largest in ecotone. Along the direction of the oasis to desert, distribution of vegetation community was decreased; along the direction of the river water flowing, there was little difference; across the direction of the watercourse, distribution of vegetation community was distinctly decreased in ecotone. The 0-180cm soil layer was closely related with plant community distribution patterns in desert-oasis ecotone. The environmental gradient of coupling relationship between water and salt played the most important role in the forming of community distribution pattern or species abundances variation in the communities. Conduct and Cl- in soil salt variables and available K in soil nutrient variables were the significant factors in all environmental factors. The total contribution rates of the environmental factors were 52.08%, the contribution rates of soil salt factors, soil nutrient factors, water factors and special factors were 16.589%,11.104%,9.940%,7.148%and 7.247%, respectively. From a management standpoint, it is most important to adopt corresponding ways according to the difference of soil salt in different region, and keep dynamic balance of water and salt in the process of restoration of damaged ecosystem in desert-oasis ecotone in the lower reaches of Tarim River.(5) Reconfiguration of vegetation community on the basis of habitat status is useful to build self-sustained mechanism of vegetation community in a short time. The reconfiguration not noly gives an indication in choosing the most appropriate community types in the process of restoration of damaged ecosystem in desert-oasis ecotone, but also fully embodies the corresponding environmental characteristics of different community types. The results of reconfiguration of vegetation community indicated that Tamarix spp. community reconfigured was located near oasis, its area was 2990.18hm2 and occupied 26.89% in whole vegetation loss zones; Populus euphratica Oliv. community reconfigured was located at the middle of watercourses and the lower of watercourses near Yengisu, its area was least, 429.80hm2 and occupied 3.87% in whole vegetation loss zones; Alhagi sparsifolia Shap. community reconfigured was located at the middle of ecotone and most of northern region, its area was largest, 7121.21hm2 and occupied 64.04%in whole vegetation loss zones; Karelinia caspica Less. community reconfigured was located at the upper of watercourses region, its area was largest, 577.96hm2 and occupied 5.20% in whole vegetation loss zones.