| Vegetation coverage is an important parameter for the description and assessment of landscape or ecosystem. The study on the estimation of vegetation coverage and its dynamic change, as well as the change of landscape indices of the hierarchy of vegetation coverage are important indicator of the change of ecological environment.This paper introduced the basic theories of estimation of vegetation coverage by remote sensing method, its research progresses and the developing trends. Then, the improved model for estimating the vegetation coverage deriving from normalized difference vegetation index (NDVI) was established, based on the method of dimidiate pixel model. Based on the TM/ETM+ images of 1989, 2001 and 2006, the vegetation coverage in different phases and areas of Chongming county were estimated by the improved model. The estimation results of 2006 were verified through field investigation, which showed that the estimated vegetation coverage by this model was effective in Chongming county.Based on the estimation of vegetation coverages of three years, the average vegetation coverages of the Chongming county in 1989, 2001 and 2006 were statistically analyzed, the hierarchical structure of vegetation coverage of Chongming county in recent 17 years were obtained, and the changes of vegetation coverages in different areas and land-use type were quantitatively analyzed. The change of vegetation coverage was not obvious in general, but the changes in different areas and different land-use types were obvious, which revealed the unbalance in vegetation coverage among areas and land-use types in the process of urbanization.The average vegetation coverages were 72.32%,67.62% and 59.25% in Chongming, showing decreased trend. Through dynamic analysis on the negative and positive changes of the vegetation coverages in the town, farm and wetland area, as well as the farm, wetland, forest, water body, building area in the periods of 1989-2001 and 2001-2006, we found that: from 1989 to 2001, the percentage of negative change of vegetation coverage was about 10.17%, and that of positive change, 20.58%. The areas of the positive changes of vegetation coverage in the township and wetland were greater than those of negative changes, but in the farm area was on the contrary. From 2001 to 2006, the percentage of negative change was about 29.1%, and that of positive change, 12%. The areas of positive changes of vegetation coverage in the farm and wetland were greater than those of negative changes, but in the town area was on the contrary. This showed that vegetation coverage of the town area deceased, and the farm area, inceased, along the progress of urbanization. In the course of urbanization, the change of vegetation coverage of various types of patches showed the trends of ecological degradation to a certain extent.The grade of vegetation coverage directly represented the comprehensive information of the grade of vegetation canopy density. The hierarchical structure of vegetation coverage implied the information of the quality of vegetation system, and directly influenced on the ecosystem services of the vegetation. The process of urbanization is the most direct factor promoting the changes in the vegetation grade of Chongming county. By studying the vegetation coverage and it's pattern during 17 years by means of translation of TM/ETM+ remote sensing images and analysis on the tempo-spatial characteristics of the hierarchical structure of the vegetation coverage, the process of disturbance of urbanization on the natural landscape and the changes in environmental quality were reproduced. The results showed that from 1989 to 2006, along with the rapid development of economy, the vegetation system in Chongming county experienced a process from serious damage to gradual restoration. From 1989 to 2001, the vegetation in both town and farm area were destroyed seriously. From 2001 to 2006, this tend still existed in the town area, but in the farm area, the vegetation was increased, and the hierarchical structure of the vegetation coverage was improved, which positively contributed to the restoration of the vegetation system in Chongming county.
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