Pattern dynamics of vegetation coverage of plateau valley-city in the western China: A case study in Xining

Abstract

The geographical and climatic characteristics of Plateau Valley-City in the Western China are very typical and fragile. The urban ecosystems and environment have been damaged by urbanization recently years. Vegetation coverage plays a key role in the ecosystem of Plateau Valley-City, and it is very sensitive to urbanization. As a typical plateau Xining experienced rapid urbanization and implemented the policy of Converting Cultivated Land into Forest/same time. Study on pattern dynamics of vegetation coverage of Xining urban plan zone and its causative factors is of significance for promoting Western Development and urbanization, and contributes to creating a pleasant urban eco-environment in the Western China. Landsat TM data are the most economical and temporal continuous remote sensing images, however, vegetation pattern analysis on urban scale needs high resolution images. To resolve the problems and provide a more accurate result, Landsat TM data of 1995 and 2009 were unmixed by Linear Spectral Mixing Model (LSMM) in Xining plan zone to calculate proportion of vegetation cover, based on Vegetation-Impervious surface-Soil (V-I-S) model. Linear Spectral Mixture Model comprises five main processes: Minimum Noise Fraction (MNF), Pixel Purity Index (PPI), end-members collection by n-D visualizer, linear spectral unmixing and accuracy test. End-members include vegetation, high albedo surface, low albedo surface and soil. By using statistical analysis, transfer matrix and grid analysis, we evaluated the pattern dynamic of vegetation coverage, and discussed the effect of urbanization, Converting Cultivated Land into Forest/ Grass and climate change to vegetation changes in each Valley terrace area and hills area. The results indicate that: at overall level, the average vegetation coverage kept about 30% and showed a downtrend in the study period, meanwhile the regional differences had a little decrease. The areas of no vegetation coverage, moderate and high abundance vegetation coverage showed increasing trend, while the areas of low and full abundance vegetation coverage showed decreasing trend. The vegetation coverage in northwestern and southwestern area emerged in an increasing trend and it was mainly from low and moderate vegetation coverage. Huangzhong County is the main area affected by the policy, and finished Converting Cultivated Land into Forest/ Grass and afforestation of 54.91km2 and 247.98km2 from 2000 to 2005. It suggested that the policy was very effective. Vegetation dynamics of hills areas may be affected by climate change because that they had same trend to the vegetation dynamics effects of climate change in Tibetan Plateau. Vegetation coverage around urban built-up area changed obviously along with urbanization direction and intensity. Areas of moderate and high abundance vegetation coverage presented a degenerate trend, especially in the urban built-up area influenced by urbanization. The main reason is that urban construction land rapid increased after 2000 and occupied moderate and high vegetation coverage. Effects of human activities on vegetation coverage may be more intense comparing to climate change because that their effect trends on vegetation are opposite. The changing trend of vegetation coverage grade can be summarized that low grades developed to high grades because of converting cultivated land into forest/ grass and medium grades presented a degenerate trend because of urbanization. It is necessary to take measures to protect the vegetation around built-up area, and avoid irreversible degradation.