Spatial-Temporal Patterns of Grassland Gradation in Southern part of the Headwaters of the Three Rivers,Qinghai in China

Grassland is a main land type in the study area,the status and trends of its degradation become a major issue of the ecological construction in China,and have been raised to the level of the needs for national ecological economic strategy.Many studies showed that grassland ecosystem degradation is,in the succession process of the climate-land cover-human activities,an indication of out of balance.Grassland degradation will lead to a series of environmental problems such as lower grassland production,land desertification,loss of biodiversity,soil degradation,loss of carbon sinks.Study areas located in the south of the head of Yangtze river.The total area is about 34 069.31km2,mainly covered by grassland(71.49%) and swamp(10.11%).The low and moderate covered grassland is 71.16% of the total grassland area.The increasing destruction of the land in this area have been kept increasing the risk of environmental degradation.Based on the TM/ETM in 1977,1990 and 2004,geographical map(1∶100 000),grassland map(1∶1000 000),and field data,the authors extract grassland degradation distribution characteristics.The Arcgis 9.1 software was used to convert data types and do the overlay.reclassification,and zonal statistic analysis.Results show that grassland degradation is the most important land cover change in the study area,which occupied 11.84% of the whole region in the former phase and 21.54% in the later phase.The areas of degraded grassland are more in the later phase than in the former phase.Grassland in the former phase became better in some regions because of the climate and glacier ablation.The degraded grassland mainly occurs in the low altitude with relatively higher population density.