北京地区不同地形条件下的土地覆盖动态

LAND COVER DYNAMICS OF DIFFERENT TOPOGRAPHIC CONDITIONS IN BEIJING

利用MSS／TM影像和1：250000高程数据分析了1978—2001年北京地区土地覆盖的变化。为研究人类活动对土地覆盖类型及植被变化的影响，根据植被的分布规律和人类活动影响，利用数字高程模型（DEM）将研究区遥感影像分为不同海拔段，分别建立标志进行解译。通过转移概率矩阵计算、不同地形因子影响下的土地覆盖类型分布的GIS分析，得到如下结果：1）1978-2001年间北京市土地覆盖变化主要发生在平原和低海拔、小坡度的平缓地区，表现为农业用地向城镇用地的转变；高海拔地区主要为天然植被所覆盖，土地覆盖变化相对较小，主要是灌丛向林地的转变。2）地形因子显著影响土地覆盖类型的分布及变化。随着海拔的升高和坡度的增大，农业用地和城镇用地减少，林地和灌丛逐渐增加。坡向对植被的分布也有较大影响。

Topographic conditions play an important role in controlling land cover dynamic processes. In this study, remotely sensed data and geographic information system were applied to analyze the changes in land cover along topographic gradients from 1978 to 2001 in Beijing, a rapidly urbanized mega city in China. The study was based on five periods of land cover maps derived from remotely sensed data, Landsat MSS for 1978, Landsat TM for 1984, 1992, 1996 and 2001, and digital elevation model （DEM） derived from a 1 ： 250 000 topographic map. The entire area was divided into 10 land cover types： conifer forest, broadleaf forest, mixed forest, shrub, brushwood, meadow, farmland, built-up, water body and bare land. The results are summarized below. 1 ） Shrub, forest, farmland and built-up consist of the main land cover types of the Beijing area. The most significant land cover change from 1978 to 2001 was a decrease in farmland and an expansion of built-up areas. Farmland decreased from 6 354 to 3 813 km^2 over the 23 years, while built-up areas increased from 421 to 2 642 km^2. Forest cover increased from 17.2% to 24.7% of the total area. The conversion matrix analysis indicated that transformation of farmland to built-up area was the most significant process and afforestation was the primary cause of replacement of shrub to forest. 2） Topographic conditions are of great importance to the distribution of land cover types and the process of land cover changes. Elevation has a strong impact on the distribution of land cover types. The area below 100 m mostly consisted of farmland and built-up areas, and the area above 100 m is mainly covered by shrub and forest. Shrub has a maximum frequency in elevations between 100 and 1 000 m, while forests were dominant in areas above 800 m. According to the analysis of land cover changes in different elevation ranges, the greatest change was below 100 m due to urbanization. The main land cover change process that occurred above 100 m was transformation from shrub to forest. This result was constant with the vertical change of natural vegetation distribution in Beijing. 3） Slope has a large influence on the distribution of land cover types. Farmland and built-up areas are distributed primarily in flat areas, while shrub and forest occupy steeper lands as compared with other land cover types. Forest frequency increased with increasing slope. Land cover change processes differed with slope gradients. In the plain area, land cover change occurred as the result of urbanization. With increasing slope gradients, afforestation, which converted shrub to forest, was the primary land cover change process.