拉萨地区土地利用变化

根据1990年、1995年和2000年3期西藏拉萨地区土地利用现状调查数据,利用GIS空间分析方法,系统地分析了1990年至2000年间拉萨地区的土地利用时空变化特征。得出: ① 10年来拉萨地区的土地利用类型转变主要发生在人类活动比较集中的城镇附近和河谷地区,很多地段的天然植被由人工植被所取代,植被覆盖度和生物产量明显提高,有效地改善了这些区域的土地覆盖状况,这些变化是这一期间实施的农业综合开发中旨在改变区域生态环境的人工植树造林和改良草场等人为有目的地改变土地利用类型的直接结果;② 10年内面积增幅最大的是林地,增加了2.56%;③ 土地利用类型变化最广泛的是牧草地,由牧草地变成耕地、园地、林地、居民点及水域的,其中牧草地变成林地的面积最大,占变化面积的94.09%;④ 耕地变成林地的面积占耕地移出总量的54.86%,变成居民点的占移出面积的38.25%;⑤ 水域变成林地的面积占变化面积的93.13%。

Land Use Change in Lhasa Area, Tibet from 1990 to 2000

Based on the results of the first land use survey in Tibet Autonomous Region carried out in the late 1980s, land use map of Lhasa area in 1990 was compiled for the main agricultural area in Lhasa valley using aerial photos obtained in April, May and October 1991 and Landsat imagery in the late 1980s and 1991 as remotely sensed data sources. Using these remotely sensed data, the land use status of Lhasa area in 1991, 1992, 1993, 1995, 1999 and 2000 were mapped through updating annual changes of cultivated land, artificial forest, grass planting, grassland restoration and residential area and so on. According to land use status of Lhasa area in 1990, 1995 and 2000 at five-year intervals, the spatial and temporal land use dynamics in Lhasa area from 1990 to 2000 are further analyzed using GIS spatial models in this paper. The main conclusions are as follows: (1) Land use change in Lhasa area during the period 1990-2000 happened mainly near the major towns and the Lhasa valley. Much natural vegetation was replaced by artificial forests; vegetation coverage and biomass have obviously increased. These changes are directly related to human activities aiming to improve regional environment, such as tree planting, grassland restoration during the implementation of the integrated agricultural development project at that time. (2) The most remarkable increase in area is forest as the forestland increased by 2.556% (2713.48 hm2) from 1990 to 2000. The most comprehensive changes of land use in Lhasa area from 1990 to 2000 is rangeland conversion. Rangeland was converted to cultivated land, horticultural land, forestland, settlement and water body. Among them, the largest area of land use changes is rangeland conversion to forestland (2338.25 hm2), being 94.093% of the area relating to land use change. Some 186.53 hm2 of cultivated land have been converted to forestland over the period 1990-2000, being 54.857% of the total area relating to cultivated land conversion into other land use types. Some water bodies changed into cultivated land, forests and rangeland. Some 243.13 hm2 of water body were converted to forests, being 93.126% of the area relating to land use change, which happened mainly from the bottomland conversion to the artificial forests. (3) Because Lhasa area is a semi-arid and environmentally vulnerable region on the Tibetan Plateau, effective measures should be taken to improve local environment in a short term through bioengineering technologies such as artificial forestation, grassland restoration and so on. These measures can improve local microclimate, conserve water, prevent land desertification and soil and water loss through modifying vegetation cover types and increasing vegetation coverage.