北京平原区粮菜林三地类时空转变特征

该文利用遥感与地理信息系统技术研究北京平原区农用地内部粮田、菜地和林地三大类农地利用类型的变化。结果表明:2004年至2014年这10 a间,粮田转变为林地与菜地是北京平原区农地利用变化主流。粮田转变为林地面积是969.97 km~2,其土地利用变化重要性指数高达72.11%,在各类农地利用变化类型中居首。而粮田转为菜地则主要是粮田转变为设施菜地,10 a间粮田转变为设施菜地面积是163.53 km~2,其土地利用变化重要性指数为12.16%,在各类农地利用变化类型中居次;同时,10 a间北京平原区农地利用变化存在空间分异特征。主城近郊区与远郊平原区农地利用变化主要是粮田转为林地,两大分区中该变化类型土地利用变化重要性指数分别为84.18%、76.21%,明显高于其他农地利用变化类型土地利用变化重要性指数。在城市新区,粮田则分别转为林地与设施菜地;10 a间北京平原区农地利用空间分布均具有圈层特征,由近郊向远郊,农地内部结构都呈现林地为主类型,到粮田、菜地与林地混合利用类型,再到粮田与林地组合利用类型的空间格局。而10 a间随着主城区的外扩,上述农地利用圈层特征出现了由近郊向远郊外移、且林地与蔬菜面积占比增加的现象。研究认为:农地内部粮、菜、林3地类的转变和农地利用圈层外移是农户追求经济效益最大化与政府为改善生态环境所致;而改善生态不应以牺牲粮田为代价,应充分利用粮田的生态系统服务功能以实现生态绿化目的。

Spatial-temporal change characteristics of cropland, vegetable land and forest land in Beijing plain region

Abstract: This research used the techniques of the remote sensing (RS) and geographic information system (GIS) to obtain the data of agricultural land use classifications by analyzing and interpreting the multi-spectrum remote sensing images, which were extracted from Landsat TM image in 2004 and Landsat OLI image in 2014. According to the situation of agricultural land use in Beijing plain, the agricultural land use were classified as the grain crop fields, vegetables cultivated in greenhouse, vegetables cultivated in the open fields and forest land. Then it analyzed spatial-temporal change characteristics of the agricultural land use in Beijing plain over the past decade (2004 to 2014) by combination of the quantitative geography model and geographic information system (GIS). The results showed that the grain crop fields were changed into forest land and greenhouse vegetable fields, which was the major trend. The area of grain crop fields changed into forest land was 969.97 km2 whose land use change intensity index was 71.11%, at the top of all kinds of land use change intensity indexes. The area of grain crop fields changed into greenhouse vegetable fields was 163.53 km2 during the past 10 years, whose land use change intensity index was 12.16%, and ranked at the second among all kinds of land use change intensity indexes. Meanwhile the agricultural land use change had spatial characteristic in Beijing plain over the past 10 years. The leading type of agricultural land use changes was that grain crop fields turned into forest land in the suburban area and the distant suburb plain of Beijing, where land use change intensity indexes of grain crop fields into forest land were 84.18% and 76.21%, respectively and higher than the other agricultural land use change intensity indexes obviously. The overwhelming urbanization was encroaching upon a lot of grain crop fields, which leaded the agricultural land use to concentrate on the forest land in the suburban districts. At the same time grain crop fields changed separately to forest land and greenhouse vegetable fields in the new urban area of Beijing, which prompted to the equalization of agricultural land use structure. The research area kept the specific spatial pattern of agricultural land use that the forest land was the leading classification in close proximity to the city center, the mixed classifications including grain crop fields, greenhouse vegetable fields and forest land were main structure in suburban area of Beijing plain and the combination type consisted of grain crop fields and forest land mainly appeared in the distant suburbs of Beijing plain. The spatial structure of the agricultural land use showed as a specific spatial pattern including 3 layer circles and had experienced some changes over the last decade. Firstly, the whole layer circles of agricultural land use had migrated outwards towards the suburbs with the rapid expansion of the central city over the time. Secondly, the spatial distributions of forest land in close proximity to city center and the vegetables cultivated in greenhouse in suburban areas of Beijing plain both had expanded outward due to the area of forest land and vegetables cultivated in greenhouse increased. Although grain crop fields and forest land were the main agricultural land use types in the distant suburbs of Beijing plain in 2004, whose proportion of grain crop fields and forest land had exchanged in 2014. Forest land had become the main type of agricultural land use in the distant suburb of Beijing plain instead of grain crop fields over the time. The study identified that the spatial structure and changes of 3 layer circles of the agricultural land use objectively reflect the different land rent, which drove the peasant household in different locations of suburbs to make different choices about the cultivation, operation and management of agricultural products. Specifically, the peasant households who lived in urban and suburban areas preferred vegetables to grain crops and preferred cultivating vegetables in greenhouse to cultivating vegetables in the open fields, because they hope to get more economic income by changing agricultural products and ways of operation and management. On the other hand, forest land which mainly provided ecological service function was closer to the city center than the grain crop fields which mainly provided production function, which meant the agricultural function had changed from production function mainly to productive and ecological functions. The research results provide a reference for the optimization of spatial distribution of agricultural land use in Beijing plain.