Evolution analysis of cultivated land structure characteristics on gully area of loess plateau in 1983－2009

To clear evolution law of quantitative and spatial structure for cultivated land in the gully area of the Loess Plateau before and after conversion of cropland to forest, taking Changwu county for example, the quantitative and spatial structure of the cultivated land in 1983, 1993, and 2009 in Changwu county were quantitatively analyzed using GIS technology and quantitative indexes based on the fully understanding of connotation of land use structure. The study results showed that the area of cultivated land in Changwu county in 1983, 1993, and 2009 was 292.1, 181.3, 159.2 km2, respectively. The reduction of cultivated land mainly occurred in the slope grades of 0-2°, >15-25° and >25°, respectively, with 29.6%, 48.9% and 60.1% from 1983 to 1993 and with 15.6%, 15.7% and 48.8% from 1993 to 2009. The transform between cultivated land and other land types was highly frequent. Cultivated land in gentle slope grades of 0-6° mainly converted to orchard land and residential, mining and traffic land, and cultivated land in steep slope grades of >15° mainly converted to forest land and unused land (waste grassland). The structural analysis showed that the quantitative distribution of the cultivated land in slopes tended to be reasonable, and the patches on gentle slope developed to a spatial structure of core-area-cluster and be uniform, and that on steep slope tended to be scattered distribution as core area. An optimized process was experienced in the last 30a in the study area, the structure evolution of cultivated land was helpful to optimize resources and its allocation, and develop agriculture intensification system. The abandonment of cultivated land in the former period from 1983 to 1993 primarily relied on the planting benefit comparison by farmers themselves, while the conversion of cultivated land to other land use types and the spatial distribution in the latter period from 1993 to 2009 occurred under much scientific planning. This paper provides a useful reference for management and policy formulation of regional land use.