Abstract: Based on the remote sensing images in 1990, 2000, 2004, and 2007 and the 1∶50000 DEM, and by using software ArcGIS 9.3 and Fragstas 3.3, this paper studied the evolution of cultivated land in Caotangxi watershed, a center region of Three Gorges Reservoir Area. The database of the cultivated land in Caotangxi watershed was established, and the slope, elevation, and terrain niche index were produced. Then, the slope gradient, elevation gradient, and terrain niche gradient were calculated with the database of the cultivated land so as to discuss the distribution characteristics of the cultivated land at different terrain gradients according to the dimensionless distribution index. The results showed that the cultivated land in the watershed was mainly distributed in the slope gradient range of 15°-35°, elevation range of 500-1000 m, southeast and south slopes, and in middle and high terrain gradient. An obvious change in the area and spatial distribution of cultivated land took place in the study area in 1990-2007. With the increasing terrain gradient index, the distribution area of cultivated land increased first and decreased then. The remarkable reduction in the aggregation index of cultivated land was mainly in the fourth and fifth terrain niche gradients (1.2-1.5 and 1.5-2.0, respectively), showing that the aggregation of cultivated land in middle and high terrain gradient decreased, the cultivated land was fragmented, and the ecological environment was becoming well.

Key words: Sassafras tsumu, altitude, genetic structure., genetic diversity