Abstract: Based on GIS techniques and Landsat images, the transformation source, extent and speed of forest land use, and the spatiotemporal differentiation characteristics of forest land transformation were obtained on the township scale for Anding District, Dingxi City in 1995, 2000, 2005, 2010 and 2015. From 1995 to 2015, the total area of forest land transformation was increasing, with the grassland and cultivated land being the main sources. The forest land transformation extent and transition speed showed a tendency of decrease first and then increase from 1995 to 2015. The spatial distribution of forest land transformation differed, and the transformation area of forest land in each town was also different. The change of forest land transformation differed with the increasing slope. Both the net conversion and transformation extent of forest land showed a trend of increase first and then decease, but with opposite transformation speed. The major transformation of forest land occurred in the area with slope larger than 15°.

Key words: tundra zone, nitrogen deposition, carbon and nitrogen mineralization, Changbai Mountains.