Abstract: The Sanjiang Plain is the largest fresh water wetland distribution area in China and the center of waterfowls breeding and habitat area in Asia,but over the past 50 years,more than 73% of its wetland had lost because of agricultural development,and as a result,the wetland biodiversity declines dramatically,and the remnant wetlands are in a very fragment state.Based on historical maps,remote sensing data and GIS techniques,this paper selected two watersheds to analyze their wetland landscape fragmentation process during 1950～2000.It was indicated that land reclamation resulted in a decrease of 98% wetland corridors in Qixing River,90% in Naoli River,87% in the middle reach of Bielahong River,and 94% in the lower reach of Bielahong River;The amount of isolated wetlands in watershed increased dramatically;The maximum patch areas of wetland decreased by 92.6% in Naoli River watershed and 74.6% in Bielahong River watershed,and the mean wetland patch area in the two watersheds decreased by 99%.Before 1983,the wetland landscape was in an extensive area distribution state (the index of patch density was <0.1),but after 1983,it fragmented dramatically,with the index of patch density larger than 1.5.The shape fragmentation indices of wetland decreased from 1950 to 2000,indicating a very big change in wetland patch shapes in the watersheds.The area fragmentation indices of wetland also increased from 1950 to 2000,especially after 1983,showing that the wetlands were in a serious fragmentation state.The wetland landscape fragmentation changed from a landmass and island model to a satellite model,and finally to a completely isolated model,which indicated the great changes in spatial structure of wetland in the Sanjiang Plain.

Key words: Watershed, Wetland landscape, Fragmentation process