Distribution Features of Natural Wetland in the Sanjiang Plain from 1954 to 2005

Since 1950s,because of economic development and national demand for grain,the Sanjiang Plain experienced some times' reclamation,the farmlands increase rapidly,and the Plain has become the national important commodity grain base.Meanwhile,the area of wetlands in Sanjiang Plain decreases rapidly.Reverse succession and degradation of wetland ecosystem lead to deterioration of ecological environment in the Sanjiang Plain and have caused serious threat to sustainable development of the Sanjiang Plain.Wetland protection possesses great significance for sustainable development of national economy and society.To understand spatial distribution features of natural wetlands is very important for wetland protection.Based on ArcGIS 9.1 software,the data of natural wetlands was extracted from remote sensing images of the Sanjiang Plain in 1954,1976,1986,1995,2000 and 2005,then overlaid them with natural environmental factors including water system,landform and terrain.The relationship between different periods' spatial distributions of natural wetlands in the Sanjiang Plain and their natural environment factors revealed the natural wetland spatial distribution features.First,natural wetland area in the Sanjiang Plain has been decreasing from 353×104 hm2 in 1954 to 81×104 hm2 in 2005.Natural wetland rate reduced from 32.42% to 7.45% over the past 52 years.For the average in 6 periods,there are 78.5% natural wetlands distributed in floodplain.Of them,the 57.2% is in low floodplain and 21.3% in high floodplain.The ratio between the area in low floodplain and total area of natural wetlands in the Sanjiang Plain increased from 42.5% in 1954 to 64.7% in 2005.At elevations ranged from 30 m to 80 m,the ratio between the area and total area of natural wetlands in the Sanjiang Plain is 85.8%.Of them,the 62.4% is at elevations ranged from 30 m to 60 m and the 23.4% distributed at elevations ranged from 60 m to 80 m.Most of natural wetlands distributed in the slopes from zero degree to one degree.Aspect does not play significant effect on natural wetland distribution in the Sanjiang Plain.The natural wetlands in the Sanjiang Plain distributed mainly on the river buffer within 0 to 3 km from river.The ratio between the area on river buffer within 0 to 1 km from river and total area of natural wetlands in the Sanjiang Plain increased from 15.01% in 1954 to 45.94% in 2005.In 6 periods,the ratio between the area on lake buffers and total area of natural wetlands in the Sanjiang Plain is less than 6% and changes little.