Abstract:

In this study,the location and distribution of herbaceous wetlands,bottomland and water body in the East Dongting Lake were identified through the decision tree classification,based on multitemporal Landsat TM/ETM+ images.The spatial distribution and expansion tendency of herbaceous wetlands under low waterlevel conditions were also estimated using matrix transformation and Bezier curve interpolation,based on the DEM data in 1995 and waterlevel data from the Chenglinji Hydrological Station.The results were as follows.(1) During 19892011,the area of herbaceous wetlands increased 30506 km2 (from 57735 km2 in 1989 to 88241 km2 in 2011).The area ratio of herbaceous wetlands to total area of east Dongting Lake increased 2307% (from 4365% in 1989 to 6672% in 2011).In contrast,the bottomland decreased 274.28 km2(from 424.33 km2 in 1989 to 1506 km2 in 2011).The water body displayed a fluctuation status from 26105 km2 to 34251 km2.(2) The herbaceous wetlands area mainly increased on the edges of the lake basin of Changzhou,Xinshengzhou and Piaoweizhou,as well as most areas of Niutouzhou,WuguangChaixiazhou and ZhongzhouTuanzhou,whereas mainly decreased in few areas,such as Fengcheguai,Beizhou,Wugangzi and Liugangzi.The matrix transformation analysis showed that the herbaceous wetlands in east Dongting Lake was in a rapid expansion during last 23 years and the development time of “water bodybottomlandherbaceous wetlands” was shortened,which may be related to the uprising of bottomland caused by sedimentation. (3) The spatial distribution of herbaceous wetlands showed a typical unimodal and reached to a peak area when the elevation was 24-25 m,then decreased significantly with the decrease of elevation.However,during 1989-2011,the elevation which was favorable for the distribution of herbaceous wetlands and the inflection point between herbaceous wetlands and other wetland types including bottomland and water body decreased gradually,which may directly result in an expansion of herbaceous wetlands. (4) At the elevation of 22-24 m,the herbaceous wetlands area varied significantly,which might be mainly driven by sedimentation during 1989-2000,and might be influenced by waterlevel changing together with sedimentation after 2000.Our data showed that the expansion of herbaceous wetlands,i.e.the invasion into bottomland in east Dongting Lake during 1989-2011,might be mainly caused by the bottomland exposion and flooding regime changes,which usually changed according to the variation of sedimentation and hydrological conditions.This study plays an important role in understanding the development trend and formation mechanisms of the Dongting Lake wetland