Loss of wetlands due to the expansion of polder in the Dongting Plain, China, AD 1368-1980

The Dongting Plain is an area characterized by wetland losses because of long-term polder construction. The study of historical polder expansion provides an opportunity to better understand the loss of wetlands covered by polders. To reconstruct the polder expansion over time, the polder patches extracted though remote sensing were marked with the names obtained from maps and the times of construction obtained from local gazetteers, and shown in the chronological order of turning points. Then, the distribution and changes in the wetlands covered by polders during 1368-1980 were reconstructed. The following are the major findings: (1) the current polders (2010s) accumulated over many centuries. There were 5.7% of the current polder area in 1644, 14.0% in 1735, 23.4% in 1850, 55.0% in 1911, 73.6% in 1949, and 100% in 1980; (2) the wetlands were mostly lost in the northern part of the region and declined rapidly over the past two centuries. The wetland area in 1850 was 6635 km(2), which in 1911, 1949 and 1980 were 73.9%, 62.7% and 40.6% of that in 1850, respectively; (3) there were differences in the rate of wetland loss. The fastest time of wetland area disappearance was in 1949-1980, and 45.8 km(2) of the wetlands had been lost each year; and (4) there was a spatial difference in wetland losses caused by polder expansion. In the northern part of the area, the wetland loss was mainly in 1851-1980, and the polders constructed in this period covered 89.5% of the polder area. In contrast, in the southern part of the area, the change in the wetland area was relatively small in each period, and the polder constructed before and after 1850 covered 48.3% and 52.7% of the polder area, respectively.