ESTABLISHMENT AND EXPANSION OF WILLOWS AFTER DAM OPERATION IN A MONSOONAL SANDY STREAM

초록

After the dam operation, the downstream riparian ecosystems have been dramatically modified from their natural conditions in East Asian monsoonal climate. We investigated establishment and expansion of riparian vegetation due to the operation of dams in a sandy stream, the Nakdong River in Korea. Andong and Imha Dams were built along the Nakdong River in 1976 and 1992 respectively. Flood magnitude of the Nakdong River has been reduced since the built of Imha Dam: the maximum average daily flow was changed from 548 m3/s of the predam to 280 m3/s of the post-dam. The downstream riparian vegetation was rapidly expanding after the flow regulation: the vegetation area was 7% of the total floodplain zone in 1988, 31% in 1995, three years after the completion of Imha Dam, and moreover 44% in 2009. The fluvial landform had been also changed and stabilized after the dam operation. During this period most of the vegetation area was occupied by willows, especially Salix subfragilis and S. chaenomeloides in the Nakodng River. The seed dispersal of willows in 2010 began in early May, peaked in mid-May and lasted 8 weeks. In the second week on May, the amount of seed dispersal was 4,704 m-2 day-1 which comprised 81% of the total seed rain. The seed dispersal was finished in the middle of May for S. subfragilis and the end of June for S. chaenomeloides. The altitude of the riparian zone where S. chaenomeloides had been established rose about 70 cm during the 16 years dam operation. During seed dispersal period in 2010, average water level drawdown speed was –17 cm/day. Therefore, upstream dams reduced both physical damages and draught stresses on the seedlings of willows. Moreover, frequent precipitation in the period of germination and establishment of willows could supply soil moisture which is critical for seedling survival. All things considered, the results of this study confirm that the establishment and expansion of willows are actively carried out d