Effects of cascade hydropower dams on the structure and distribution of riparian and upland vegetation along the middle-lower Lancang-Mekong River

The extensive number of hydropower dams being planned in southwest China has attracted much attention in recent years. Eight cascading dams along the middle and lower reaches of the Lancang-Mekong River basin were selected to assess the riparian and upland vegetation. A total of 24 transects and 126 quadrats perpendicular to the river channel were surveyed from upstream to downstream. By using two-way indicator species analysis (TWINSPAN), the vegetation types in this region were classified into 21 vegetation classes. The ecological gradient analysis was completed using canonical correspondence analysis (CCA) and demonstrated that the dominant environmental factors impacting vegetation distribution were the variations in latitude and altitude. The vegetation impact index (VII) was developed as a quantitative index to assess the impact of dam inundation and operation on the upland and riparian vegetation. The values of VII showed that the most endangered vegetation communities were the shrub and herb communities in riparian habitats along this river. The effects of cascading hydropower dams on riparian and upland vegetation distribution were more complex than those of single dams. Cascading hydropower dams can enhance habitat fragmentation, reduce the distribution ranges (latitude and altitude) of primary vegetation and reduce the complexity of the vegetation types along the river as well as induce the loss of primary vegetation in the whole watershed.