Effects of cascade hydropower dams on the structure and distribution of riparian and upland vegetation along the middle-lower Lancang-Mekong River
Highlights
► We studied the vegetation associated dams construction along Lancang-Mekong River. ► Latitude and altitude were the dominant factors impacting vegetation distribution. ► VII showed the most endangered vegetation were the shrub and herb communities. ► Dams can enhance habitat fragmentation of primary vegetation along this river. ► The effects of dams on vegetation were more complex than those of single dams.
Introduction
Dams and reservoirs serve important functions for mankind, such as water supply for agriculture, industries and municipalities, seasonal flood regulation, electricity production, and navigation (Rosenberg et al., 2000, WCD, 2000). The most extensive period of dam construction and operation was concentrated in 20th century (Mallik and Richardson, 2009). Almost 60% (172/292) of large river systems were affected by dams (Nilsson et al., 2005). Worldwide, 19% of the demand for electricity is provided by hydropower dams, and one-third of the countries in the world rely on hydropower dams, which provide more than half of their electricity (WCD, 2000). Due to rapid growth in energy demand, China is currently undergoing a climax period of hydropower dam construction. There are 4,860 dams greater than 30 m and 117 dams greater than 60 m that have been completed or were under construction prior to 2006 in China (Jia et al., 2006). Cascading hydropower dams, defined as a series of dams from upstream to downstream (Fang and Deng, 2011), can maximize the utilization of water resources for hydropower generation, flood control and navigation. This is the major form of hydropower exploitation in southwestern China.
Dam construction and operation, especially of large dams, can profoundly impact watershed and river ecosystems (Poff et al., 1997, He et al., 2004). Large dams can significantly change shoreline and riparian vegetation in both the impoundment region and downstream reaches (New and Xie, 2008), even though critical environment processes and diverse habitats for flora and fauna are supported by riparian regions (Beauchamp et al., 2007, Mallik and Richardson, 2009). A number of studies have demonstrated that the riparian vegetation can be remarkably impacted by damming, leading to habitat heterogeneity, declines in species richness and native species and exotic species invasion (Nilsson and Svedmark, 2002, Stave et al., 2005, Tealdi et al., 2011). The changes in ecological processes associated with dam construction in both aquatic and terrestrial riparian ecosystems have become a key focus of river studies (Gordon and Meentemeyer, 2006). In addition to the hydrologic regime, geographic location, climate, vegetation zonation, pre-dam species composition and species biological characteristics can determine the responses of the riparian flora to hydropower operation (New and Xie, 2008). Moreover, short-term vegetation changes are different than the long-term response due to riparian succession after dam operation (Nilson et al., 1997). However, there is no universal model for the quantitative assessment of the interrelationship between riparian vegetation, hydrologic regime and other factors associated with damming (Bombino et al., 2006, Tealdi et al., 2011).
At present, the Lancang-Mekong River, a famous international river in Asia, is one of the large rivers that remain mostly undammed on the mainstem except for eight cascading hydropower dams constructed along the upper Mekong River in China (Nilsson et al., 2005, Grumbine and Xu, 2011). As of September 2010, another 11 dams have been proposed on the lower Mekong River in Laos (Grumbine and Xu, 2011). With unique geomorphologic features and climatic variation, the Lancang-Mekong River basin is one richest biodiversity hotspots in the world (Thompson, 2008). From its source to the sea, the river flows across more than seven biomes, from alpine meadow to tropical rain and monsoon forests (Wu, 1980, Wu and Zhu, 1987, Nilsson et al., 2005). In this richly diverse river basin, eight cascading dams have been constructed in the middle-lower reaches of the Mekong in Yunnan Province, China (Table 1 and Fig. 1). Hydropower construction and planning has attracted a lot of attention and disagreement among conservationists in recent years (Nilsson et al., 2005, Grumbine and Xu, 2011). The effects of cascading hydropower dam construction on watershed ecosystems are more severe than those of a single dam along this international river (Zhai et al., 2010). The integrity of both river and riparian habitats can be altered by dam construction, leading to changes in the structure and functions of river and riparian ecosystems (Wei et al., 2008). To date, some research has been performed to explore the effects of hydropower construction on the downstream hydrologic regime, water quality and sediment trapping efficiency (Fu et al., 2006, He et al., 2006, Kummu and Varis, 2007, Wei et al., 2008, Kummu et al., 2010, Zhao et al., 2011). However, few researchers have quantitatively evaluated the effects of cascade hydropower dams on the structure and distribution of riparian and upland vegetation. Consequently, this study was conducted in the middle-lower reaches of the Lancang-Mekong River with the following objectives: (1) assessing the changes in the structure and distribution of the riparian and upland vegetation after the construction of the dams; (2) identifying the key driving factors associated with these changes; (3) developing a feasible assessment tool to quantify the impacts of dam construction on riparian and upland vegetation; and (4) providing insights into ecosystem impact assessment for cascading hydropower dams that may be useful elsewhere in the world.
Section snippets
Natural profile of the study area
The Lancang-Mekong River, situated in southeastern Eurasia with a length of 4,909 km, originates from Guyong-Pudigao creek (5,160 m above sea level) near the foot of Mt. Jifu on the Qinghai-Tibet Plateau and discharges into the South China Sea (Liu et al., 2007). Its watershed area is about 760,000 km2, discharge is approximately 14,500 m3/s, and volume is 457 km3/a (MRC, 2010). It is also a famous transboundary river and usually divided into two parts: the Upper Mekong River Basin, including the
Vegetation type classification
The TWINSPAN analysis classified the 126 quadrats into 21 vegetation types (Table 2). The numbers of tree, shrub and grass communities were 10, 7 and 4, respectively. The majority of vegetation distributed on uplands was tree and shrub, whereas riparian areas supported grass and shrub vegetation. For tree communities, 8 of the 10 were distributed on upland and 2 were distributed on upland and riparian areas. In shrub communities, 3 of the 7 were distributed on riparian and others on upland
Vegetation distribution patterns and environmental gradients
This study investigated the vegetation distribution pattern and ecological gradient along this river at a large spatial scale. The TWINSPAN hierarchical classification method was employed to classify the upland and riparian vegetation along the middle-lower reaches of the Lancang-Mekong River into 21 types. The CCA ordination results indicated that the geographical factors, including latitude, altitude and longitude, were the dominant factors impacting the vegetation distribution pattern along
Conclusions
Inundation was the most significant effect caused by cascading dam construction and operation on riparian and upland vegetation. The vegetation impact index (VII) developed in this study can be used as a tool to quantify the direct effects induced by inundation. Native riparian and upland vegetation played important roles in maintaining the regional ecological integrity and riparian ecosystem health. Measures, such as ecological planning and design, should be developed before dam construction
Acknowledgements
This research was supported by the Key Program of the National Natural Foundation of China, key project (Grant No. 50939001) and Nonprofit Environment Protection Specific Project of China (Grant No. 201209029-4). The authors express great thanks to the anonymous reviewers for their time and efforts.
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