Abstract
Construction of dams across rivers is a common practice worldwide. However, obstructing free-flowing rivers could result in many negative impacts on riparian ecosystems, and are highly site-specific. In the present study, we evaluate the potential impacts of river regulation on the riparian vegetation by enumerating the vegetation at different distances from the dam/reservoir complex from upstream to downstream reaches of a tributary of the river Mahaweli in Sri Lanka. The tree-dominated riparian vegetation has been evaluated using belt transects located in the immediate and 8 km away from the reservoir/dam towards both upstream and the downstream of the tributary (US-0, US-8; DS-0, DS-8, respectively). A total of 150 species belonging to 58 families were identified, of which 43% were tree species. Approximately 77% of species were found exclusively in the upstream while 8% were recorded in the downstream. Both canopy and understory layers showed a significant decline in terms of richness, abundance, diversity and stem density from upstream to downstream. Leuceana leucocephala (Lam.) de Wit, an invasive tree species, showed higher relative abundance in downstream reaches, while riparian species such as Terminalia arjuna (Roxb.) Wight & Arn. and Pongamia pinnata (L.) showed no marked decline from upstream to downstream. The conditions triggered by reduced water discharges and severe river bank erosion may have contributed to these changes. The results indicate that the extreme water management practices in storage reservoirs have incurred negative impacts on the composition of the riparian vegetation. Introduction of well-coordinated flow management practices may help to mitigate some of these negative impacts.
Highlights
Riparian ecosystem occupies the ecotone between terrestrial and aquatic territories and is unusually complex and dynamic in nature compared to other terrestrial ecosystems (Nilsson and Berggren, 2000), possibly due to many natural and anthropogenic disturbances (Naimen et al.,1993; Hanlon et al, 1998; Nilsson et al, 1988)
Extreme drought conditions that prevail in the immediate downstream due to low discharges influence plant survival, growth and species composition (Stromberg and Patten, 1991; Rood, et al, 1995; Stromberg et al, 1996), while changes in overbank flooding and floodplain sedimentation strongly influence the riparian vegetation in the downstream (Bendix and Hupp, 2000)
The diversity was higher in the upstream compared to downstream reaches of Ulhitiya Oya (ULH)
Summary
Riparian ecosystem occupies the ecotone between terrestrial and aquatic territories and is unusually complex and dynamic in nature compared to other terrestrial ecosystems (Nilsson and Berggren, 2000), possibly due to many natural and anthropogenic disturbances (Naimen et al.,1993; Hanlon et al, 1998; Nilsson et al, 1988). The river regulation imposes inundation of habitats and establishment of new riparian zones in the upstream (Nilsson and Berggren, 2000), while the downstream reaches experiences lack of groundwater recharge and subsiding groundwater table (Nilsson and Berggren, 2000). Such hydrological changes have led to extinction of some riparian species, decrease abundance and alter species composition over time (Stromberg et al, 1996; Merritt and Cooper, 2000). Extreme drought conditions that prevail in the immediate downstream due to low discharges influence plant survival, growth and species composition (Stromberg and Patten, 1991; Rood, et al, 1995; Stromberg et al, 1996), while changes in overbank flooding and floodplain sedimentation strongly influence the riparian vegetation in the downstream (Bendix and Hupp, 2000)
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