Do Exclusive Riverine Plants Move Away From The Riverine Forests Of Mahaweli River Basin Due To River Regulation In Sri Lanka?

Presented by: Shalini Rajakaruna
University Of PeradeniyaAuthors
S.L. Rajakaruna1, K.B. Ranawana2, A.M.T.A. Gunarathne3 And H.M.S.P. Madawala3

Introduction: What conservation problem or question does your study address?
Altered hydrology reduces groundwater table in riverine zones of downstream of dams creating severe impacts on riverine vegetation. Mahaweli Development Project in Sri Lanka involved construction of five dams across River ‘Mahaweli’. With the advent of this project, river and its watershed were subjected to changes. This talk discusses decreasing of abundance of exclusive riverine plants in the riverine forests of Mahaweli river basin due to river regulation.

Methods: What were the main research methods you used?
Two unregulated rivers (Badulu Oya and Upstream of Ulhitiya Oya) and two regulated rivers (immediate downstream of Minipe anicut and Ulhitiya reserviour) were selected for the study. Lack of data on vegetation in pre-regulated conditions led us to compare unregulated tributaries with regulated ones assuming that the conditions in there are somewhat similar to pre-regulated era (Bain et al., 1998; Nilsson et al., 1991) Two sampling blocks were selected from each study site. Each block encompassed a 100 m long stretch along the banks. Four 10 m x 30 m belt transects were established perpendicular to the river. Each belt transect was divided horizontally into three 10 m x 10 m sub-plots. The closest of the three sub-plots to the river flow was named as the lower zone, the furthest was known as upper zone, the sub-plot in the center as the middle zone. The vegetation was sampled under the categories of canopy, understory and ground vegetation.

Results: What are your most important results?
In regulated rivers, the abundance of exclusive riverine species were lower than that of un-regulated rivers, mainly in the understory vegetation. In regulated rivers the exclusive riverine species are highly replaced by terrestrial species like Bauhinia racemosa. Abundance of Hydnocarpus venenata an endemic riverine species was significantly lower and abundance of Leucenea leucocephala a terrestrial invasive was significantly higher in regulated streams

Discussion: What are your important discussion points and what is the relevance of your results to conservation (if any)?
Findings indicated that prolonged or repeated dry conditions caused by regulated water flows impact the riverine species and may even cause the complete removal of some water loving plant species confined to riverine forests in future. The exigency in restoration with riverine species and control of invasives for conservation of the riverine forests along regulated rivers was identified as a result of the findings of this investigation.