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Sri Lanka has experienced a tremendous boost in large-scale irrigation development. TheUdawalawe irrigation extension project of the Mahawelli Authority will result in theconstruction of a left bank main canal from the Walawe River and the establishment of anirrigation system spanning an area of approximately 15.000 ha. A systematic study onbiodiversity was carried out in the Lower Walawe Irrigation extension area, from February toAugust 2001. This paper summarizes the structure and composition of natural vegetation typesin the study area, during the pre-development stage. A total of 45 transects (each 50mx5m) weredemarcated within 16 sites along the dry to arid zone climatic gradient in the study area. Eachtransect was divided into 10IILx5m sub plots to facilitate sampling and individual plants overlOcm in girth were recorded. The data was analysed using ecological indices (Shannon-Wienerindex and Simpson's Index) and multivariate analyses using the PCORD4 software The area consists of 3 major natural vegetation types; dry thorny scrubland (DTF), rock outcropforests (ROF).and degrade dry zone forest (DDF). A total of 151 plant species, belonging to 52families were recorded from the above vegetation types. The most diverse families wereFabaceae [Relative Diversity (RD)=l1.3], Poaceae (RD=9.3) and Euphobiaceae (RD=8). Interms of density, the most abundant species in the study area were Flueggea leucopyrus(Euphobiaceae) (23.3%) and Phyllanthus polyphyllus (Euphobiaceae) (8.6%) Comparisons of the diversity among vegetation types showed that the degraded dry zone forest"vas high in diversity (Simpson's diversity index; D=17.6, Shannon-Wiener diversity indexH'=3.6) than rock outcrop forests (D=10.7,H=3.1) and scrubland (D=9.9,H=304). Consideringthe species composition and density of flora, a Detrended Correspondence Analysis (DCA)showed the DDF as a distinct cluster, while the ROF exhibited an overlap with the DTS. ATWINSPAN cluster analysis showed that the DTS and DDF separate into two distinct clusters,which correspond to the dry and arid climatic zones. However, the ROF remained as a singlemajor cluster.

THE STRUTURE AND COMPOSITION OF NATURAL VEGETATION TYPES IN LOWER WALAWE BASIN IRRIGATION EXTENTION AREA