in Sri Lanka - Ministry of Environment

Present Status of Dry-zone Flora in Sri LankaAnoma Perera,Department of Botany, University of Peradeniya, Peradeniya.A. Introduction to the ecosystemThe dry land of Sri Lanka which is classically described as the dry and intermediate zones ofthe country, extends over about 66% of the total is land area, but nearly 85% of the naturalvegetation-cover of the country exists within this region (Legg and Jewell, 1995). Three majornatural terrestrial vegetation types are found in the dry land of Sri Lanka; viz, dry forests, savannaand grasslands. Depending on the variability in their location, climate, edaphic characteristicsand biotic impacts, it appears that several sub-categories of the above vegetation types canbe identified.Dry forest ecosystemsSri Lankan dry forests are a part of the tropical rainforest biome. These exhibit characteristicfeatures of tropical semi-deciduous or tropical seasonal forests of the world vegetation typesdescribed by Longman and Jenik (1987), in Archibold, (1995). Tropical semi-deciduous forestsgrow in low moisture conditions (e.g. annual rain fall < 1200 mm) and are mainly found inBundala National Park (Figure 1), southern part of Yala National Park and north-eastern partof Wilpaththu National Park. Presence of deciduous or leaf exchanging species in the forestcanopy is a salient feature of these forests.In contrast, tropical seasonal forests (Figure 2) occur in areas where comparatively higherrainfall (mean annual rainfall of 1200-1500 mm) is received and the major portion of the dryforests of the country falls into this forest category. These forests grow taller than tropical semideciduousforests and show a clear stratification. Moreover, some evergreen species may bepresent in the canopy. However, the deviations in recorded physical features such as soil typeand elevation (Alwis and Eriyagama, 1969) as well as the slope of the terrain appear to resultin spatial heterogeneity in the soil moisture contents. This has resulted in the formation ofdifferent forest communities which deviated from the typical Manilkara hexandra-Chloroxylonswietenia-Drypetes sepiaria community of lowland tropical seasonal forests.It has been reported that most of the dry forests in the lowlands had grown after the destructionof hydraulic civilization in the area around 13 th century (Brohier, 1941; de Rosayro, 1961) andtherefore, are secondary in origin. Also, these forests are heavily disturbed by human activitiessuch as timber logging and frequent shifting cultivation (de Jong et al., 2001; de Rosayro, 1961;Perera et al., 1995; Perera, 2001a) resulting in secondary forests or successional forests atdifferent ages (de Rosayro, 1961; Perera, 1998, 2001a). Natural regeneration of Sri Lankandry forest species is reported to be very poor (de Rosayro, 1959; Holmes, 1957; Perera et al.,1995; Perera, 2001a; Rutnam, 1959) which is attributed to the poor quality and quantity of theseed rain (Perera, 1998, 2004), soil seed bank (Perera, 1998a, 2000, 2004, 2005) and theseedling bank (Perera, 2001b, 2004). Repeated disturbances in secondary forests and lack ofregeneration of high forest species may result in scrub jungle plagioclimaxes through deflectedsuccession but these are not a true climax vegetation type of the dry land of Sri Lanka. Such165