The Influence of Cyclones on the Dry Evergreen Forest of Sri Lanka

Abstract

I assessed damage caused by a cyclone in November 1978 to a dry evergreen forest in Sri Lanka. Damage included defoliation, breakage of twigs, branches and trunks, tree fails and post-cyclone tree mortality. Within the 3 km2 study area there was a trend for increasing damage with forest height. Tree species in the upper forest layers had significantly more falls and post-cyclone mortality than trees in the more sheltered subcanopy and shrub layers. Mortality was significantly greater among trees which lost 40 percent or more of their branches and trunks than among trees with lesser crown damage. Defoliation and twig loss were extreme in the discontinuous upper layers and probably contributed to the greater tree mortality evident there. Overbrowsing by folivorous primates after the cydone may have contributed to the demise of some preferred feeding trees. Total tree loss was 46 percent from the upper forest layers, 29 percent from the subcanopy, or 40 percent from all tree layers. Five upper layer tree species were subject to tree losses of 80 to 100 percent representing a virtual elimination of 22 percent of species from the upper layers or 12 percent from the evergreen forest formerly covered 80 percent of the island's land area, and often has been described as old secondary climax in recognition of past disturbance. Extrapolations from meterological data indicate that 33 to 44 percent of the range of dry evergreen forest may be subject to cyclone damage per century. The species composition of dry evergreen forest on a local and wider geographical scale typically is fairly uniform in the subcanopy layer, but variable in the upper layers. I suggest that recurrent cyclone damage may be an important factor contributing to succession in dry evergreen forest and to the variation in species composition of the upper layers. TROPICAL CYCLONES OR HURRICANES are known to cause extensive damage to human life and property in the coastal regions of the Bay of Bengal in the Indian Ocean. Although natural plant and animal populations are also subject to cyclone damage, published reports addressing this phenomenon are rare and concern areas outside the Indian region (e.g., Webb 1958, Whitmore 1974). A recent cyclone which swept through the dry evergreen forest of Sri Lanka provided an opportunity to study the effects on phenology, forest structure and floristic composition. Quantitative data concerning these features were collected before the cyclone, allowing their comparison before and after the cyclone. The objectives of this paper are threefold: first, to assess the nature and extent of cyclone damage to the forest; second, to examine the causes of tree mortality in the 42 months following the cyclone; and third, to consider the effect of recurrent cyclones on the geographic variation in species composition of the dry evergreen forest of Sri Lanka. The long-term recovery of the forest from cyclone damage will not be examined in this report. Data concerning phenology are considered briefly, however, in order to show the immediate effects of the cyclone. NOMENCLATURE AND NATURAL HISTORY The forest types of Sri Lanka have been classified according to several schemes (e.g., Chapman 1947; Holmes 1956; Koelmeyer 1957, 1958; Gaussen et al. 1964; Fernando 1968). Mueller-Dombois (1968) offers an evaluation of these schemes and additional refinements have been made recently by Greller and Balasubramaniam (1980) and Greller et al. (1980). The forest type considered here is commonly known in Sri Lanka as the forest. It has also been referred to as Evergreen Forest (Champion 1935, Chapman 1947, Holmes 1956), Mixed Evergreen Forest (de Rosayro 1950; Koelmeyer 1957, 1958; Andrews 1961), Semideciduous Forest (Gaussen et al. 1964, 1965), South Tropical Moist Deciduous Forest (Chapman 1947, Koelmeyer 1957), Lowland Seasonal Rain Forest (Perera 1975) and Semi-evergreen Forest (Dittus 1977). The last label had been selected to conform to Walter's (1971) worldwide classification of tropical vegetation types, although Walter does not specifically refer to the dry-zone forest of Sri Lanka. In the literature the dry-zone forest is most frequently referred to as Dry Evergreen Forest and therefore I will follow this tradition here. Taxonomic nomenclature follows Abeywick-rama (1959) and Dassanayake and Fosberg (1981, 1983). I Received 2 August 1983, revised 11 May 1984, accepted 14 June 1984. 2 Field address: Smithsonian Institution Primate Project, 4/4 Galkanda Road, Anniewatte, Kandy, Sri Lanka. BIOTROPICA 17(1): 1-14 1985 1 This content downloaded from 207.46.13.127 on Wed, 12 Oct 2016 04:32:28 UTC All use subject to http://about.jstor.org/terms