The Ecology of a Semi-Evergreen Forest Community in Sri Lanka

Abstract

Semi-evergreen forests cover the dry zone plains of Sri Lanka and constitute four-fifths of the island's vegetation. In a sample area of 3 kM2, in the Polonnaruwa Sanctuary, 63 tree species were found; 46 were characteristic of other dry zone forests, and 17 occurred only under special edaphic and biotic conditions. As in other dry zone forests, Drypetes sepiara (Euphorbiaceae) prevailed with a relative density of 21.3 percent. The Shannon index of diversity was 4.23 bits per individual, of which 79.4 percent was attributable to evenness; most species had few individuals. Dominance was shared between species typical of the subcanopy and canopy. Measures of diversity and of dominance between species placed the semi-evergreen community in Polonnaruwa as a type intermediate between tropical rainforest and deciduous monsoon forest. Measures of diameters (DBH) and estimates of the height of trees indicated that all species with typically very large trees had few individuals that were distributed more or less evenly through all the size classes. Typically smaller species had large numerical representation. These facts are discussed in light of dominance relationships and regenerative patterns in the community, and are related to possible evolutionary trends. The distribution of most species was clumped, but that of certain rare species was random. Clumping at 2,500 m2 plot size usually meant clumping at smaller-sized plots. Clumping on a large scale reflected local differences in species dominance. Five shrub associations were distinguished by density and constitute dominant and co-dominant species. Differences were related to the amount of light penetrating through the tree canopy and to edaphic factors. Glycosmis pentaphylla (Rutaceae) was the dominant shrub species in the climax association which flourished under a fairly closed tree canopy and lacked an herbaceous layer. ALTHOUGH EVERGREEN RAINFORESTS and monsoon forests of tropical Asia have received considerable attention from eaologists (e.g., Richards 1962, Ashton 1964, Ogawa et al. 1961, 1965), the semi-evergreen forests have been less intensely studied. In Sri Lanka the forests have been surveyed primarily from an economic perspective (e.g., Andrews 1961). This paper aims first, quantitatively to describe some structural features of a community of semi-evergreen forest in Sri Lanka; second, to examine the dominance relationships between species in this community. These relationships are briefly related to possible successional patterns in the dry zone of Sri Lanka, and to possible evolutionary trends among plants in general. The semi-evergreen forest has been described by Walter (1971: 209) as one in which the upper tree layer is defoliated for some of the year [during the droughti,' while the lower tree layer retains its foliage. According to certain structural features, annual rainfall and drought duration, it is a type intermediate between tropical evergreen rainforest and deciduous monsoon forest. The tropical island of Sri Lanka has a gradation of forests from lowland and montane rainforests to lowland scrub forests and savanna (fig. 1). The majority of the forests may be roughly classed as semievergreen, however. These cover the most extensive physiographic feature of Sri Lanka, the lowland plains in the dry zone. Several dassificatory schemes have been proposed for the vegetation of Sri Lanka (e.g., Koelmeyer 1958, Gaussen et al. 1964, Fernando 1968). Mueller-Dombois (1968) evaluates these schemes and offers an up-to-date reclassification according to long-term climatic information. The community of forest described herein is located within the Polonnaruwa Sanctuary in the northwestern dry zone at 070 56' N and 81? 00' E. Polonnaruwa was the capital of Sri Lanka for varying periods between the 8th and 13th centuries. It was completely abandoned by the end of the 13th century, and natural vegetation reclaimed the land for the next 700 years. The area was resettled only at the beginning of the 20th century. Most of the natural vegetation has been maintained within the Sanctuary, although the shrub layer has been removed in areas immediately surrounding archaeological sites. The cutting of trees is prohibited, but poaching, particularly of young trees of construction-pole size, is not completely prevented. Parts of the study area are completely devoid of young trees and shrubs. These areas have been largely omitted in the present sample. The study site encloses approximately 3 km2, is bounded by water and cultivation, and is continuous with more extensive expanses of forest by a narrow