Abstract:
Soil fertility status and ecology (floristic richness, girth-class distribution, complexity-index, importance value index, and species area curve) of the understorey vegetation were studied in a tropical rainforest in southern Sri Lanka. At three spatially separated areas, (sites A, B, and C) sixteen contiguous plots per site, each 25 x 25 m were established along a strip of vegetation spanning across the entire altitudinal range. Overall floristic and phytosociological heterogeneity within the forest was associated with the combined effects of the variations of topography, aspect and relief, altitudinal gradients, and accompanied differences in surface run-off, erosion and other edaphic conditions. Compared with the sites A and C, the soils of Site-B were of relatively at higher fertility status (lesser amounts of larger particles, better water holding capacity, higher pH and phosphorus concentration). A total of 131 undergrowth woody species (below 30 cm stem circumference) belonging to 95 genera and 43 families waere enumerated from a sampling area of 3 ha, comprising 1 ha from each sampling site. About 38% of the species collected were endemic to Sri Lanka. Of all the species, about 70% were tree species, and the balance, were typical undergrowth species. The species area curve indicated that a sampling area of 25x100 m is sufficient to obtain the maximum number of species in any natural community of this forest. Based on the relative frequency, and basal area, the most dominant species of the forest was Macaranga digyna. However, Polyalthia korinti is the dominant species based on relative density. Based on the IVI of the undergrowth species, three sampling sites represented different plant communities viz., (site A) M. digyna dominated plant community, (site B) M. digyna, Aporosa lindleyana, and Axinadra zeylanica co-dominated plant community, and (site C) Semecarpus gardneri, A. zeylanica, and Gyrinops walla co-dominated plant community. Euphorbiaceae (IVI= 69.9) was the most dominant family of the undergrowth followed by Dipterocarpaceae, Anacardiaceae, Annonaceae and Dilleniaceae with IVI of 37.6, 29.0, 20.4 and 20.3, respectively. On average, the density of woody stems of the undergrowth vegetation of the forest was 3,500 stems/ha. Majority of the individuals was represented by a very few common species. Approximately, 59% of the total number of species and 44% of the endemic species were represented by less than 25 individuals/ha. The proportions of stems of the three stem circumference classes (2-9.4, 9.5-19.4, and 19.5-29.4) at three sites appeared to show a consistent ratio of 6:5:3
