FACTORS AFFECTING THE VEGETATION SUCCESSION OF MOIST·DECIDUOUS FORESTS AT SIGIRIYA SANCTUAR~SRILANKA

Abstract

This study examines the vegetation succession of moist deciduous forests at the Sigiriyasanctuary and some factors affecting for this. Thirty eight 20x20m2 experimental plotswere established in sites ranging in disturbance history from areas abandoned for onemonth after swidden farming to forests not known to have been recently disturbed. Thevegetation was enumerated while the canopy openness, soil moisture and soil temperaturewere measured. Seed rain was monitored while the soil seed bank was enumerated threetimes over a year in four month intervals. Seedling bank was also enumerated in lx2m2quadrat in each experimental plot. Seed predation was examined by keeping seeds ofChloroxylon swietenia seeds in the experimental plots and by continuous observations.

Succession appears to be a process of progressive change in species dominance and lossof less shade-tolerant subordinates rather than accumulation of new colonists. Both seedrain and soil seed bank in all these young successional forests contain many agriculturalweed species but the density of seeds of climax species is negligible. Consequently, theseedling bank is very poor and contains many agricultural weed and light demandingshrub species. The soil temperature in young successional forests is higher while the soilmoisture is lower than the older successional or high forests. After 18-20 years, the treesare released and shade-out light demanding species. This results in the death of most ofthe light demanding shrubs and as a consequence, root competition is reduced and morespace is available for the establishment of new immigrant climax forest species. The seedrain, soil seed bank and the seedling bank of 20-25 year old forests contain some seeds ofclimax forest species.

High forests contain many climax forest tree ,IUd shrub species but seeds a.id seedlings ofvaluable timber tree species are rare in them. High seed predation is one reason for thissituation. Seedling bank is very fragmented and mostly contains individuals of climaxforest shrub species.

It seems that the studied moist deciduous forests are somewhat degraded and thevegetation succession after a large scale disturbance is a slow process. Therefore,protection and rehabilitation of these forests are vital activities for their perpetuity.