THE VARIATION IN LEAr STRUCTURE O~ SEEDLINGS IN THE FAMILIES DIPTEROCARPACEAE, CLUSIACEAE, AND MYRTACEAE UNDER DIFFERENT LIGHT ENVIRONMENTS

Abstract

Light has been characicriscd as the most important environmental factor that affect the leafstructure of tree seedlings in tropical rain forest in South and Southeast Asia. Many studieshave been carried out on leaf structure of early-successional and late-successional specic-,and also among the species within the same successional status. These studies have helpedin creating a clear understanding of ecological distribution in relation to each species. Thepresent study examined the leaf structure or six canopy tree species that are considered to berelatively shade tolerant in lowland rain forests in Sri Lanka.

Seedlings of Dipteracarpus tevlcnticns, D. hispidus (Diptcrocarpaceae), Mesua f{'ITNI. M.nagassarium (Clusiaccac). Sy:rgillillfirnllll'/1 and S. rubicundum (Myrtaccac) were grown indifferent controlled environmental shelters that simulated a range of light environmentsfound in the ground storey of the rain forest. Two treatments exposed seedlings to differentduration of full sunlight. These simulated the groundstorey Photosynthetic Photon FluxDensity (PFD) that were comparable to the centres of about 200 m2 and 400 m2 canopyopenings. Three other shelters exposed seedlings to uniform diffuse light conditions thatsimulated amounts and qualities of radiation similar to that of the forest undcrstorey (PFD50. R:FR ratio 0.46: PPO 350. R:FR ratio 0.97; PFO XOO,R:FR ratio 1.05). A control lighttreatment proved full sun light quality (PPFD 200n, R:FR ratio 1.27) for the seedlinggrowth.

The attributes measured were thickness of leaf blade. upper and lower epidermis. palisademesophyll and number of layers, and stomatal frequency. Results show that S\'~\'gil/lllspecies have highest in leaf blade, palisade cell thickness and stomatal frequency followedin order by MC.I'I.w>Dipterocarpus. This suggests that S\'~ygillill species are more lighttolerant but less in drought tolerance. The thickest upper and lower epidermis inDipterocarpus reveal that they are more efficient in water use. Svzygiun: and Dipteroiarpusexhibit rows in palisidac mcsophyll when exposed to full sun. These findings elucidatesome of the relationships between leaf anatomy and the species ecology across the foresttopography.