The Variation in Leaf Structure of Seedlings in the Families Dipterocarpacead, Clusiaceae and Myrtaceae under Different Light Environments

Abstract

Light has been characterized as the most important environmental factor that affects the leaf structure of tree seedlings in tropical rain forests in South and Southeast Asia. Many studies have been carried out on leaf structure of early-successional and late-successional species and also among the species within the same successional status. These studies provide a clearer understanding of the ecological distribution of each species. The present study examined the leaf structure of six canopy tree species that are considered to be relatively shade tolerant in lowland rain forests in Sri Lanka. Seedlings of Dipterocarpus zeylanicus, D. hispidus (Dipterocarpaceae), Mesua ferrea, M. nagassarium (Clusiaceae), Syzvgium firmum and S. rubicundum (Myrtaceae) were grown in different controlled environmental shelters that simulated a range of light environments found in the groundstorey of the rain forests. Two treatments exposed seedlings to different duration of full sunlight. These simulated the groundstorey Photosynthetic Photon Flux Density (PFD) that were comparable to the canopy openings of about 200 rrr' and 400m2• Three other shelters exposed seedlings to uniform diffuse light conditions that simulated amounts and qualities of radiation like that of the forest understorey (PFD 50, R:FR ratio 0.23; PFD 350, R:FR ratio 0.97; PFD 800, R:FR ratio 1.05). A control light treatment provided full sun light quality (PFD 2000, R:FR ratio 1.27) for seedling growth. The attributes measured were thicknesses of leaf blade, upper and lower epidermis, palisade mesophyll and number of layers, and stomatal frequency. Results show that Syzygium species have the highest leaf blade, and palisade cell thickness and stomatal frequency followed in order by Mesua > Dipterocarpus. This suggests that Syzygium species are more light tolerant but less drought tolerant. The thickest upper and lower epidermis in Dipterocarps reveal that they are more efficient in water use. Syzygium and Dipterocarpus exhibited double rows of cells in palisade mesophyll when exposed to full sun. These findings elucidate some of the relationships between leaf anatomy and the species ecology across the forest topography.