Tuesday, October 10, 2006

The Variation In Leaf Structure Of Seedlings In The Families Dipterocarpaceae, Clusiaceae And Myrtaceae Under Different Light Environments

B.M.P. Singhakumara1, H.K. Gamage1 and P.M.S. Ashton2
1Department of Forestry and Environmental Science, University of Sri Jayewardenepura, Nugegoda.
2School of Forestry and Environmental Studies, Yale University, New Haven, USA.

Light has been characterized as the most important environmental factor that affect the leaf structure of tree seedlings in tropical rain forest 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 have helped in creating a clear understanding of ecological distribution in relation to each species. The present study examined the Icaf 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), Syzigium firmum and S. rubicundum (Myrtaceae) were grown in different controlled environmental shelters that simulated a range of light environments found in the ground storey of the rain forest. Two treatments exposed seedlings to different duration of full sunlight. These simulated the ground storey Photosynthetic Photon Flux Density (PFD) that were comparable to the centres of about 200 m2 and 400 m2 canopy openings. Three other shelters exposed seedlings to uniform diffuse light conditions that simulated amounts and qualities of radiation similar to that of the forest understorey (PFD 50, R:FR ratio 0.46; PFD 350, R:FR ratio 0.97; PFD 800, R:FR ratio 1.05). A control light treatment proved full sun light quality (PPFD 2000, R:FR ratio 1.27) for the seedling growth.

The attributes measured were thickness of leaf blade, upper and lower epidermis, palisade mesophyll and number of layers, and stomatal frequency. Results show that Syzygium species have highest in leaf blade, palisade cell thickness and stomatal frequency followed in order by Mesua>Dipterocarpus. This suggests that Syzigium species are more light tolerant but less in drought tolerance. The thickest upper and lower epidermis in Dipterncar/n4s reveal that they are more efficient in water use. Syzigium and Dipterocarpus exhibit rows in palisidae mesophyll when exposed to full sun. These findings elucidate some of the relationships between leaf anatomy and the species ecology across the forest topography.

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