Tuesday, October 10, 2006

Seedling Survival And Growth Of Mesua And Dipterocarpus Species In A Sri Lankan Rain Forest

B. M. P. Singhakumara and P. M. S. Ashton
1Department of Forestry and Environmental Science, University of Sri Jayewardenepura.
2School of Forestry and Environmental Studies, Yale University New Haven, USA.

Seedlings of Dipterocarpus (D. hispidus and D. zeylanicus) and Mesua (M. ferrea and M. nagassarium) were investigated for their survival and growth in different microenvironments in a lowland rain forest in south-west of Sri Lanka. These species occur together as canopy dominants of late-successional rain forest. Partitioning of the regeneration niche can be one explanation for the co-existence of ecologically similar canopy tree species within a forest. This study tested that differences in disturbances across the topography of lowland rain forest cause different survival and growth rates in closely related tree species among and within canopy openings of different size.

Seedlings were planted in plots located in five zones representing a range of forest around storey micro-environments found adjacent to and across canopy openings of three sites- valley, mid slope and ridge top. Plots were located from north to south across each gap (i) under storey 10 m to the south of the gap, (ii) southern gap edge, (iii) centre, (iv) northern edge and (v) under storey 10 m to the north of the gap. Experiments were designed to monitor survival and growth of planted seedlings for two years. At the end of two years survival was calculated, height increment recorded and destructive samples were taken to measure dry mass gain of root, stem and leaves.

Seedlings of Mesua ferrea, M. nagassarium and D. zeylanicus showed high survival on all sites and in all gap/ canopy conditions. D. hispidus showed the lowest survival in all sites. Growth measures attained maxima in the centre gap/ canopy condition in the three sites for all species. Results also demonstrated clear differences among species. Dipterocarpus exhibited greater growth responsiveness than Mesua species. These differences appeared to be related to availability of soil moisture and ground storey radiation regimes. This study demonstrated that closely related species have specific growth characteristics that allow each to establish and grow better than its relatives in particular forest microenvironments.

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