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.
