EFFECT OF LIGHT AND SOIL MOISTURE ON SEEDLING LEAF ANATOMY OF FOUR CANOPY TREE SPECIES IN A LOWLAND RAIN FOREST, SRI LANKA

Abstract

This study examined the variation in leaf structure among seedlings of four canopy tree species that have been characterized as late-successional and relatively shade-tolerant. Seedlings of Dipterocarpus zeylanicus Thw., D. hispidus Thw., Mesua ferrea L., and M. nagassarium (Burm.f) Kosterm., were grown for two years within replicated environmental shelters that had various light and soil moisture treatments. These treatments reflected the range of microenvironments that have been observed within the mixed-dipterocarp forest of southwestern Sri Lanka. Three light treatments exposed seedlings to uniform diffuse light conditions simulating radiation at the forest understorey and forest edge (PPFD, 50, 350, and 800 mmol m-2 s-1). Two treatments also exposed seedlings to direct radiation comparable to the centers of 200m2 and 400m2 canopy openings. A control treatment exposed seedlings to full sun (1600 mmol m-2 s-1). For each light treatment seedlings were grown in either soil watered to field capacity or soil that was consistently <30% of field capacity. After two years, leaf samples were taken for each species and light-soil moisture combination. Measures were made of leaf blade thickness, stomatal frequency, and thicknesses of upper epidermal, palisade mesophyll, and lower epidermal cell layers. Significant differences in measures of leaf structure were shown among species and among the various treatments.

In general all species showed leaf dimensions and cell layers that increased with increase in amount of light and decrease in availability of soil moisture. Largest dimensions were therefore recorded from leaves exposed to full sun but grown in relatively dry soil. Mesua ferrea exhibited the thickest leaf blades followed by M. nagassarium, D. zeylanicus and lastly D. hispidus. Dipterocarpus zeylanicus had greater thicknesses of epidermal and palisade mesophyll layers than M. ferrea. This suggested that M. ferrea has a much thicker spongy mesophyll layer than the other species. Unlike Mesua, both Dipterocarpus species exhibited double rows of cells within the palisade mesophyll and lower epidermal layers in certain treatments. This phenomenon increased in frequency with amount of light. Greatest densities of stomata were measured for D. hispidus followed in declining order by M. ferrea, M. nagassarium and D. zeylanicus. Difference in shade anddroughttolerance among species in relation to site specialization is discussed.