An assessment of vegetation and canopy structure of moderately exploited natural forest area in Yagirala forest reserve

W M P S B Wahala,Department of Forestry and Environmental Science, University of Sri Jayewardenepura, Sri Lanka
W A J M De Costa, Department of Crop Science, University of Peradeniya, Sri Lanka. and
D M S H K Ranasinghe, Department of Forestry and Environmental Science

Study assessed the vegetation composition and structure and the forest canopy structure in terms of Leaf Area Index (LAI), Mean Leaf Angle (MLA) and canopy openness in different elevational classes of moderately exploited natural forest area which covers about 82% of total natural forest cover in Yagirala forest reserve, a tropical lowland rain forest selectively logged by State Timber Cooperation in late 70’s.

Canopy architecture termed as angle distribution of foliage elements (Chen et al. 1992), can be quantified by the leaf area index (LAI) and mean leaf angle (MLA). In this study Hemispherical photographic method was used to characterize canopy architecture at three elevational classes (i.e. valley, mid-slope and ridge top). At each elevational class, hemispherical photos of the forest canopy were taken at each sampling point at a height of 1m above the ground along transects up to 200m at 50m intervals. Hemispherical photographs were analyzed using HemiView 2.1 canopy analysis software.

A vegetation survey was carried out to determine floristic composition of dominant species and families, which contribute more to the forest canopy. The enumeration was carried out using 0.05 ha circular plots at three elevational classes, totally covering 0.6ha of the area. Individuals taller 1m were enumerated and species, diameter at breast height (dbh) and total height measurements were recorded and relative basal area, relative frequency, relative density and Importance Value Index (IVI), diameter class distribution were estimated.

Leaf area index (LAI) and mean leaf angle (MLA) did not show significant variation between three elevational classes. LAI mean value of low elevation areas show high value of 2.256 and mean value of high elevation areas show low value of 2.087. Average MLA value for the moderately exploited area is 29.14. Canopy openness given in terms of visual sky fraction is also not significantly diferent between three elevational classes. The results give an estimation of homogeneity of canopy openness within the moderately exploited natural forest.

59 species were identified belong to 29 families, and 1804 stems per hectare were recorded. According to the IVI, Myristicaceae, Dilleniaceae, Rutaceae, Euphobiaceae and Ebenaceae were recorded as the most abundant families. Myristica dactyloides Dillenia retusa Acronychia pedunculata and Bridelia moonii were recorded as the most abundant tree species, which contribute more to the canopy cover.