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.

Predicting the optimal rotation length of Teak plantations using a simulation model

L H P Gunaratne and T N Subramanium
Department of Agricultural Economics and Business Management,
University of Peradeniya, Sri Lanka.

The traditional problem in Forest Economics has been mainly dealt with the estimation of optimal rotation length for sustainable management. However, given the limitation of the availability of time series data on growth and management in forest plantations in Sri Lanka, such estimations are not feasible. Against this background, this study attempted to develop a simulation model and predict rotational interval of teak plantation under different management conditions.

A field survey was conducted to identify the plantation related parameters in the Kurunegala District. Meantime, a simulation model was developed with Microsoft Visual Basic and Microsoft Excel using the equations specified in the Teak Management Plan (1997) to supplement the field data. The past data obtained from the Forest Department was used to parameterize the model. Using the age and the height of a plantation at a particular time, the model was capable of predicting the plantation related parameters such as dbh and timber volume over the time.

According to the values predicted by the simulation model, four different categories were identified based on the site index, which represents the level of growth of a stand. Rotational interval was estimated using the Faustman rotational model at different discount rates. Thereby it was estimated that the rotational intervals for the four categories at discount rates 10%, 15% and 20% as 19, 17 and 15 years respectively. The internal rate of return obtained through the simulation was above the market rate of 12%. Though the model yielded shorter rotational intervals than the present field values, the approach could be used in future with more reliable field data.

Estimation of change of taper with age for Tectona grandis L. f. (Teak)

M A Nafees, Divisional Forest Office, Ampara and
S M C U P Subasinghe, Department of Forestry and Environmental Science, University of Sri Jayewardenepura, Sri Lanka.

Taper is the change of diameter over a specified length along the tree stem, which varies not only by species and site type, but also by diameter at breast height (dbh), height and other tree growth parameters. Taper is important for the foresters to estimate the upper stem diameters especially in volume calculations. The main objective of the present study was to find out the variation of taper with different ages for Tectona grandis L. f. (teak). For this reason, three teak plantations of 22, 27 and 35 years of age were selected from the Block 01 in Kotagoda plantation, Ampara Forest Division. Selection of these three plantations from a single Block was done to minimise the effect of the site quality on tree growth. Data were collected from randomly laid out 14 circular plots of 0.05 ha. The trees in sample plots were measured for dbh, total height and sectional diameters at 5m and 10m along the stem. In order to estimate the taper variation, the equation developed by Kozak et al (1969) for Douglas fir in Western Colombia was used. This equation is given below and it is considered as one of the best models developed to estimate the upper stem tree diameters. (Equation)

; where d = diameter at height h and H = total height

At the first stage, the data were fitted to the selected equation separately for each age and the regression parameters were estimated separately without changing the basic structure of the original equation. It was observed that the parameter b0 and b2 were positive values and increased from age 22 to 27 and then decreased from that to age 35. The parameter b1 was a negative value which decreased from age 22 to 27 and then increased from that to age 35. When the results were examined further, it was observed that the taper is decreasing with the age proving that the teak tree stems become more conical with the age, i.e., the diameter difference increases with age for a fixed length along the stem.

Construction of a growth model to predict the individual stem volume of Tectona grandis L.f. (Teak) in Sri Lanka

S M C U P Subasinghe
Department of Forestry and Environmental Science,
University of Sri Jayewardenepura, Sri Lanka

Stem volume is one of the most important variables in commercial forestry since all the management decisions are taken on the volume production of trees. Also volume is the most difficult variable to measure and therefore it has to be predicted using a reliable method. Therefore, a growth model was newly constructed in this study to predict the individual stem volume of teak (Tectona grandis L.f.) planted as even-aged monocultures in Sri Lanka.

In order to construct the model, data were collected from 11 plantations in dry zone (Anuradhapura, Hambatota and Puttlam Forest Divisions) and intermediate zone (Kurunegala Forest Division). The selected plantations vary in age from 27 to 44 years. Stratified random sampling was used to collect the data with 0.02 ha circular sample plots. It was decided to use the Newton’s formula for volume calculations and for this reason, the standing tree stems were divided into 3-5 sections and end diameters, mid diameter and length of each section were measured. The final section was assumed as a cone. The stem volume was then calculated by adding all the section volumes together. In addition to that, breast height diameter, total height and crown height were also measured. Using those data, tree volume, basal area and top height were calculated which were necessary for the analysis.

A theoretical model was developed to predict the individual tree volume using the relationship of tree volume with form factor, basal area and total height. Three site factors and four transformations which are biologically accepted were used to enhance the quality of the models. When tested for the site differences using top height/age index, five classes were identified from the 11 plantations. At the first stage of analysis, the data of each site class were fitted separately to the basic model structures. As a result, eight common models were identified for all five site classes with R2 over 91.0 and good standard residual distributions. However, the regression parameters were different for different site classes even for the same model structure.

In order to eliminate the difficulty of using different parameter sets for different site classes, data collected from all plantations were pooled and fitted at the second stage to the selected eight models and new parameter sets were estimated. Then the normal residuals were calculated separately for five site classes using those models and tested using one way ANOVA. Only one model indicated the non-significant residuals for all site types, i.e., v1/2 = 0.5730 (basal area * total height)1/2 + 0.0235 [1/(top height / age)]. That model indicated negligible bias (-0.002) and very high modelling efficiency (0.91). When validated with the reserved data which were not used for model building, it indicated a good distribution of normal residuals. Finally it was concluded that the selected model proved its ability of predicting the unbiased volume of individual teak stems of all site classes in Sri Lanka and therefore it is recommended to use in the field.
(Financial support from the University of Sri Jayewardenepura is acknowledged.)