Establishment of relationships of growth at 7-year old mahogany trees with selected site factors

Establishment of relationships of growth at 7-year old mahogany trees with selected site factors in low country wet zone, using GIS as a tool

Himesha Randeni and Upul Subasinghe

Paper presented for the 14th International Forestry and Environment Symposium 2009

Mahogany (Swietenia macrophylla) is an exotic tree, which is heavily adapted to the climatic conditions of wet and intermediate zones of Sri Lanka. Although the state sector manages mahogany with longer rotations, private sector expects to achieve the maximum timber yield within a shorter period. Due to the land scarcity, many of these mahogany plantations have been established in barren and rubber uprooted lands which were heavily degraded. Therefore the soil conditions and site factors directly affect the growth of the mahogany within short rotations.

The present study was carried out in a 7 years old mahogany monoculture plantation established in Gomaragala, in low country wet zone of Sri Lanka to find out the effect of soil and site factors to the mahogany tree growth. Extent of this forest is 20.7 ha and it is managed by a private plantation company. This forest has been divided into 2 lots for the purpose of administration and further divided into 240 plots of 20 perch each.

In order to identify the relationships, tree dbh and height were used as growth parameters. Slope, bedrock establishment and terrain were selected as selected site factors. The growth parameters (i.e.dbh and height) were measured for all the trees in plantation (one measurement from each plot). Slope and terrain as geographical parameters were measured of all 240 plots in the entire plantation.

Since the regression based methods were not adequate for both qualitative as well as quantitative parameter analysis, GIS based analysis was decided to use for the present study, using ArcView 3.3. In order to create digital maps, the survey plan of the selected forest was digitized and georeferanced using 10 ground control points collected by a GPS data receptor. Then the georeferanced base map was digitized to demarcate all the plots and other land marks. After that different maps were prepared in vector form separately for each parameter. However, for the analysis, all these vector layers were converted to raster layers. Raster layers were then reclassified and overlaid two or three layers at a time with the growth parameters to identify the effects. Then map analysis was completed to make decisions regarding tree growth in different site factors.

Results revealed that there are significant relationships between tree growth and the selected site parameters. However, the best conditions for the mahogany tree growth in the particular area are the slope between 11⁰-24⁰, and shallow bedrock prevalence and stony terrain.

Prediction of individual tree volume of Alstonia macrophylla

Construction of a precise growth model to predict the individual stem volume of Alstonia macrophylla
by
Rangika Bandara and Upul Subasinghe

In order to reduce the pressure on existing natural forests of Sri Lanka, Forest Department promoted growing timber species as plantations and in homegardens. Among the suggested species, Alstonia macrophylla (Hawari nuga) has recently become popular due to its faster growth rate, ease of estabishment and timber value.

However, at present, there is no method for estimating the stem volume of this species. Therefore mathematical models were built in this study to predict the individual stem volume of the selected species grown as plantations and in homegardens.

Hawari nuga is sidely found in wet zone of Sri Lanka and therefore the study sites were selected from Kalutara and Galle Districts. For the non-forest areas, sampling was carried out in individual basis and ten 0.02 ha circular plots with slope correction were randomly laid out for each plantation in order to measure the necessary parameters.

Dbh, total height and height to the crown base of the trees were measured. Necessary measurements were also taken to calculate the stem volume using Newton's formula.

A theoritical model structure was developed using the relationship between form factor, volume, basal area and total height. Regression analysis was used to fit the data into the model. Untransformed as well as transformed combinations of the model structures were tested in order to select the best models. After examining the statistical performances of the resultant models, two models were finally selected for plantation grown trees and open grown trees. Those models had very high modelling efficiencies and negligible bias. When validated with the reserved data at the beginning of the modelling procedure, the selected models proved the capability of predicting stem volumes of Alstonia macrophylla grown in plantations and homegardens.