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.

Evaluation of Teak Growth in Farmer Woodlots in Sri Lanka

Evaluation of growth of teak (Tectona grandis) managed as farmer woodlots in dry and intermediate zones of Sri Lanka with reference to selected soil physical and chemical properties
by
Thilina Jayarathne and Upul Subasinghe

The replanting activities using new species were started in Sri Lanka as early as 1680s with the introduction of teak. Since then teak has become widely planted in dry and intermediate zones of the Island. One of the main methods of establishing teak plantations was by the farmer woodlots with a participatory approach.

However, the growth studies of those woodlots have not widely been conducted in Sri Lanka to identify the social issues and the management problems. Therefore this study was conducted to identify the growth of teak under diffferent soil parameters of farmer woodlots and to identify the possible relationships among growth parameters and soil properties. For this purpose, 17 teak plantations were selected for the data collection with age varying from 8 to 13 years. Those plantations were seleced from Anuradhapura and Polonnaruwa Forest Divisions to represent the dry zone and from Kurunegala Forest Division to represent the intermediate zone. Three farmer woodlots from each plantation were randomly selected and three sample plots were laid in each woodlot. Dbh and total height were measured for all trees in each sample plot. Bulk density, pH, mechanical properties, colour, soil nitrate, phosphorus, potassium, magnesium and calcium were measured using standard methods at the laboratory for the collected soil samples.

After statistical and graphical analysis of those variables, it was observed that both height and dbh indicated exponential relationship with plantation age. The sand% of the selected plantations varied from 68.6 to 96.4%. The clay% varied from 0.3 to 7.5%. Silt% changed from 3.2 to 27.2%. The observed soil types were belonged to sand, sandy loam and loamy sand. The heighest bulk density was 2.313 gcm-3 while the lowest was 0.896 gcm-3. The observed nitrate values varied from 2.133 to 7.466 ppm. The highest and lowest values of phosphorus concentration were 254 and 56 ppm respectively. Potassium varied from 4.126 to 8.326 ppm. However, calcium was not detected for most of the soil samples. It was important to note that the pH values were below 7.0 for all plantations.

When the sand% of soil increased, the growth of teak showed a decreasing trend and higher growth rates were observed with increasing clay%. Moreover, both height and a growth index developed for this study showed an increase with high amounts of nitrogen. Moreover, it was observed that soil nitrogen and phosphorus tend to increase with increasing clay% of soil. Growth of teak in most farmer woodlots are in between height classes II and III according to the yield tables published by Sri Lanka Forest Department.