CHANGE OF BREAST HEIGHT DIAMETER WITH AGE OF Swietenia macrophylla(MAHOGANY) EVEN-AGED MONOCULTURES

Abstract

Growth models are important tools for growth and yield prediction in forest plantations. Species specific growth models facilitate to investigate appropriate management guidelines and silvicultural strategies. Mahogany is one of the imperative timber species in commercial plantation sector in Sri Lanka. Most of the mahogany plantations are managed in order to receive a higher financial return while maintaining of shorter rotations. However, growth models in such aspects are lacking in Sri Lanka for the above aspects. Therefore this study is important, as it investigated the distribution of breast height diameter (dbh) of mahogany trees growing in different site types with their age.

For this study, data were collected from sixteen mahogany monocultures in Kalutara, Ratnapura  and Matale districts. In order to represent growth of the whole plantation, each plantation was visually categorized in to good, moderate and poor sites. Twenty trees were selected from each site and altogether sixty trees  were selected  from each plantation. A theoretical basic model structure was developed between dbh and age for each site types, in order to establish a model to find out the relationship between dbh and age of mahogany. That basic structure was tested with linear regression analysis and non-linear regression analysis such as exponential and logistic functions. In addition to the untransformed explanatory variables, several transformations were also used for identification of the best model. The best equation was selected using R2value and analyzing residual distribution. Further, a common model was tested for all site types with pooled data as it is difficult to use different models for different site types. However, further analysis proved that this attempt was not successful. Therefore it was decided to build growth specific models to express relationship between dbh and age of mahogany.

For the three finally selected models for three site types (good, moderate and poor), dbh has an exponential relationship with height. The R2values were above 70% and the residual distributions were good. Finally, a height based site/growth classification system was also introduced in this study, so that the appropriate model can be used for the selected site type.

Key words: Mahogany, Growth models, Yield prediction