Prediction of the Early Growth of Plantation Grown G. walla

Abstract

Certain tree species of Thymalaeaceae family produce highly fragrant, valuable resin called agarwood inside the stems, branches and roots due to a defence mechanism to protect internal tissue damage from invading microorganisms. Gyrinops walla is the only native tree species bearing the agarwood resin production ability which is growing in the low and mid elevations of wet climates of Sri Lanka. After some years of discovering the ability to produce agarwood resins in of G. walla, private sector investors planned to establish small and medium scale plantations using this species. However, information on G. walla growth rates under plantations conditions were not available, which are essential for the effective management. Therefore the present study aimed at constructing height and diameter prediction models for the early stages of G. walla plantations. For this purse, monthly measured height and diameter data of 40 plants of an even-aged G. walla plantation were used. Several non-linear and 2nd and 3rd order polynomial models were initially tested, keeping age as the single explanatory variable. Among them, the best performances were given by the 2nd order polynomial models for both height and diameter variables. Both models had R2
over 99.0 and root mean square error and mean absolute difference less than 0.10, proving high accuracy. Fitted line plots also did not indicate deviations of the residuals. Though the models built in this study are recommended for predicting the early plantation growth of G. walla, future research should be conducted to validate them till the maturity of the trees.

Keywords: G. walla, polynomial regression, growth modelling, plantation, height, diameter