Modelling Total Height of Eucalyptus grandis Hill ex Maiden

Abstract

Eucalyptus grandis Hill ex Maiden (rose gum) is an introduced species to Sri Lanka from Australia. At present it has drawn attention of both public and private sectors due to the use as fuel, railway sleepers and sawn timber. This study focused on prediction of total height of E. grandis with age, which is an essential requirement in plantation management. Data were collected from 26 even-aged plantations, covering all favourable regions of Sri Lanka for the growth of E. grandis. At first theoretical equations were formulated with four possible transformations. Then parameters were estimated by fitting data at three different stages. R2 values and standard residual distributions were used as preliminary evaluations. Initially it was tried to model height using tree age as the only explanatory variable. Both linear and exponential functions were used at this stage. The resultant models, however, were not successful for both functions due to low R2 values. Therefore next attempt was made after partitioning the data into different site types using a simple site index. Three different site types were identified at this stage. Then linear and exponential functions were separately fitted to each site type to estimate parameters for different site types while keeping the same basic equation forms. This attempt was also not successful due to low R2 values, large outliers (for some site types) or incompatibility of the resultant models with biological reality. After the above unsuccessful attempts, the last step was conducted by pooling the data again and incorporating a second explanatory variable, site index. Other than multiple linear functions, exponential and logistic functions were modified by adding the second explanatory variable at this stage. Based on R2 and standard residuals, seven suitable models were selected. Then the estimated height values were fitted against an age series to test the distribution and compatibility with biological reality. Finally, after both qualitative and quantitative evaluations, the best model was selected to predict the height growth for E. grandis in Sri Lanka for all site types.