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Diameter increment models for the mixed swamp forests of Sarawak Chai, Francis

Abstract

Information on the growth and yield of Mixed Swamp Forests in Sarawak is inadequate. Tree diameter increment models were built as an initial effort towards developing a growth and yield modelling system for forests in Sarawak. First, the level of modelling for better predictions of diameter increment of commercial trees was investigated. The three levels of modelling were: (1) developing a single equation for all commercial species, (2) a separate equation for each of two groups, (3) a separate equation for each individual species. Second, linear and nonlinear models for seven important commercial species were developed. Diameter increment was modelled as a function of both tree and stand predictor variables. Only one nonlinear function, based on theoretical concepts, was attempted. Third, the usefulness of crown position and previous diameter growth for predicting current diameter growth was examined. Several objective measures such as mean bias, mean squared bias, and mean absolute deviation were calculated from validation data to indicate accurracy of predictions. Modelling at the individual species level resulted in better predictions than at the single group or two group level. The linear weighted models tested as better predictors than the nonlinear model. Previous diameter growth, in the absence of better predictor variables, was found to be more useful than crown position for predicting current diameter growth.

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