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A cubic tree taper model

Summary A flexible tree taper model based on a cubic polynomial is described. It is algebraically invertible and integrable, and can be constrained by one or two diameters, neither of which need be diameter at breast height (DBH). Fitted to radiata pine data, this taper model ranked better than five more highly parameterised benchmark models for overall accuracy and utility. Analyses of optimum constraint locations showed that a single diameter constraint was best positioned around centroid height (25–30% of tree height), and the best two-diameter constraints were at 10% and either 30% or 60% of total height. Compared with a single DBH constraint, optimal one-diameter and two-diameter constraints improved predictive accuracy by 32% and 47% respectively.