Use of upper stem diameters in a polynomial taper equation for New Zealand radiata pine: an evaluation

Abstract

Polynomial taper models are the tree profile equation types that are most commonly used to describe stem profiles of New Zealand radiata pine (Pinus radiata D.Don). Among these equations are model forms that include an upper stem diameter measurement as a predictor. Such taper functions may be more costly to use given the need to measure the additional stem diameter. Therefore, it is important to have an insightful understanding of the contribution of the additional diameter measurement to taper model prediction bias and precision to better inform decisions on whether to develop advanced technology for, or invest in, tree upper stem diameter measurements(s) for taper equation use. Prediction precision and bias for diameter and volume were evaluated for a regular polynomial taper model with dbh and total height as tree-level predictors (DH model) and eight taper models that included an upper stem diameter as an additional tree-level predictor (DH+ models). Two sets of radiata pine stem sectional data from 66 stands across New Zealand were used in the investigation. Seventy percent of the trees in the larger one of the two sets were used for model fitting. The rest of the trees in the larger data set, and those in the remaining data set, were used for model validation. A DH+ taper equation that included an upper stem diameter measured at half the distance between breast height and the tree tip exhibited the lowest prediction error for stem diameters. Most of the reductions in diameter prediction error, from use of this equation, occurred in the upper half of the tree bole. Tree-level volume predictions from both DH and DH+ taper models were unbiased across the range of tree sizes investigated, but DH+ models exhibited greater precision in volume prediction. Including an outside bark upper stem diameter, measured at half the distance between breast height and the tree tip, in a polynomial taper equation for New Zealand radiata pine may result in a considerable improvement in tree volume prediction precision especially for total aboveground volume. Use of a diameter measured at 6 m may not result in prediction precision that is different from that of a DH taper equation.