Comparison of model forms for estimating stem taper and volume in the primary conifer species of the North American Acadian Region

The performance of ten commonly used taper equations for predicting both stem form and volume in balsam fir [Abies balsamea (L.) Mill], red spruce[Picea rubens (Sarg.)], and white pine[Pinus strobus (L.)] in the Acadian Region of North America was investigated. Results show that the Kozak (2004) and Bi (2000) equations were superior to the other equations in predicting diameter inside bark for red spruce and white pine, while the Valentine and Gregoire (2001) equation performed slightly better for balsam fir. For stem volume, the Clark et al. (1991) equation provided the best predictions across all species when upper stem diameter measurements were available, while the Kozak (2004) and compatible taper equation of Fang et al. (2000) performed well when those measurements were unavailable. The incorporation of crown variables substantially improved stem volume predictions (mean absolute bias reduction of 7-15%; root mean square error reduction of 10-15%) for all three species, but had little impact on stem form predictions. The best taper equation reduced the predicted root mean square error by 16, 39, and 45% compared to estimates from the widely used Honer (1965) regional stem volume equations for balsam fir, red spruce, and white pine, respectively. When multiple taper equations exist for a certain species, the use of the geometric mean of all predictions is an attractive alternative to selecting the best equation.