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

Abstract• 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.Résumé• Les performances de dix équations de la décroissance de la tige, couramment utilisées pour prédire à la fois la forme du tronc et le volume pour le sapin baumier [Abies balsamea (L.) Mill], l’Épinette rouge [Picea rubens (Sarg.)], et le pin Weymouth [Pinus strobus (L.)] ont été étudiées dans la région de l’Acadie en Amérique du Nord.• Les résultats montrent que les équations de Kozak (2004) et de Bi (2000) étaient supérieures aux autres équations pour la prédiction du diamètre sous écorce pour l’épinette rouge et le pin Weymouth, tandis que l’équation de Valentine et Gregoire (2001) était légèrement meilleure pour la forme du tronc du sapin baumier.• Pour le volume de la tige, l’équation de Clark et al. (1991) fourni les meilleures prévisions pour toutes les espèces lorsque les mesures du diamètre de la partie supérieure de la tige étaient disponibles, tandis que l’équation de Kozak (2004) et l’équation compatible de défilement de Fang et al. (2000) conviennent bien lorsque ces mesures n’étaient pas disponibles.• L’incorporation de variables de couronne a amélioré sensiblement les prédictions du volume des troncs (réduction moyenne des biais absolu de 7–15 % ; réduction de l’erreur quadratique moyenne de 10–15 %) pour les trois espèces, mais avait peu d’impact sur les prédictions de la forme du tronc.• La meilleure équation de décroissance a réduit l’estimation de l’erreur quadratique moyenne de 16, 39, et 45 % par rapport aux estimations largement utilisées avec les équations régionales d’Honer (1965) pour l’estimation du volume de la tige respectivement pour le sapin baumier, l’épinette rouge et le pin Weymouth.• Lorsque plusieurs équations de défilement existent pour certaines espèces, l’utilisation de la moyenne géométrique de toutes les prédictions est une alternative intéressante pour la sélection de la “meilleure” équation.

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