Branch models for white spruce (Picea glauca (Moench) Voss) in naturally regenerated stands

Abstract

To describe the branching characteristics of white spruce trees (Picea glauca [Moench] Voss) in naturally regenerated, pure and mixed-species stands of the Canadian boreal forest, predictive models were developed for: (1) the number of branches (⩾5mm diameter), (2) maximum branch diameter, (3) the diameter of all other branches, and (4) the angle of branch insertion (⩾5mm diameter). An additional set of equations were developed for models (1) and (4) for branches ⩾12.5mm in diameter in order to evaluate the factors affecting the recovery of No 1 grade cut lumber. Tree and stand-level variables common to distance independent growth and yield simulators were tested in the models and a generalized hierarchical mixed-effect approach was used to estimate the parameters. Estimates for the branch characteristic were obtained for 1m sections of the live crown since distinct branch whorls were unidentifiable. The relative or absolute depth into the crown (m) were significant variables in all the models, reflecting the importance of branch age and diminishing light levels with increasing crown depth. Increasing social position within a stand, measured by the basal area of larger trees (Bal), was positively related to an increase in the number of branches ⩾5mm per section but did not significantly influence the number of branches ⩾12.5mm, which was more strongly related to crown length and tree slenderness (ratio of height to diameter at breast height). Crown length was a significant variable affecting maximum branch diameters, whereas tree slenderness was important for the diameter of all other branches. The rank of a branch relative to the largest branch within a 1m stem section was related to the diameter of smaller branches, angle of insertion for branches ⩾5mm as well as branches ⩾12.5mm. No species composition or explicit stand-level variables (other than Bal) were significant in any of the models. Within many of the models, the presence of tree-level variables which are known to be sensitive to changes in competition and tree social status suggest that branch characteristics for white spruce within pure and mixed-species stands of the boreal forest are affected by local changes in light availability.