Percent stocking models for four major Alberta tree species

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Summary Based on stem-mapped permanent sample plot data, percent stocking dynamics were modelled for four major tree species in the province of Alberta, Canada: trembling aspen ( Populus tremuloides Michx.), white spruce ( Picea glauca (Moench) Voss), lodgepole pine ( Pinus contorta Dougl. var. latifolia Engelm.) and black spruce ( Picea mariana (Mill.) B.S.P.). Percent stocking was defi ned as the percentage of 10-m 2 plots occupied by at least one tree with a minimum height of 1.3 m. The difference equation technique was applied for model fi tting. Site index was a signifi cant predictor in all models. Heterogeneous residual variances present in the aspen, white spruce and lodgepole pine models were corrected by applying a power-of-the-mean variance function. Serial correlation due to repeated measurements was considered through a linear log covariance structure. A percent stocking index, defi ned as the percent stocking at 50 years total age, was derived from the fi tted percent stocking models and used for assessing species interactions in mixed-species stands. The presence of other species did not appear to affect percent stocking changes over time for aspen and black spruce. However, white spruce percent stocking was reduced when growing together with aspen, and lodgepole pine percent stocking was negatively affected by the presence of aspen, white spruce and black spruce.