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Modelling juvenile height in mixed species, even aged interior cedar-hemlock stands Froese, Robert Edgar

Abstract

Height models for juvenile even aged mixed-species stands were developed for locations near Nelson, British Columbia. Separate models were developed for each conifer species found, as well as for paper birch (Betula papyrifera). The objectives were to develop models for: 1) the number of years to reach breast height; 2) height achieved at a given age above breast height (height yield); 3) separate models for all trees (average height) and the largest diameter dominant tree free of damage or suppression on a 0.01 ha plot (top height); and, 4) site and stand attributes incorporated as independent variables. Numerous biophysical variables were tested as predictors of juvenile height. Statistically significant models for years to breast height and for height yield were developed for most species; most using variables other than site index. For modelling height yield, two ad hoc methods for including independent variables were used. For trees with multiple measurements ad hoc models were contrasted with a two-stage parameter prediction approach. The best ad hoc models used functions of site index; however, the parameter prediction approach produced models with equal or better fit to the best ad hoc models, including those using site index alone. Few consistent trends in model form were observed for both years to breast height and height yield, and many models were not biologically tenable and should not be applied operationally. Because all combinations of independent variables were not equally sampled, the data may have not been sufficient to capture trends or variables may be acting as analogues for other causal factors. Regardless, the measured variables were useful predictors of juvenile height, even with small data sets. This promising result demonstrated the validity of the approach and the potential for precise height models not based on site index.

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