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UBC Theses and Dissertations

Examination and modelling of tree form and taper over time for interior lodgepole pine Muhairwe, Charles K.


Forest management, concerned with maintaining or increasing the output of the different forest resources, is increasingly becoming more intensive. Therefore, relevant, accurate, timely, and cost-effective forest resource information on the current inventory and future growth potential is critical. This information must also be utilized over time in the most effective way for planning purposes. Such information can he provided via forest inventories and growth and yield studies. Understanding and modelling of taper changes over time will provide some of the necessary information in an efficient manner. The two objectives of this study were: (1) to investigate changes in tree form and taper over time as affected by changes in tree, stand and site variables, and (2) to develop a dynamic taper function for dominant and codominant trees of interior lodgepole pine (Finns contorta Dougi.) based on the results of this investigation. To meet these objectives, two different sets of data were used: permanent sample plot data from Alberta and detailed stem analysis data from Interior British Columbia. The permanent sample plot data were used to develop a model to predict stand density and to select models to predict total tree height and diameter at breast height. The stem analysis data were used to examine tree form and taper changes over time, and to select, fit, and test the dynamic taper model. Instead of developing an entirely new taper model, existing taper functions were investigated as possible candidates for both objectives. Two static taper models, the simple ta.per equation (Husch et al. 1982, p. 99) and the variable-exponent taper equation by Kozak (1988), were selected. The first objective of this study was achieved. Tree shape and taper were found to change along the stem at one time and over time with changes in tree and stand variables, such as the ratio of diameter at breast height to total tree height, crown length and crown ratio, and stand density. It was also found that trees have a simple parabolic shape at young age. However, as trees age or increase in size, different portions of the stem take different shapes because of unequal diameter growth along the stem. Stand density and crown size appear to the determining factors in tree shape and taper changes and any changes in these two factors will determine tree shape and taper. The second objective of this study was also achieved. By incorporating tree. stand, and site factors into a simple static variable-exponent taper model by Kozak (1988), a dynamic taper function was developed. This dynamic taper function tracked the behaviour of very complex tree shape and taper changes over time with reasonable accuracy. The function was tested using a validation data set and it provided consistent estimates of diameter inside bark along the stem over time. The model was fitted using ordinary linear and nonlinear least squares. Feasible generalized least squares was considered, but not used, because of the difficulties in obtaining a consistent estimate of the error covariance matrix. The dynamic taper modelling approach will be a useful tool in forest management because the taper models will enable forest managers to simulate stand development in order to achieve specific objectives. Dynamic taper modelling appears to be a feasible and practical idea, and it is recommended that dynamic taper models for other species and crown classes he developed to incorporate in individual tree growth models.

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