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UBC Theses and Dissertations

Simulation of growth, yield and management of aspen Bella, Imre E

Abstract

A semi-stochastic model was developed to simulate tree growth and stand yield information required for managing aspen (Populas tremuloides (Michx.)). The model is based upon new approaches for evaluating inter-tree competition effects, representing actual tree spatial arrangement, defining interactions between increments of height and d.b.h. and competition measures, and representing random components of variation in tree growth and mortality. In building the model, components of tree growth and mortality were identified and described mathematically or represented directly in the computer. Inter-tree competition, the most important component in the model, was extensively studied. The maximum zone of influence of a tree was derived from estimates of fully open grown crown width. A new hypothesis was advanced and mathematically expressed to describe competition effects on tree growth by exponential terms of ratios based on relative tree size. The model simulates height increments as if each tree was dominant or open-growing. The rate of height growth is a function of site quality. Simulated "potential" height increment is reduced, according to the tree's competitive status, to obtain height increment. D.b.h. increment is based on data from open grown aspen then reduced in proportion to tree competition. Reductions from the maximum rate of growth are based on tree growth and mortality expected in normal aspen stands. In the model mortality is directly related to the tree's competitive status and inversely to its current increment, including random variation, in relation to a specified threshold value. The model was calibrated with data from a normal stand growing on an above average aspen site in Saskatchewan. Input data were from a suitable permanent sample plot having fairly uniform clonal structure. After a few calibration runs and model refinements, simulated stand growth statistics showed satisfactory correspondence with actual growth on the permanent sample plot, and with comparable yield table statistics. Simulations were made also for normal stands growing on poor sites and on the best sites in the same region. The model was generally satisfactory and could replace normal yield tables. After certain extensions, the revised model also simulated growth and development of initially open stands reasonably well. The model also was used to simulate aspen stand growth and productivity in terms of tree component dry matter weights for normal stands on average sites with weight regressions determined in an associated study. For maximum production of wood fibre, the optimum rotation was 33 years for either volume or weight. Although tested only for pure aspen stands, the model can be modified for use with other species. With further refinement, it may be possible also to simulate the growth of mixed stands and uneven-aged stands.

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