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Assessing and promoting windfirmness in conifers in British Columbia Mitchell, Stephen Jarvis

Abstract

Windthrow is a natural disturbance agent which disrupts forest and stand level plans in British Columbia (BC). The objectives of this thesis were to: i) review the factors contributing to windthrow risk in BC, ii) investigate patterns of stem growth in conifers following thinning and consider their use for diagnosing windfirmness; and iii) present a diagnostic method of assessing windthrow risk suited to the heterogeneity of BC's forests. Stem analysis was used to reconstruct the post-thinning stem growth patterns for 25 Sitka spruce (Picea sitchensis (Bong.) Carr.) from a coastal stand which was thinned in 1980, and 45 Douglas-fir {Pseudotsuga menziesii var. glauca (Beissn.) Franco) from a stand on the interior plateau with 1978 and 1984 thinned portions. Both stands had very high initial densities and sample trees represented a range of stem slenderness in the thinned and control portions. Following thinning, height increment was temporarily reduced, radial increment increased and the allocation of radial increment became more basal in each of the three thinning treatments. The post-thinning stem slenderness curves were reverse-S in shape with initially more slender trees showing the greatest decline. Critical turning moments of 36 Douglas-fir trees winched to failure were related to tree size. With a uniform wind profile over the crown length, critical wind speed declined with slenderness. The relationship between safety factor and slenderness varied between treatments depending on the attenuation of the within-canopy wind profile. The short term growth responses, their relationship with initial stem slenderness and the shape of the slendemess adjustment curves suggested a pattern of form re-equilibration following thinning. It appears that observation of the magnitude and duration of post-thinning stem form adjustment is a useful diagnostic tool for scheduling subsequent thinning entries in stands with high initial slendemess, and for identifying more vulnerable trees. Forest managers in BC use a system of site and stand diagnosis during the preparation of stand level prescriptions. A diagnostic framework for windthrow risk is presented which can be incorporated into the prescription process. In this framework, the principle of acclimative growth is used in assessing the stand hazard and treatment risk components of windthrow risk. Recommendations are made for a comprehensive program of windthrow management.

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