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Modelling windthrow risk in coastal variable retention using tree, neighbourhood, and stand attributes Scott, Robyn Elizabeth

Abstract

The adoption of the retention system in much of coastal British Columbia, Canada, has brought with it concern over the windfirmness of the retained trees. The present state of knowledge concerning windthrow risk factors is inadequate for the purposes of risk prediction in partial cuts and in structurally complex forests, such as those that exist in Clayoquot Sound. This study examined the relationship between the occurrence of windthrow after partial-cut harvesting and various stand, neighbourhood, and tree attributes. Measurements of 1215 trees were obtained from 234 sample plots in retention system cutblocks in Clayoquot Sound on the west coast of Vancouver Island, and of 1891 trees from 115 plots in the Silviculture Treatments for Ecosystem Management in the Sayward (STEMS) study site near Campbell River. At the Clayoquot site, 16.5% of trees were windthrown, while at the STEMS site, 5.3% of trees were windthrown. Logistic regression models were fit for both areas using tree, neighbourhood, and stand variables to predict the probability of individual trees being windthrown. The best-fit models for the Clayoquot and STEMS datasets correctly predicted windthrow status of 72% and 94% of the sampled trees, respectively. The proportion of damaged trees at the Clayoquot site increased with increasing tree height-diameter ratio, crown density (sparse, moderate, full), and an index of fetch distance equal to distance multiplied by removal level, and decreased with increasing percent live crown and post-harvest density. At the STEMS site, windthrow decreased with increasing tree diameter. It is recommended that forest managers plan to retain at least 20% of original stand density in areas where windthrow is a concern, and focus retention efforts on trees with low height-diameter ratios, sparse crowns, and high percent live crown.

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