Quantifying wind damage associated with variable retention harvesting in coastal British Columbia

Abstract

Conservation of biological diversity is a key criterion in all sustainable forest management certification schemes. A common strategy for maintaining stand-level diversity after forest harvesting on the coast of British Columbia is variable retention. This approach leaves diverse amounts, types and patterns of tree retention, ranging from single trees to large patches of the original forest. Variable retention contributes to diversity of stand structure; however, wind damage may have a significant effect on the value of retention for different organisms. Wind damage also influences perception of the approach by resource managers and the public. This study quantified the extent of wind damage on forest edges, patches and dispersed individual trees after harvesting retention cutblocks. We also investigated the qualitative and quantitative factors associated with wind damage. The geographic distribution of the study facilitated evaluation of regional variation in windthrow. We sampled 172 harvested areas over a 6-year period from southern Vancouver Island (VI) to Haida Gwaii (Queen Charlotte Islands). The study showed regional differences in wind damage for cutblocks after 2–5 wind seasons. Wind damage was measured as the total percentage of windthrown trees, broken stems and leaning trees within 25 m of forest edges. Average wind damage to cutblock edges (16%) ranged from 11% on southern VI to 25% on the northwest coast of VI. There was a similar regional trend with wind damage to trees retained in large patches and in smaller groups. The average damage along the edges of retention patches over 1 ha (24%) was lowest for southern VI and the BC coastal mainland (16%) and highest for Haida Gwaii (45%). Wind damage to entire groups averaging 0.22 ha in size (37%) varied from 23 to 53% among study areas. For strips of retained timber, wind damage averaged 32% in the 25 m edge sampled. Windward edges were more vulnerable to windthrow than other boundary exposures and damage differed significantly among biogeoclimatic subzones. Topographically exposed locations such as ridge crests and upper slopes experienced more wind damage than middle and lower slopes. The amount of wind damage also increased with increasing stand height and fetch distance. The distance that windthrow penetrated a stand edge was affected by some of the same factors as percent damage. Penetration along cutblock edges varied regionally from 6 m to 20 m. Our results suggest that designing variable retention for local site, stand and wind exposure conditions can reduce the potential for wind damage.