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

Background/Question/Methods

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 varying 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 impact on the value of retention for different organisms. 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 site and regional differences in wind damage.

We sampled 172 harvested blocks over an 8-year period from southern Vancouver Island (VI) to Haida Gwaii. The 4648 sample plots represent 366 km of external cutblock boundaries, 26 km of large patch edges, 197 ha of small retention patches and 50 km of riparian and other strip edges. Damage was measured as the total percentage of windthrown trees, broken stems and leaning trees within 25m of forest edges.

Results/Conclusions

The study showed regional differences in wind damage after at least 2 wind seasons. Average damage to external cutblock edges (16%) ranged from 11% on southern VI to 25% on the northwest coast of VI. There was a similar regional trend with damage for patches, groups and strips. The average damage along the edges of larger patches (24%) was lowest for southern VI and the BC mainland (16%) and highest for Haida Gwaii (45%). Average damage in small groups (39%) varied from 20% to 45%; damage to strips of retained timber averaged 31%.

 Windward edges were more vulnerable to damage than other boundary exposures. Topographically exposed locations such as ridge crests had more wind damage than valley floors. The amount of damage also increased with increasing stand height and cumulative fetch. Windthrow penetration into stand edges varied from 6m on southern VI to 20m on northwestern VI. Although wind damage is typically viewed as negative from a visual and economic standpoint, wind plays an important role as a natural disturbance agent. Many forest species depend upon downed wood as habitat. Still, the wind damage we documented raises concerns about rates of windthrow that are higher than natural levels.