Empirical modelling of windthrow risk in partially harvested stands using tree, neighbourhood, and stand attributes

Adoption of the retention silviculture system in much of coastal British Columbia, Canada, has brought concern over the windfirmness of retained trees. Knowledge concerning windthrow risk factors is inadequate for risk prediction in partially harvested stands and in structurally complex forests. In this study, we present a methodology to predict the occurrence of windthrow after partial-cut harvesting in areas with complex stand and terrain conditions. Anticipating that post-harvest wind exposure (fetch) would be a key attribute, we developed and tested several GIS-based methods of characterizing fetch after partial harvest. The methods were applied in a case study in Clayoquot Sound on the west coast of Vancouver Island. Measurements of 1215 trees were obtained from 234 sample plots in retention system cutblocks, which were harvested from 1 to 6 years previously. Sixteen percent of the sampled trees were windthrown. Logistic regression models were fit using tree, neighbourhood, and stand-level variables to predict the probability of individual trees being windthrown. The best-fit models correctly predicted the status of 72 and 74% of the sampled trees. The proportion of damaged trees increased with increasing tree height-diameter ratio and crown density, and with increasing fetch. The proportion of damaged trees decreased with increasing tree percent live crown and post-harvest stand density. We recommend that forest managers plan to retain at least 20% of original stand density in locations where windthrow is a concern, and preferentially retain trees with low height-diameter ratios, sparse crowns, and deep live crowns.