Empirical modeling of cutblock edge windthrow risk on Vancouver Island, Canada, using stand level information

Abstract

Winter storms routinely damage stands along clearcut boundaries on Northern Vancouver Island, disrupting forest management plans. Empirical modelling of windthrow risk is more suitable than mechanistic modelling for the western hemlock ( Tsuga heterophylla (Raf.) Sarg.), redcedar ( Thuja plicata (Donn)) and amabilis fir ( Abies amabilis Forbes.) stands in this area which vary in composition, structure, density and age. Cutblock edge windthrow in a 900 km 2 study area was mapped using 1:15000 scale aerial photographs. A geographic information system (GIS) database for 3000–50 m long by 40 m deep cutblock edge segments was assembled using readily available topographic and inventory information. Logistic regression models for predicting probability of damage for various levels of damage severity were evaluated. Key variables in these models included site quality, stocking, boundary orientation, time since harvest and topographic exposure. The models correctly predicted the outcome of 71–76% of test segments, and adequately fit using χ 2 tests for test data grouped by stand characteristics. These models can be entered into GIS map calculations to produce landscape level maps of windthrow hazard, or into spreadsheets for evaluating the probability of damage for alternate cutblock design scenarios.