Development and long-term evaluation of harvesting patterns to reduce windthrow risk of understorey spruce in aspen-white spruce mixedwood stands in Alberta, Canada

Summary The Hotchkiss River Mixedwood Timber Harvesting Study developed new approaches to harvesting systems for western Canada's boreal mixedwood forests. Conventional harvesting equipment was used to test 11 harvesting and silvicultural systems over a 530-ha site. These include one- and two- pass modifi ed uniform shelterwoods, two- and three-pass strip shelterwoods, two-pass alternate strip shelterwoods and four-pass progressive strip shelterwoods. These were used to test varying levels of wind protection designed to protect and minimize wind damage to understorey (immature) white spruce residuals following harvest of the aspen overstorey. Ongoing monitoring of wind dynamics and associated windthrow patterns since 1992 have provided clear management practice guidelines for reducing windthrow of immature spruce and residual aspen following harvesting. There are thresholds related to spruce height and distance from aspen residuals, beyond which windthrow damage increases signifi cantly. The infl uence of topography, timing of harvest and the spatial confi gurations of multiple harvests on windthrow dynamics have been clarifi ed through this research. These results have provided valuable information to help forest planners to utilize harvesting and silvicultural systems that best reduce windthrow damage to understorey spruce, following harvest of overstorey aspen.