Windthrow following four harvest treatments in an Engelmann spruce - subalpine fir forest in southern interior British Columbia, Canada

We used transect surveys at a large-scale experimental site at Sicamous Creek, B.C., to measure the effects of five treatments on windthrow: 10-ha clearcuts, arrays of 1-ha patch cuts, arrays of 0.1-ha patch cuts, individual-tree selection cuts, and uncut controls. We also examined edge effects and conditions predisposing trees to windthrow. Windthrow of subalpine fir (Abies lasiocarpa (Hook.) Nutt.) in the 2.7 years following harvesting increased from 0.6% of basal area per year in uncut forest to 0.8-1.8% per year in harvested treatments, with highest rates in individual tree selection units and lowest rates in 0.1-ha patch-cut arrays. Engelmann spruce (Picea engelmannii Parry ex Engelm.) showed similar patterns of windthrow but lower rates (0.2-0.7% of basal area per year in harvested treatments). Windthrow was concentrated near north and east edges of 1-ha and 10-ha openings but was dispersed throughout the more uniform treatments. Windthrown trees did not differ from random trees in diameter but had lower height/diameter ratios, probably reflecting the greater windthrow observed in subxeric sites on complex, elevated topography. The rates and distribution of windthrow in different harvest treatments have implications for ecological processes, salvage, long-term windthrow potential, and mitigation possibilities.