The Relative Importance or Snow Avalanche Disturbance and Thinning on Canopy Plant Populations

Snow avalanche return intervals on two avalanche paths in the southern Canadian Rockies were estimated from scarred trees and shrubs. The interval between avalanches increased exponentially down each path. The tree and shrub diameters at which avalanches could cause breakage were predicted using both the mechanics of large delections in tapered beams and the resulting stem bending stress, and these predictions were confirmed by comparison to actual diameters at breakage on avalanche paths. Lodgepole pine (Pinus contorta) and Engelmann spruce (Picea engelmannii) bend when small, but were broken by avalanches when larger than °6 cm in diameter at the base. Glandular birch (Betula glandulosa) and willow (Salix glauca) never grew large enough to break. Breakage was influenced by size rather than wood elasticity of strength. Information on thinning mortality was reconstructed from live and dead stems in two Engelmann spruce populations and one lodgepole pine population. Dead trees wer cross—dated, using ring widths, to determine time of death. Avalanche mortality of trees was more important than thinning mortality when the average interval between avalanches was <150 yr. The shift from shrub— to tree—dominated growth habit down the avalanche path occurred when the average interval between avalanches was less than 15 to 20 yr.