Species, tree size, and overstory environment affect likelihood of ice storm damage to understory trees in a mature Douglas-fir forest

Glaze events (i.e., ice storms) are a potential source of disturbance whose effects have not been heavily researched in forests in the Pacific Northwest. This study examines the effects of species, size, and overstory environment on occurrence, source, and severity of damage sustained by planted understory trees in a mature Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest as a result of the glaze event that struck the central Oregon Coast Range in November 2014. Understory tree species were Douglas-fir, grand fir (Abies grandis (Douglas ex D. Don) Lindl.), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and western redcedar (Thuja plicata Donn ex D. Don). Overall, western redcedar was damaged least frequently, and western hemlock was damaged most frequently. Redcedar was less susceptible to crown damage and more susceptible to bending damage than the other three species, while hemlock showed the opposite trend. We found significantly lower (P < 0.001) likelihoods of ice loading damage to understory trees from increasing overstory basal area under both conifers and hardwoods, but significantly higher (P = 0.006 and P < 0.001 respectively) likelihoods of damage from falling debris. The opposition of these trends resulted in no change in likelihood of damage occurrence with increasing overstory conifer density (P = 0.544), while likelihood of damage was higher under increasing hardwood density (P < 0.001). In examining the influence of tree size, we found that increasing diameter at breast height resulted in a significantly lower likelihood of bending (P = 0.009), but a significantly higher likelihood of crown loss (P < 0.001). Finally, we saw significantly higher likelihoods of both bending (P < 0.001) and crown loss (P = 0.009) with increasing height:diameter ratio. These results suggest that managers may want to consider overstory environment, species, and understory tree size when planning for the risk of future glaze events.