Greater risk of physical damage caused by debris fall to understory plants and tree seedlings in old-growth forests than in young forests

Falling canopy debris causes injury and mortality of tree seedlings and understory plants in a wide variety of forests. Canopy structure and dynamics differ between young and old-growth forests: old forests are taller and have more aboveground biomass and greater annual mortality of bole biomass. I predicted that risk of damage caused by debris fall in the understory is greater in old-growth forests than in young forests. I tested this prediction by tracking for 1 year the fates of artificial seedlings placed in young (stand age 31 to 61 years) and old-growth (stand age circa 500 years) Pseudotsuga-Tsuga forests. The risk of physical damage caused by debris fall in old-growth forests was significantly greater than in young forests (P = 0.001). Seedling models were damaged by falling debris at a rate of 4.4%·year^-1 and 0.8%·year^-1 in old-growth and young forests, respectively. More seedling models were damaged by fallen coarse woody debris in old-growth forests than in young forests, although this trend was not significant (P = 0.134). Approximately 25% of seedling models in both young and old-growth forests were damaged by something other than fallen canopy debris, most likely snow accumulation.