Dynamics of snags in aspen-dominated midboreal forests

This research focuses on the dynamics of snags within aspen-dominated (Populus tremuloides Michx.) boreal forests within Alberta. Data from Alberta Lands and Forest Service permanent sample plots (PSP) were analyzed to determine densities, size distributions, input rates, falldown rates, and longevity patterns of snags (greater than or equal to 10 cm DBH). The mean density of snags in 20 to 39-yr-old stands was 18.1 snags per ha increasing to 61.6-99.8 snags per ha in stands up to 100 + yrs. In general, size distributions of trees and snags exhibited a reversed J-shaped size distribution. In stands < 40 yrs of age, size distributions of snags lagged behind trees. However, as the stand aged and self-thinning of small trees was supplemented by the death of canopy codominants, the size distribution of snags began to increasingly overlaps with that of trees. overall snag input rates varied from 0.08-8.2% of trees per annum, larger values were associated with older stands and/or smaller trees. Snag falldown rates varied from 9-21% of snags per annum depending upon stand age. Falldown rates exhibited a U-shaped pattern with rates decreasing in 60 to 79-yr-old stands. Snag longevity patterns exhibited a negative sigmoidal shape with an initial period of relative stability for approximately 5 yrs after death, afterwards the probability of falldown increased with a greatest falldown occurring between 10 to 20 yrs after death. (C) 1998 Elsevier Science B.V.