Characteristics and Origins of Deadwood Material in Aspen-Dominated Boreal Forests

This study examines the densities, physical structure, and origins of snags and downed woody material (DWM) in young (20-30 yr), mature (50-65 yr), and old stands (120+ yr) in aspen-dominated (Populus tremuloides Michx.) boreal forests of northeastern Alberta. Nearest neighbor and line intercept techniques were used to sample trees, snags, and DWM. Overall snag densities ($\geq 10$ cm diameter at breast height) were greater in mature and old stands. Overall coarse DWM $\geq 11$ cm diameter) counts were greater in young and old stands; volume of coarse DWM was greater in old stands. Analysis of spatial components of variation suggested that fires produced initially heterogeneous patterns in the densities of trees and dead woody material. The degree of spatial heterogeneity within and among stands decreased as stands developed to maturity. As a stand developed from maturity to old age, densities of trees and dead woody material retained spatial homogeneity within stands; however, they became more heterogeneous among stands. Comparisons of size distributions and decay patterns indicated that in young stands approximately $(\bar{X}\pm$ 1 SE) 19.1 $\pm 13.0$ snags/ha (53.9%) and $48.5 \pm 11.9$ m$^3$\ha of coarse DWM (79.2%) were derived from the prefire cohort of trees. Mature stands had $3.8 \pm 4.0$ snags/ha (5.8%) prefire snags. However, $50.4 \pm 6.4$ m$^3$\ha coarse DWM were classified as prefire in origin (65.7%). In old stands, snags and coarse DWM were probably generated from the postfire cohort of trees. In general, the diversity of sizes and decay patterns of snags and DWM within stands was largely due to the mix of prefire materials and the postfire materials generated by self-thinning and senescence/death of large trees. To simulate some of the conditions that occur after natural disturbances such as wildfires, timber harvest strategies should, in part, attempt to maintain the biological legacy associated with deadwood materials.