Tree mortality and snag dynamics in North American boreal tree species after a wildfire: a long-term study

Temporal patterns of tree mortality and snag dynamics after fire were investigated over 10 years in a permanent plot design established immediately after a wildfire in an eastern boreal forest landscape of north-western Quebec, Canada. Post-fire tree mortality, snag persistence, tree fall patterns and variables influencing snag dynamics were assessed in deciduous, mixed and coniferous stands that experienced low- and moderate-severity fires. Temporal patterns of tree mortality for the three species revealed that mortality was delayed through time. Most post-fire tree mortality occurred within 2 years following fire but continued until the end of the 10-year observation period. Jack pine was the most persistent snag species, followed by trembling aspen and black spruce. Factors influencing the persistence of snags were multi-scaled and generally species-specific. Fire severity was the only common factor influencing snag persistence among all species, with snags located in severely-burned stands being less susceptible to falling. Trembling aspen snag persistence increased with basal area and diameter. Salvage logging in the vicinity affected black spruce. Fall patterns also differed among species. Bypasses of the snag stage (i.e. when a living tree falls directly to the forest floor) as well as uprooting of snags were common.

Effect of thinning and prescribed fire restoration treatments on woody debris and snag dynamics in a Sierran old-growth, mixed-conifer forest

Forest managers have little information of the effects of common restoration treatments, thinning and burning, on dead woody material (DWM) dynamics in fire-suppressed forests. Fine woody debris (FWD; 0.6–29.9 cm), coarse woody debris (CWD; ≥30.0 cm), and snags (≥5 cm) were inventoried and mapped in eighteen 4 ha plots before and after applying thinning (overstory, understory, and no thinning) and burning (burn and no burn) treatments. The combination of burning and thinning reduced FWD and CWD quantity and mean piece size, removed highly decayed logs, and increased small (5.0–24.9 cm) snag recruitment. In contrast, thin-only treatments produced similar results but increased FWD and did not remove many small snags. There were no differences in DWM response between the two thinning treatments. Log and snag spatial patterns prior to and following treatment were similar. These results indicate that burning in combination with thinning is more effective at reducing surface FWD and CWD, and removing small trees than are burn-only and thin-only treatments. Although large snags and logs were consumed in the burn, long-term recruitment of these habitat structures relies on managers retaining large-diameter trees. Repeated burns need to be conducted after initial restoration treatments to understand natural patterns of DWM.

Snag dynamics in partially harvested and unmanaged northern hardwood forests

We used data from hardwood-dominated permanent sample plots in Ontario to estimate the probability of a tree falling during the 5 year period in which it dies ("tree fall"), and likewise the 5 year probability of snag fall. Tree fall probabilities ranged from 5% to 31% across species, with smaller dead trees more likely to be downed than larger ones. Expected half-lives (median time from death to fall) for 25 cm diameter snags varied from 5 to 13 years among species. Fall rates were higher for 10 cm diameter snags but relatively constant for 20–60 cm diameter snags. Recent harvesting substantially increased the probabilities of both tree fall and snag fall, with the former effect most pronounced for small individuals. We used these estimated fall rates to simulate snag dynamics in uneven-aged sugar maple (Acer saccharum Marsh.) stands. Mean snag densities were 32 and 50 snags/ha in selection-management and old-growth scenarios, respectively. Fifty-four percent of this difference was attributable to the lower density of live trees in the selection-management scenario, while 31% was attributable to losses of snags during harvesting. Silvicultural practices that strategically increase tree mortality rates, together with snag retention, as far as safety permits, during harvesting, could represent an effective approach to snag management under the selection system.