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

Abstract

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.