Fuel mass and stand structure 13 years after logging of a severely burned ponderosa pine forest in northeastern Oregon, USA

摘要

Stand structure and fuel mass were measured in 2011, 13 years after logging of a seasonally dry, ponderosa pine dominated forest that had burned severely in the 1996 Summit Wildfire, Malheur National Forest, northeastern Oregon, U.S.A. Data are compared to those taken one year after post-fire logging (1999), and analyzed in the context of a second fire (Sunshine Fire) that burned through one of the four treatment blocks in 2008. Three treatments were evaluated in a randomized block experiment: unlogged control, commercial harvest (most dead merchantable trees removed), and fuel reduction harvest (most dead merchantable trees removed plus most dead trees & 10 cm diameter). Density and basal area of live trees trended downward between 1999 and 2011, but this delayed mortality was limited to smaller diameter stems. Dead tree basal area (snags) in unlogged stands declined over 12 years by 74%, compared to an 84% decline in logged stands over the same period. Higher snag fall rate in logged stands over time, combined with snag removal during logging in 1998, resulted in snag densities that were 22% of unlogged stands by 2011. In unlogged mixed-species stands, Douglas-fir snags fell at the same rate as did ponderosa pine snags, but fir snags fell at a faster rate in logged stands. Slash fuel tripled over twelve years in unlogged stands, such that mean slash mass levels became equal for all treatments. Unlogged stands accumulated nearly double the mass of coarse woody debris as logged stands, with most of the difference observed in the sound component of coarse woody fuel. By 13 years after logging (2011), density of regeneration averaged higher overall for unlogged stands, but was highly variable and patchy across all study units. Finally, the Sunshine Fire burned through nearly one complete block of the experiment in 2008, and erased most of the effects of the 1998 logging operation: snags were reduced to & 0.5 ha(-1) for all treatments, forest floor and slash fuel mass declined to near zero, log mass declined to about 25% that observed in stands unburned by the Sunshine Fire, and shrubs, seedlings, and saplings were nearly eliminated. While the intermediate-term effects of the 1998 logging were still evident in 2011, the 1996 Summit Fire remained the most dominant recent disturbance evident on the landscape.