Changes in fuelbed characteristics and resulting fire potentials after fuel reduction treatments in dry forests of the Blue Mountains, northeastern Oregon

摘要

In many fire-prone forests in the United States, changes occurring in the last century have resulted in overstory structures, conifer densities, down woody structure and fuel loads that deviate from those described historically. With these changes, forests are presumed to be unsustainable. Broad-scale treatments are proposed to reduce fuels and promote stand development on trajectories toward more sustainable structures. Yet little research to date has identified the effects of these treatments, especially in low elevation dry ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) forests. We report initial fuelbed conditions and changes immediately and 4-6 years after fuel reduction treatments in an operational-scale, replicated (N =4), completely randomized experiment in northeastern Oregon. Treatments included a single entry thin from below conducted in 1998, a late season burn conducted in 2000, a thin followed by burning (thin+burn), and a no action treatment which served as a control. Between 1998 and 2004, litter mass declined about 4.4Mghap# in thin units, about 4.1Mghap# in thin+burn units, and about 1.5Mghap# in burn units. Duff mass did not change with treatment. Mass of woody fuels in the 1 and 10h timelag classes increased in thin units immediately after treatment, but fell to nearly pre-treatment levels by 2004. Mass of woody fuels in the 100h timelag class increased about 1.6Mghap# in thin units, and decreased about 0.7Mghap# in burn units and about 0.1Mghap# in thin+burn units. There was no difference in the change in total woody fuel mass among all treatments. About 62% of the residual trees in the burn and the thin+burn units were charred; charring extended up the bole about 1.1m. About 50% of the residual trees in the burn and thin+burn units had scorched lower crowns. Mean scorch height was 4.3m. Log density was greater in control units compared to actively treated units and greater in the thin units compared to burn or thin+burn units. We used the Fuel Characteristic Classification System (FCCS) to construct a representative fuelbed for each unit from inventoried data and to calculate three indices of fire potential as measures of the change in fire hazard resulting from our treatments: surface fire behavior, crown fire behavior, and fuels available for consumption. Projected flame length, rate of spread, and reaction intensity are derived as metrics of future surface fire behavior. These results are discussed in the context of management options for restoration of ecosystem health in similar low elevation dry ponderosa pine and Douglas-fir forests.