Thinning and Prescribed Fire Effects on Fuels and Potential Fire Behavior in an Eastern Cascades Forest, Washington, USA

Prescribed fire and low thinning were applied to dry forests dominated by ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) in the eastern Washington Cascades. Experimental design was an unbalanced analysis of variance with 4 control units, 4 thin units, 2 burn units and 2 thin/burn units. Thinning was designed to reduce basal area to 10-14 m2 ha-1 in a non-uniform pattern and burning was a low intensity spring burn. Burn coverage was spotty, ranging from 23-51%, and considered ineffective in reducing fuels at the time of application by management and research personnel. Both thinning and burning had effects on vegetation and fuels variables. Thinning reduced canopy closure, canopy bulk density, and basal area, and increased canopy base height. Burning had no influence on these canopy variables. Thinning increased 10-hr timelag (0.62-2.54 cm) fuels. Burning decreased 1-hr (0-0.62 cm) and 10-hr timelag fuels, forest floor depth and mass, and increased fuelbed depth. There were interactions between thinning and burning for 1-hr and 10-hr timelag fuels, and fuelbed depth. These differences in fuel properties did not translate into differences in simulated wildfire behavior and tree mortality. Thinning did increase potential surface fire flame length under 97 percentile weather, and active crown fire potential decreased on thinned units, but basal area survival did not significantly differ between treatments under 80 and 97 percentile weather. The scale at which data are presented has a large influence on interpretation of results. For example, torching fire behavior, expressed as an average at the unit level, was low, but 17% of the individual plots (about 30 plots total per unit) across all treatments did exhibit potential torching behavior.