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

Abstract

Prescribed fire and low thinning were applied to dry forests dominatedby ponderosa pine(Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) in the easternWashingtonCascades. Experimental design was an unbalanced analysis of variancewith 4 control units, 4thin units, 2 burn units and 2 thin/burn units. Thinning was designedto reduce basal area to 10-14 m2 ha-1 in a non-uniform pattern and burning was a low intensityspring burn. Burncoverage was spotty, ranging from 23-51%, and considered ineffectivein reducing fuels at thetime of application by management and research personnel. Both thinningand burning hadeffects on vegetation and fuels variables. Thinning reduced canopyclosure, canopy bulkdensity, and basal area, and increased canopy base height. Burninghad no influence on thesecanopy variables. Thinning increased 10-hr timelag (0.62-2.54 cm)fuels. Burning decreased1-hr (0-0.62 cm) and 10-hr timelag fuels, forest floor depth and mass,and increased fuelbeddepth. There were interactions between thinning and burning for 1-hrand 10-hr timelag fuels,and fuelbed depth. These differences in fuel properties did not translateinto differences insimulated wildfire behavior and tree mortality. Thinning did increasepotential surface fireflame length under 97 percentile weather, and active crown fire potentialdecreased on thinnedunits, but basal area survival did not significantly differ betweentreatments under 80 and 97percentile weather. The scale at which data are presented has a largeinfluence on interpretationof results. For example, torching fire behavior, expressed as an averageat the unit level, waslow, but 17% of the individual plots (about 30 plots total per unit)across all treatments didexhibit potential torching behavior.