Root disease and wildfire are important disturbance agents in western North American forests. It has been speculated that root disease increases potential fire behavior, since root disease kills trees thereby contributing to available fuels. However, little quantitative evidence of this effect has been offered. This research quantifies the effect of Armillaria root disease on potential wildfire behavior in a managed ponderosa pine (Pinus ponderosa) forest near Glenwood, Washington. Armillaria, like other conifer root diseases, creates slowly expanding centers of tree mortality. Forest structure and composition data were collected from paired plots within and outside of mortality centers. Differences in forest structure and composition variables were identified using pairwise t-tests with alpha = 0.05. Two fuelbeds were created and fire behavior was modeled using the Fuel Characteristic Classification System (FCCS). FCCS calculates standard fire behavior metrics as well as fire behavior potentials calculated on a unit-less 0-9 scale which reflects the ability of a fuelbed to support a given fire behavior.
Overstory canopy cover was significantly reduced in plots with Armillaria relative to healthy plots (53 versus 78%), and there was no difference in abundance of ladder fuels. This led to a decrease in crown fire potential (FCCS crown-fire potential of 3 versus 4) in the diseased fuelbed. Significantly greater 100 and 1000 hour fuel loadings were detected in the diseased plots (6.9 versus 2.5 Mg/ha, and 1.6 versus 0.7 Mg/ha respectively), but there were no differences in 1 and 10 hour fuels. Significantly lower needle loading (2.3 versus 3.7 Mg/ha), and forest floor covered by needles (72% versus 93%) were detected in diseased plots; however, these plots had more grass litter cover (18% versus 3%). Few differences were detected in the shrub or herb layers. Increased reaction potential was predicted for the diseased fuelbed (882 versus 824 kW/m2), however rate of spread (ROS) and flame length were reduced (1.4 versus 1.9 m/min, and 1 versus 1.2 m respectively). The simultaneous increase in reaction potential and decrease in ROS and flame length in the diseased fuelbed is counterintuitive. Increased 100 and 1000hour fuels in the diseased fuelbed initially act as a heat sink, absorbing the energy of the flaming front. This study suggests that root disease may decrease wildland fire severity and highlights the need to consider multiple competing effects when assessing the impact of disturbances on wildland fire.