Effect of Armillaria Root Disease on Stand Structure, Composition, and Potential Fire Behavior in a Managed Ponderosa Pine Forest near Glenwood, WA
Johnson, Nathan G.
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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 and contributes to available fuels. However, little quantitative evidence of this effect on wildfire 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. Plant community composition was largely the same between plot types, with slightly more grasses in the diseased plots. There were several differences in stand structure: diseased plots had lower basal area and density in the overstory and midstory canopy, diseased sites had more snags in the older age classes, and diseased plots had higher 100 and 1000 hr fuels. Two fuelbeds were created based on these differences 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. There was no difference in abundance of ladder fuels. This led to a decrease in crown fire potential in the diseased fuelbed. Significantly greater 100 and 1000 hr fuel loadings were detected in the diseased plots but no differences in 1 and 10 hr fuels. There were also differences in forest floor litter. Few differences were detected in the shrub or herb layers. Increased reaction potential for surface fire was predicted for the diseased fuelbed, however rate of spread (ROS) and flame length were reduced. The simultaneous increase in reaction potential and decrease in ROS and flame length in the diseased fuelbed is counterintuitive. Increased 100 and 1000 hr 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.
- Forestry