Bark beetle outbreaks may influence forest fire risk

Beetle-killed trees are easy fuel for fire compared to healthy, living ones. This observation has led many scientists to predict that bark beetle outbreaks should lead to an increased fire risk. But the relationship between beetles and fire is complex.[[Bigler, Christof, Dominik Kulakowski, and Thomas T. Veblen, “Multiple disturbance interactions and drought influence fire severity in Rocky Mountain subalpine forests,” (PDF) Ecology 86, no. 11 (2005): 3018–3029.]] [[Bebi, Peter, Dominik Kulakowski, and Thomas T. Veblen, “Interactions between fire and spruce beetles in a subalpine Rocky Mountain forest landscape,” (PDF) Ecology 84, no. 2 (2003): 362–371.]] [[ Negron, Jose F., et al., “US Forest Service bark beetle research in the western United States: Looking toward the future,” (PDF) Journal of Forestry 106, no. 6 (2008): 325–331.]]  

Once beetle-killed trees lose their needles, for example — an outcome that can take several years — they may offer less fuel than live trees. But as new trees sprout and grow in beetle-damaged areas, they can promote fire, by acting as “ladders” to carry fires from the surface up to the treetops. On the other hand, they can also inhibit fires by increasing humidity at ground level.

One recent study by Heather Lynch and colleagues in Yellowstone National Park found that beetle damage from a 1972 – 1975 outbreak was correlated with the area burned in the Park’s massive 1988 fire, but damage from the more recent 1980 – 1983 outbreak was not.[[ Lynch, Heather J., et al., “The Influence of Previous Mountain Pine Beetle (Dendroctonus ponderosae) Activity on the 1988 Yellowstone Fires,” (Abstract) Ecosystems 9, no. 8 (2007) 1318-1327.]] They argue that their study and others indicate that beetle damage increases fire risk by changing the ages, sizes and kinds of trees in forests, not by creating a buildup of dead wood. Their work suggests that there may be a time lag of ten years or more between beetle outbreaks and fire vulnerability.

Climate warming in the West is helping bark beetles to spread, and recent outbreaks have been severe.