Mountain pine beetle outbreaks do not increase the likelihood of forest fires

A 2010 Greater Yellowstone study by the University of Wisconsin (Monica Turner, Martin Simard) and Colorado State U. (William Romme and Jacob Griffin) advocates less cutting to decrease fires. Some experts say that our Northern Rockies forests of lodgepole pine are in an alternating dance of fire and beetle kill. The more beetle killed trees, the more fires. “There were surprisingly little data to back up that conventional wisdom,” said co-auther Turner.

Their 2010 Greater Yellowstone Ecosystem (GYE) study gave a different perspective.
Conventional wisdom about mountain pine beetle (MPB) and forest fires across the west often assumed that the beetle-killed trees increased rapidly spreading forest fires. In the study, however, MPBs may actually decrease the likelihood of crown fires in Greater Yellowstone. Extreme weather conditions may play a more prominent role in the spread of forest fires.

Both pine beetle outbreaks and forest fires have increased in extent and severity over the last 30 years. The recent increase in their attacks on lodgepole pine in the Greater Yellowstone area led to widespread tree deaths, presumedly increasing active crown fires.
“This is not the case,” says Simard who studied GYE 30 years. “Our study suggests that, when beetle-killed trees lose their needles, the amount of canopy fuel is reduced by 50 percent, and as a result, fuel connectivity and the probability of active crown fire are also reduced.”

Dr. Monica Turner, Professor of Ecology, told CCN, “I’ve been working with GYE for 25 years, so I have a lot of experience with that system (especially within Yellowstone National Park, proper). Crown fires in the GYE are driven primarily by summer weather conditions (we usually say that the fire regime is controlled by climate more so than by fuels, which are generally available; this is different from many southwestern ponderosa pine forests, in which the fire regime was historically limited by fuel). With respect to beetle-killed forest, there are two outbreak phases where concern remains about whether fire risk is elevated compared to forest that was not affected by the beetles.” The first is the early “red phase”, a risk because needles are drier and more flammable than green needles.

“However, it is important to remember that the forest stand is a mixture of red and green needles, and so it can’t be assumed that all the needles are red.” In the 2011 Science Journal, Turner commented on the startling results, “contrary to conventional wisdom, …the probability of active crown fire appeared to be reduced.”
Simard and colleagues sampled surface and canopy fuels including ladder fuels, in 35 lodgepole pine stands in Greater Yellowstone that were either undisturbed or attacked by MPBs between one and 36 years ago. Computer models predicted potential fire behavior in these stands and considered other factors that could be contributing to active crown fire spread, including moisture conditions and extreme weather.

“Fires in subalpine conifer forests, including in Yellowstone, are mainly driven by climate,” says Simard. “Hot, dry, windy conditions lead to active crown fires, in which the fire spreads from tree to tree indiscriminately.”
Turner told CCN, “Also, the (red) trees begin shedding needles immediately, so the amount of fuel in the canopy declines. Whether the net effect of these processes changes actual fires has yet to be evaluated rigorously in a real setting. Our modeling studies suggest that the risk of crown fire will not increase in lodgepole pine or Douglas fir, but there are other views out there. It is agreed, however, that "red phase" is relatively short.

Once the stands are in the ‘gray phase’ (which is when salvage harvest if most likely) there is largely agreement among forest ecologists that the risk of crown fire will not increase and will likely go down (because of less fuel and less fuel continuity). When the stands are about 30 years post-outbreak, there is a lot of downed wood because the trees have fallen.”
Although the probability of active crown fires is reduced, the study shows passive crown fires may be more likely to occur 25 to 35 years after an outbreak of beetles; however, such fires are less intense and spread more slowly than active crown fires. In passive crown fires, flames move from the ground to the tree crown and cause torching as flames climb up ladder fuels to the canopy. However, these types of fires do not directly spread to neighboring tree crowns according to the report.

In response to CCN’s question regarding opening up the forest canopy for decreasing fires and risks, Turner said, “With respect to management of old-growth lodgepole pine forests, I am not aware of a basis for creating openings to reduce crown fire in the landscape (of course, removing all trees would reduce occurrence of a crown fire in that opening by definition).”
Turner gave an example of how open areas don’t work that way in big fires. “In extreme weather conditions (e.g. the 1988 Yellowstone Fires) which is when the big fires occurred, not even the Grand Canyon of the Yellowstone stopped the fires from spreading.” Firebreaks work with moderate fires not big ones.

USFS Greater Yellowstone Area team leader Amy Waring responds, “At this point in time, we are not taking a position on the conclusions presented in the Simard, et al paper.  It is not on point with our purpose and need for the Greater Red Lodge Red Lodge Forest and Habitat Management Project (GRLA).  We are not trying to lower the probably of crown fire following a mountain pine beetle epidemic.” 

Waring continued, “The Greater Red Lodge project area does not currently have epidemic levels of mountain pine beetle, nor does the project area have extensive amounts of beetle killed trees.  Mountain pine beetle is present at endemic levels, and there are scattered pockets of trees that are infested with mountain pine beetle and/or have already been killed by beetles (occasional ¼ to ½ acre patches of high mortality).   The existing condition of our project area is dominated by mature lodgepole pine. The vegetation pattern appears as a homogeneous cover type and size class broken up by small areas of past timber harvest, suppressed wildfires, wind damage, and endemic bark beetle incidences. Stand densities (trees per acre) are high.  Spruce, subalpine fir and Douglas fir are regenerating and establishing underneath the lodgepole pine, creating multiple canopy layers.  The Forest Service has identified the area as moderate to high hazard for both mountain pine beetle and wildfire.”

“These findings have important implications for Greater Yellowstone,” concluded Turner of the study. “This study suggests that management of beetle-killed forests primarily to reduce a perceived increase in fire likelihood would not be warranted. Rather, the data indicate that the mountain pine beetle reduces the probability of active crown fire for up to 35 years by thinning the forest. However, it will be important to pay attention to how both of these key natural disturbances respond to a changing climate.”
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