by George Wuerthner
Lodgepole pine is one of the most common trees in the Northern Rockies. For instance, 80% of the trees in Yellowstone National Park are lodgepole pine. However, it is also a common tree in the Cascades, Sierra Nevada, and into British Columbia.
One of the important drivers in lodgepole pine ecology is periodic beetle kill from the mountain pine beetle. Bark beetles are like wolves that thin an elk herd down to its carrying capacity. Typically only the older trees are suitable for attack, so mortality in lodgepole forests is usually less than 50% of trees. The remaining trees, freed up from competition grow much faster and for a while are able to resist any future beetle attacks.
Bark beetles lay their eggs in the inner layer of tree bark where their larvae develop, then eat the living layer. A fungi also enters the tree with the bark beetles. These two factors often kill the tree, leading to a common sight of red-needled trees covering hillsides.
Since beetle mortality usually occurs in a mosaic with patches of dead trees and patches of live ones, the overall ecosystem biodiversity increases. Species dependent on dead trees like cavity nesting birds benefit from beetle kill, while those that might need some live trees—say thermal cover for elk in winter—also benefit. Thus beetles can be thought of a “keystone species” that creates habitat for many other species. Some research suggests that beetles create greater biodiversity overall as a consequence.
BARK BEETLES DO NOT INCREASE FIRES
Bark beetle numbers surge during drought periods because trees stressed by drought are unable to cast off beetles. One of the common assumptions behind logging/thinning projects being promoted around the West is that beetle kill will increase fire risk. Proponents of these projects argue that the solution is to log forests to preclude beetle kill by reducing densities and/or to remove existing dead trees to reduce fuels.
However, a host of studies demonstrate that beetle killed forests are no more likely to burn than green forests. Indeed, some studies suggest that for a period of time after a bark beetle outbreak, forests are less likely to burn.
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| A lodgepole pine fire (Forest Service) |
This is easily explained by fuels. One of the big misconceptions about wildfire is that fuels drive them and the more biomass, so the thinking goes, the more likely you are to have a major fire. But the “fuels” that carry wildfires are the small flashy fine fuels like pine needles, cones, and small branches, not the boles of trees. That is why there are “snags” left after a fire. Most of the tree is not consumed or burned in a wildfire. So once a beetle kill tree loses its needles and the small branches break off in winter storms, they are actually less flammable than live green trees.
In fact, green trees, due to their abundance of resin-filled needles and branches, will burn more intensely than dead wood under extreme weather conditions of low humidity, high temperatures and high winds. These are the kind of weather conditions that drive large wildfires.
There is a nuance here, however. As the young trees unaffected by bark beetles grow up in the understory of remaining trees, they do provide more “ladder” fuel that can sometimes increase fire spread for a few decades until the canopy closes and fire risk is again reduced—assuming that conditions for fire spread exist at all during those decades and there are ignitions.
Of course, the other factor in the beetle/fire story has to do with timing of fires in lodgepole pine forests. Lodgepole pine tends to burn at long intervals of hundreds of years. That is because the right combination of wind, humidity, and ignition simply do not exist every year, and often not for decades or centuries. While beetles may kill trees, the likelihood that those particular trees will be in the path of a fire is a low probability.
LOGGING WON’T STOP FIRES
A number of studies have demonstrated that there is no greater increase in fires in beetle kill areas on average than other sites. In some cases, at least until the younger trees start to fill in the forest, fire risk is actually reduced.
Despite this evidence, the Forest Service continues to advocate logging/thinning on the flawed assumption that a reduction in beetle kill trees will preclude large wildfires. Not only is this not the case, but in reality we need large wildfires for the ecological work they do. Even if it were possible to reduce fires we would not want to do this.
PROTECT HOMES
Some 98% of all beetle outbreaks are in remote areas and the likelihood that they will encounter or threaten homes is extremely small. Nevertheless, it is well established the best way to protect homes from wildfire is not by thinning the forest, but by keeping homes from being built in the “fire plain” in the first place. For those homes already in the fire plain, reducing the flammability in the home ignition zone (200 feet is all that is need) surrounding a home is the only proven way to safeguard homes.
REFERENCES:
Area burned in the western United States is unaffected by recent mountain pine beetle outbreaks
Sarah J. Hart, Tania Schoennagel, Thomas T. Veblen, and Teresa B. Chapman
www.pnas.org/cgi/doi/10.1073/pnas.1424037112
The European spruce bark beetle Ips typographus in a national park: from pest to keystone species
Jo¨rg Mu¨ ller Æ Heinz Bußler Æ Martin Goßner Æ
Thomas Rettelbach Æ Peter Duelli
Biodivers Conserv (2008) 17:2979–3001
DOI 10.1007/s10531-008-9409-01
Does wildfire likelihood increase following insect outbreaks in conifer forests?
GARRETT W. MEIGS, JOHN L. CAMPBELL, HAROLD S. J. ZALD, JOHN D. BAILEY, DAVID C. SHAW, AND ROBERT E. KENNEDY
www.esajournals.org
2 July 2015 v Volume 6(7) v Article 118
Don’t Blame the Beetles
By Cally Carswell
Science
10 OCTOBER 2014 • VOL 346 ISSUE 6206
Fire severity and tree regeneration following bark beetle outbreaks: the role of outbreak stage and burning conditions
Brian J. Harvey, Daniel C. Donato, William H. Romme, Monica G. Turner
Ecological Society of America
The influence of mountain pine beetle outbreaks and drought on severe wildfires in northwestern Colorado and southern Wyoming: A look at the past century
Dominik Kulakowski, Daniel Jarvis
Forest Ecology and Management 262 (2011) 1686–1696
Management for Mountain Pine Beetle Outbreak Suppression: Does Relevant Science Support Current Policy?
Diana L. Six, Eric Biber, and Elisabeth Long
Forests 2014, 5, 103-133; doi:10.3390/f5010103
Are density reduction treatments effective at managing for resistance or resilience to spruce beetle disturbance in the southern Rocky Mountains?
Christian Temperli, Sarah J. Hart, Thomas T. Veblen, Dominik Kulakowski, Julia J. Hicks,Robert Andrus
Forest Ecology and Management 334 (2014) 53–63
Bark Beetles and Fire;: Two Forces of Nature Transforming Western Forests
FIRE SCIENCE DIGEST ISSUE 12 FEBRUARY 2012
Bark beetle outbreaks, wildfires and defensible space: how much area do we need to treat to protect homes and communities?
Glen Aronson and Dominik Kulakowski
International Journal of Wildland Fire 2013, 22, 256–265
http://dx.doi.org/10.1071/WF11070
