Mountain Pine Beetle in Lodgepole Pine, William Ciesla forestryimages 5382438
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Mountain Pine Beetle
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Ecological Forestry for Lodgepole Pine in Colorado ^
The mountain pine beetle (Dendroctonus ponderosae) outbreak in Colorado has heightened the interest and work on management in lodgepole pine forests. The bark beetle is a native insect and outbreaks are a natural part of the forest cycle. However, the public is concerned about the aesthetics, fire threat, and ecological impacts of the current, large outbreak. In addition, climate change may also have exacerbated the extent and severity of the bark beetle’s impact.
Lodgepole pine forests cover as much as 50 million acres in western North America, from Baja California to the Yukon (Lotan and Critchfield 1990). Across this large area there is a great deal of variation in lodgepole’s ecological role. In Colorado, lodgepole generally grows between 7,500 and 10,000 ft. above sea level. While lodgepole can grow in mixed stands with other conifers below 9,000 ft., it often forms extensive pure stands between 9,000 and 10,000 ft. in elevation (in mesic areas) and between 9,500 and 10,500 in drier areas. It is a shade intolerant species with rapid early growth.
Lodgepole is not a long-lived tree species. As stands age, susceptibility to bark beetle increases (in British Columbia stand vulnerability increases at 60–80 years of age (Li et al. 2005)). Trees weakened from resource competition with their neighbors are more susceptible to bark beetle attack. Drought and other abiotic stressors also increase susceptibility. Stand-replacing fires are another common disturbance in lodgepole pine forests. Even older lodgepole pines have thin bark and are easily killed by fire. Moreover, lodgepole often forms large, dense, and even-aged forests with little understory but large amounts of ladder (dead stems and other material that can carry fire into tree crowns) and canopy fuels. Lodgepole is adapted to replace itself after stand-replacing fires. Its serotinous cones open after fires, it establishes well on mineral soil, and it grows quickly in the open conditions. The historic fire return interval in lodgepole pine in the southern Cascade Mountains was on the order of 50 years (Sugihara et al. 2006), but is likely to be longer in most Colorado forests (Stohlgren and Bachand 1997). In Yellowstone National Park fire return intervals varied from 135–185 years at low elevations to 280–310 years at higher elevations (Schoennagel et al. 2003).
Large scale mountain pine beetle outbreaks have occurred often in the past. However, widespread burning in the late 19th century and fire suppression in the 20th century may have created larger stands of similar age, which are now of an age when susceptibility to bark beetle is high (Romme et al. 2006). In addition, the warming trend of the last 30 years and the 2002-2003 drought were advantageous for bark beetles. Similarly, long-term warming and drying due to climate change will encourage large populations of bark beetles in the future. Warm temperatures protect the beetles from winter mortality and allow populations to increase. Drying and drought conditions decrease tree resistance and increase the success of beetle attacks, although mountain pine beetle reproduction is more successful in healthy trees.
As with many ecosystems, thinning in lodgepole pine stands to improve individual tree vigor helps reduce susceptibility to bark beetle attack. However, there are limitations to the benefits of thinning in lodgepole. Opening up stands can result in windthrow. During large outbreaks it may not be possible to protect an individual stand through thinning, because the positive effects of thinning take at least a year to be realized and can be overwhelmed by large populations of bark beetles.
In specific high-value stands or individual trees, insecticides provide short-term protection for lodgepole pine. During an outbreak, insecticides will need to be reapplied at least annually (see http://www.ext.colostate.edu/pubs/insect/05528.html). The cost and need for reapplication makes insecticides impracticle for large areas. Another chemical tool to manage bark beetles in lodgepole pine involves semiochemicals. Semiochemicals, such as pheromones, allow bark beetles to communicate. Verbenone is an anti-aggregation pheromone that can be used to disrupt bark beetle attacks on individual trees or stands (Progar 2003).
Li, C., H. J. Barclay, B. C. Hawkes, and S. W. Taylor. 2005. Lodgepole Pine Forest Age Class Dynamics and Susceptibility to Mountain Pine Beetle Attack. Ecological Complexity 2(3):232-239. http://www.sciencedirect.com/science/
Lotan, J. E., and W. B. Critchfield. 1990. Lodgepole Pine. in R. M. Burns and B. H. Honkala, editors. Silvics of North America. Agriculture Handbook 654, USDA, Forest Service, Washington, DC.
Progar, R. A. 2003. Verbenone Reduces Mountain Pine Beetle Attack in Lodgepole Pine. Western Journal of Applied Forestry 18:229-232. http://www.ingentaconnect.com/content/saf/wjaf/2003
Romme, B., J. Clement, J. Hicke, D. Kulakowski, L. MacDonald, T. Schoennagel, and T. Veblen. 2006. Recent Forest Insect Outbreaks and Fire Risk in Colorado Forests: A Brief Synthesis of Relevant Research. Colorado Forest Restoration Institute, Fort Collins, CO. http://www.cfri.colostate.edu/docs/cfri_insect.pdf
Schoennagel, T., M. G. Turner, and W. H. Romme. 2003. The Influence of Fire Interval and Serotiny on Postfire Lodgepole Pine Density in Yellowstone National Park. Ecology 84(11):2967-2978. http://www.esajournals.org/doi/abs/10.1890/02-0277
Stohlgren, T. J., and R. R. Bachand. 1997. Lodgepole Pine (Pinus Contorta) Ecotones in Rocky Mountain National Park, Colorado, USA. Ecology 78(2):632-641. doi:10.1890/0012-9658(1997)078[0632:LPPCEI]2.0.CO;2
Sugihara, N. G., J. W. V. Wagtendon, K. E. Shaffer, J. Fites-Kaufman, and A. E. Thode. 2006. Fire in California's Ecosystems. University of California Press, Berkeley, CA.