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Fire and fuel treatments increase tree resistance to bark beetles

Date: April 10, 2018


Measuring trees killed by mountain pine beetle in the experimental study project area.
Measuring trees killed by mountain pine beetle in the experimental study project area.
The frequency of fire in low-elevation coniferous forests in western North America has greatly declined since the late 1800s. In many areas, this has increased tree density, increased the proportion of shade-tolerant species, reduced resource availability, and increased forest susceptibility to forest insect pests and high-severity wildfire. This study investigated how low-intensity fire affects tree defenses and whether fuel treatments impact resistance to a mountain pine beetle outbreak.


Previous research showed most bark beetle-caused tree mortality in burned areas occurred 1-2 years post-fire, but mortality then declined. This might be because fire was stimulating defenses in ponderosa pine trees that evolved with frequent, low-severity fire, similar to how vaccines work in humans. It was hypothesized that fire not only fosters forest conditions conducive to low-severity fire, but also periodically stimulates tree defenses to be more resistant to bark beetles. A retrospective examination of how growth and defenses have changed with fire frequency was conducted using tree ring analysis. An experimental fuel treatment study was conducted to see how tree-level defenses scaled up to stand-level resistance to a naturally occurring mountain pine beetle outbreak.

Key Findings

  • Mortality from the mountain pine beetle was highest in the denser, untreated control and burn-only treatments, with approximately 50 percent and 39 percent, respectively, of ponderosa pine killed during the outbreak. This is compared to almost no mortality in the thin-only and thin-burn treatments. Thinning treatments, with or without fire, dramatically increased tree growth and resin ducts relative to control and burn-only treatments.
  • Resin duct-related traits provide resistance against bark beetles and low-severity fire induces resin duct production. When fire ceases resin duct production declines.
  • Low-severity fire can trigger a long-lasting induced defense that may increase tree survival from bark beetles.
  • While ponderosa pine remained dominant in the thin and thin-burn treatments after the outbreak, the high pine mortality in the control and burn-only treatment caused a shift in species dominance to Douglas-fir. The high Douglas-fir component in the control and burn-only treatments due to 20th century fire exclusion, coupled with high pine mortality from mountain pine beetles, has likely reduced resilience of this forest beyond the ability to return to a ponderosa pine-dominated system in the absence of further fire or mechanical treatment.




Featured Publications

Hood, Sharon M. ; Sala, Anna ; Heyerdahl, Emily K. ; Boutin, Marion , 2015
Hood, Sharon M. ; Sala, Anna , 2015

Principal Investigators - External: 
Anna Sala - University of Montana
Forest Service Partners: 
Inland Empire Tree Improvement Cooperative
Research Location: 
Oregon, Idaho, Utah, Montana