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.