Native insect outbreaks have been critical elements of cyclical forest ecology for millennia. Tree-mortality from these outbreaks creates natural gaps in the forest canopy to allow for heterogeneity of plants and tree age diversity, improving the resilience of forests to future disturbances. Snags and downed dead logs from stand-killing events provide important habitats for many species of birds and rodents and are critical components of old-growth forests.
While a natural component of forest ecology, recent climate change conditions have led to an uncharacteristic increase in insect outbreak events. In fact, some research claims that two-thirds of trees in American forests are at risk of extensive tree mortality from insect outbreaks or disease. Despite the potential scale of these disturbances, it bears underscoring that outbreaks depend on a variety of conditions which vary by region and forest type.
There are many classes of native insects common in American forests, including bark beetles (also known as phloem-borers, woodborers, and engravers). North America hosts 550 species of bark beetles, and these beetles are regarded as the “most important mortality agent in conifers.” However, globally, the population dynamics of bark beetles are poorly understood. A few notable species include the southern pine beetle, mountain pine beetle, western pine beetle, Douglas-fir beetle, spruce beetle, and pinyon ips. Conifers are more susceptible to bark beetles than hardwoods.
The primary natural defense against damage by insects is the production of resin (oleoresin flooding). A tree’s ability to produce resin and pitch out insects is severely negatively impacted by drought and competition between trees in a stand. Other defenses include insectivores (bark beetle predators), thick, less penetrable bark, and naturally diverse, complex forests. Insects can overcome these defenses if the tree is weak enough and enough insects are recruited to the target tree to colonize. Usually, an outbreak begins with successful colonization of a weakened (often lightning-struck) tree. From there, bark beetles can colonize surrounding trees.
When forest management intervenes, it is commonly through active management practices like thinning and logging. While thinning can be effective in some cases when infestations are small, current science does not support the idea that thinning is a universally effective solution. In some cases, thinning can reduce competition between trees, thereby reducing the drought stress which can make stands vulnerable to bark beetle outbreaks. However, research suggests that once an epidemic has begun, logging will not alter the trajectory of the outbreak. Despite this extremely limited window of potential effectiveness, logging is often employed in an attempt to curb epidemics, leading to the removal of mature and old-growth trees and stands, doing more harm than benefit to the overall forest ecosystem.
Beyond removal, logging can have long-lasting negative impacts on forests and trees. For example, thinning can damage surviving trees and compact soil, negatively affecting tree growth and causing stress to stands, which in turn can increase chances of insect infestation. Thinning can also increase the likelihood of other disturbances like windthrow, wood-boring insect infestation, and root pathogen infestation, all of which increase the risk of bark beetle infestation. Logging in stands deemed vulnerable by researchers or agencies can also lead to population decline of sensitive species that prey on bark beetles and actually increases fire risk due to the removal of fire resistant mature and old-growth trees. Additionally, removing dead trees after natural disturbances like insect outbreaks can leave the forest vulnerable to future disturbance by reducing mature and old-growth characteristics. Generally, unmanaged, older forests have been found to be more resilient to insect outbreaks than managed stands.
One common justification for logging, despite these deleterious impacts, is to reduce the risk of fire. However, research, and therefore evidence, that insect-killed stands are more susceptible to severe fire remains limited. Agency scientists have noted that “extensive bark beetle killed forests can exacerbate fire risk in some cover types, but not others.” It is also well documented that fire-weather and climate are more directly correlated to fire severity than bark beetle outbreaks. A prevailing assumption has long been that insect outbreaks result in higher fire hazard due to elevated “fuel loads.” However, previous studies have shown that “burn severity is either unaffected by or weakly positively associated with [insect] outbreak severity [and] insect effects are context dependent,” meaning they depend on many different environmental variables and are not directly correlated to fire risk. It has also been noted that “factors like fuel treatments, topography, and weather are stronger predictors of fire effects.” Additionally, many studies of spruce beetle and mountain pine beetle show a small effect or no increase in fire risk. In fact, “beetle kill may actually decrease the hazard of high-severity crown fire [in Lodgepole pine] by reducing the continuity of the canopy.”
Given the inconclusive evidence available, active management to mitigate the negative environmental impacts of insect outbreaks should be approached with a high degree of caution. The efficacy of such treatments is variable, site specific, and may result in unintended impacts more detrimental to the forest system than the insect outbreak would have been, unmitigated.
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