How Are Park Operations Affected by Spruce Beetle?

Beyond the direct effects on the forest, the effects of a large-scale spruce beetle outbreak has implications for park managers and visitors alike.

NPS Illustration/M. Perez

With widespread tree mortality, questions have arisen of how the dead material will affect fire occurrence or behavior. Additionally, as large numbers of trees may die in a short period of time, park managers must plan for how, where, and when removing those dead trees is a priority.

Hazard Trees

Park managers must prioritize visitor safety. The ongoing spruce beetle outbreak is causing much higher numbers of trees to die than is usual in just a few years. Spruce beetles infest large-diameter white spruce as their preferred host – a tree species and size very common around some of the Park’s long-standing historic structures and throughout visitor service areas.

Standing dead or weakened trees will need to be removed from areas near housing, buildings, and campgrounds. While this is always the case, the rapid increase in the number of hazard trees presents a difficult management challenge. Land managers may choose to temporarily close a campground or recreation area to complete the work safely.

To mitigate increases in hazardous amounts of fuel buildup (dead or live), the Denali Fire Management Team will continue to reduce and remove vegetation through thinning and prescribed fire (i.e., pile-burns) that surround critical infrastructure and escape routes.

How does beetle-kill interact with wildfire?

In the fire-adapted boreal forest, spruce trees (alive or dead) welcome fire. Even healthy, green spruce are highly flammable. Spruce is a fire adapted species, and their needles contain volatile compounds. Additionally, live spruce has naturally low fuel moisture throughout most of the summer, also contributing to their flammability.

As tree mortality plays out over time, its effects on fire are likely to vary and change depending on local site conditions and the amount of time that has passed since disturbance by beetle.

 

As trees die from spruce beetle attack, they go through several phases that may interact with fire differently. Healthy trees fade from green into the “red phase”, and then the “grey phase” as time passes and needles and fine branches fall off the now dead trees.

NPS Illustration/S. Stehn

• In the first 1-3 years after infestation, the “red phase”, dried red or orange needles may have lost some of their volatile compounds but still cling to branches. These red needs are very dry and may be more susceptible to potential ignitions by sparks from embers or direct fire impact. Loosely attached needles and branches also may increase the likelihood of casting embers and contribute to fire spread.
• A couple of years beyond tree mortality, affected spruce enter the “grey phase” wherein the needles have fallen off and varying degrees of fine branches may also have shed. If so, this reduces the surface area for ignitions and likelihood of casting embers if a fire were to start.
• In the years during and following the “grey phase”, the amount of sunlight reaching the forest floor also changes. Over several years this may influence the plant composition and cover. In the short term is also may contribute to drying of other fuel layers – grass, shrubs, moss, and duff.

Importantly, even in the “grey phase”, the amount of fuel (i.e., burnable material) on the landscape has not changed, it has only been rearranged. As trees fall down and begin to decay, there is less wildfire risk over time.

Beetle-kill stands can affect wildfire behavior and thus fire suppression and control tactics. Firefighters have a harder time moving through sites to do their firefighting work when the forest floor is dense with dead and downed logs and woody material. The use of water to put out a fire may also be less effective when the burning material is dense and piled, rather than upright and spaced apart.

Information here was summarized from:

• Holsten, E.H., Werner, R.A., and R.L. Develice. 1995. Effects of a spruce beetle (Coleoptera: Scolytidae) outbreak and fire on Lutz Spruce in Alaska. Environmental Entomology 24(6): 1539-1547.
• Hansen, W.D., Chapin, F.S. III, Naughton, H.T., Rupp, T.S., and D. Verbyla. 2016. Forest-landscape structure mediates effects of a spruce bark beetle (Dendroctonus rufipennis) outbreak on subsequent likelihood of burning in Alaskan boreal forest. Forest Ecology and Management 369(1): 38-46.
• Hood, S.M. 2020. Fire and bark beetle interactions. In: Manzello, S.L., ed. Encyclopedia of Wildfires and Wildland-Urban Interface (WUI) Fires. Cham, Switzerland: Springer. 6 p. Available from: https://research.fs.usda.gov/treesearch/60652.

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