What Trees are Spruce Beetle Choosing?

Because tree height can be difficult to measure, standard practice is to measure trees around the trunk at a height of 1.35m (4.5 ft). Spruce beetle generally prefer trees greater than 12 cm (5 in) in diameter, but during severe outbreaks, even smaller trees are at risk of spruce beetle attack.

NPS Illustration / M. Perez

When spruce beetle (Dendroctonus rufipennis) is at endemic population levels, they tend to target weakened, aged, or damaged spruce trees greater than 12 cm (5 in) in diameter. However, when beetles are at epidemic population levels in outbreak stage, there are fewer available hosts. With limited options, healthier and smaller spruce trees are left vulnerable to attack. In outbreak conditions, entire generations of trees can be lost, and only the smallest trees may survive.

Thanks in part to long-term monitoring programs operating in National Parks, park scientists can assess impacts of spruce beetle or other forest disturbance. Researchers use this data—along with remote sensing—to learn more about forest change. Ongoing studies of vegetation change in Denali National Park and Preserve are currently documenting dramatic loss of living spruce, particularly large spruce, in some areas. For example, an ongoing study conducted by the park’s botany team at sites located within a few miles of the Denali Park Visitor Center indicated that by 2024, nearly 50% of trees >36 cm (14 in) in diameter showed signs of spruce beetle attack, but no trees investigated under 12 cm (5 in) diameter showed signed of attack (Roland et al. 2024, unpublished data).

 

Chances of surviving a spruce beetle epidemic are higher for smaller trees. Monitoring data pre-disturbance (before the current outbreak of spruce beetle) show that only 10–20% of trees over 15 cm (6 in) in diameter have survived post-disturbance by outbreak level populations of spruce beetle.

Stehn et al. 2024, unpublished data

Information here was summarized from:

Roland, C., Walton, J., and Tomerlin, M. (2024). Spruce beetle monitoring project, data exploration. Preliminary results from in-progress research supported by the NPS Inventory and Monitoring Program, Central Alaska Network, and Denali National Park and Preserve.

Stehn, S., Roland C., and others. (2024). Post-disturbance effects of spruce beetle on vegetation communities south of the Alaska Range. Preliminary results from in-progress research supported by the NPS Alaska Natural Resource Council, NPS Western Area Fire Management, and NPS Inventory and Monitoring, Central Alaska Network Programs, as well as the University of Northern Colorado.


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