Assessing the Risk of Denali's Forests to Spruce Beetle Outbreak
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By Jill Syrotchen and Sarah Stehn (last updated July, 2018)
The Problem Spruce Beetles Pose
Spruce beetles (Dendroctonus rufipennis), although native to Alaska, have a history of periodically instigating large dieback events of spruce forests in certain regions of the state. Their primary hosts are white, Sitka, and Lutz spruce, and they feed on black spruce less frequently. Spruce beetles are a type of bark beetle—they bore through a tree’s bark to feed in its sugary phloem tissue.
Phloem is the innermost layer of the bark and serves to transport compounds produced via photosynthesis to other parts of the tree. Thus severe disruption in phloem tissue starves the tree, sometimes causing death within two years.
Spruce beetles occur across most of Alaska south of the Brooks Range. In the 1990’s, the Kenai Peninsula experienced a spruce beetle outbreak that affected 1.3 million acres of land, killing most large diameter spruce trees. As of 2010, a combined 6 million acres of Alaskan land have exhibited signs of spruce beetle activity.
In a recent 2017 aerial detection survey, the USDA Forest Service mapped 400,000 acres of spruce beetle damage—over double the damage detected in 2016—signifying the start of another beetle outbreak. Usually limited by a two-year life cycle, these beetles can now sometimes complete their life cycle in one year due to warming temperatures associated with global climate change (Bentz et al. 2010). This makes spruce beetle populations, and therefore their infestations, larger and more devastating than before.
An increasing number of spruce beetles have begun to disperse from Southcentral Alaska into the Interior—including southern portions of Denali National Park and Preserve (Burr and Hutten 2017).
Detecting Spruce Beetles in Denali
In infested spruce trees, you may find one or more small holes in the lower portion of the trunk. Attacking spruce beetles bored these holes, sometimes covered with a thick, opaque resin called pitch tubes—the tree’s defense against attack. Healthy, younger spruce can successfully defend against a spruce beetle attack by pitching out the beetle, trapping it in resin before it can lay eggs within the tree. Unhealthy spruce, older spruce, or trees that have already been attacked, are less successful at pitching out attacking spruce beetles. In addition to holes and pitch tubes, infested spruce trees may begin to lose their needles or change needle color from dark green to yellow or red. A tree with red needles may signal that it was attacked the year before, now serving as a source of the present year’s infestation.
Spruce beetles usually become active and disperse to attack new trees when temperatures reach 50-60° F (10-15.6° C), around the end of May through June. After mating and boring into a suitable spruce tree, a female spruce beetle lays her eggs in galleries within the bark. The eggs soon hatch into larvae which feed, grow, and pupate within the phloem of the spruce tree. Newly pupated adult beetles then move to suitable overwintering sites within the bark at the very base of spruce trees. Older trees are the most suitable to overwinter within, as their thick bark aids in insolating and protecting the beetles.
Aerial detection surveys by the USDA Forest Service in 2017 detected spruce beetle damage extending north from southcentral Alaska into the southern reaches of Denali National Park and Preserve (Burr and Hutten 2017). The Alaska Range and its influence on weather and climate patterns appears to be acting as a barrier to severe outbreak, with little to no damage detected in the more interior climates of the northern park.
In June 2018, Denali botany staff visually surveyed trails in the park entrance area and found little to no signs of spruce beetle activity, determining that there is not a current severe threat posed to northern Denali’s spruce forests. Additionally, trapping along Highway 3 just north of the park has yet to reveal elevated beetle populations (Burr and Hutten 2017). Staff will continue to monitor for beetle activity periodically, in effort to anticipate potential widespread changes to Denali’s forest structure.
Spruce beetles are a native insect, and their dispersal is a natural process, but a few variables will greatly impact when and to where these beetles spread. For example, warming climatic conditions are thought to increase the number of beetles that complete a life generation in one year, reduce cold-induced mortality of beetles, and potentially weaken host-trees through drought stress (Bentz et al. 2010). A change in these variables may result in different outcomes for Denali’s spruce and spruce beetle populations over time.
In a minimal effects scenario, Denali’s mixed-species forests, forming mosaics of beetle-preferred host species (i.e., white spruce) and non-preferred host species will limit spruce beetle outbreak, and with a low forest disturbance rate, ecological processes can continue as usual. Healthy spruce stands can pitch out most of the attacking beetles, controlling outbreaks further. Additionally, Denali’s long, cold winters will continue to limit beetle populations.
In a greater effect scenario, spruce beetles will target patches of white spruce monoculture in Denali, killing large tracts of trees. A prevalence of stressed trees may be unable to defend against beetle infestation, potentially increasing risk of wildfires from increased fuel. Animals living in affected areas may lose habitat and food sources, and in the case they are unable to find new habitat in which to live and feed, populations of those animals may decrease. Deciduous trees, frequent after disturbance, will enjoy dominance in places where spruce once grew.
In between these scenarios, a moderate level of spruce beetle outbreak may only kill small tracts of spruce in Denali. Animals in affected areas will be able to disperse to unaffected habitat sites, causing competition for resources in unaffected areas to increase. A change in understory plant cover may occur in newly opened spaces, but forest composition will remain relatively unchanged.
Time will tell how long the southcentral Alaska’s current outbreak will last, and how far north it will spread. Denali’s diversity of habitats ensures that disturbance to the forest, even if severe, will open opportunities for other natural ecological processes to occur and contribute to the landscape mosaic future visitors and inhabitants will experience. Keeping an eye out for new disturbance vectors as they develop, such as potential severe outbreaks of the spruce bark beetle, is important to improving our understanding of disturbance effects on Denali’s forests.
Bentz, B. J., J. Régnière, C. J. Fettig, E. M. Hansen, J. L. Hayes, J. A. Hicke, R. G. Kelsey, J. F. Negrón, and S. J. Seybold. 2010. Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects. Bioscience 60:602-613.