Boreal Blights—What's Happening to the Forest?

By Sarah Stehn, Botanist & RB Smith, Scientist-in-Park Intern (last update: May, 2021)

The boreal forest is a dynamic place. Disturbances such as fire, flooding, fungal infection, and insect infestation regularly affect individual plant species, or entire plant communities.

The interplay between parasite and host is a natural part of the ecosystem – many of the fungi and insects are native to Alaska, and the affected plants are generally resilient. But changes in climate or other large-scale factors may have far-reaching effects on these complex parasite-host dynamics and can tip the balance to favor one or the other.

Have you noticed any of the ‘symptoms’ of disturbance below? Explore the links provided for more information about these widespread boreal blights in spruce trees or other species.

Blights to Spruce

"Symptoms" of spruce disturbance

(A) Photo by JLS Photography – Alaska licensed with CC-NC-ND 2.0 (B) NPS Photo/Sarah Swanson, (C) Photo by Scot Nelson via CC0 1.0, (D) NPS Photo/Davyd Betchkal, (E) Photo by Jason Moan, Alaska Division of Forestry.

(A) Round, knobby growths on the sides of trees
Sometimes, trees will grow outward protrusions of wood called “burls,” usually in response to infection, injury or some other stress. “Cankers” are sections of the tree that have died and been deformed, and many other kinds of tree dysmorphia can occur when parts of the plant are damaged in some way as it grows. While these growths can sometimes grow quite large and conspicuous, they rarely cause the tree serious damage.

(B) Bright yellow needles at tips of spruce branches
The native spruce needle rust fungus creates a severe discoloration of current year’s growth. Only needles at the tips of the branches appear yellowed, as older needles are resistant to infection.

The needle rust fungus has two hosts, Labrador Tea and White Spruce. In early summer, spores are produced on Labrador Tea leaves. Once dispersed, the spores infect newly grown spruce needles, developing and producing a second round of spores in late summer. Note that this fungus is always around, it’s just that wet and cool summers allow for high spore production and dispersal during both host stages. Outbreaks can be locally severe, but rarely cause lasting damage to the trees. This fungus is related to the spruce broom rust fungus that leaves behind the ‘witches’ broom branch structures in spruce.

For more information: Spruce Needle Rust – USDA Forest Health Protection Program.

(C) Brown or yellow “bushes” growing on spruces
Spruce broom rust is a fungal disease caused by a native parasitic fungus called Chrysomyxa arctostaphyli. It infects spruce needles, which turn yellow or pale green and form buds, producing dense “brooms” of the yellow or orange needles. The fungus alternates between two hosts, with White or Black Spruce as the primary host and Bearberry, also known as Kinnickinnick, as an alternate host needed to complete the fungus’ life cycle. In mid-summer, the fungus in the needles produces orange spores, which are so numerous a tree may release clouds of spores when shaken. Some even can smell the spores. In the winter, the brooms shed their needles and appear brown and barren. Squirrels or birds may use old brooms as a base for their nests.

For more information: Spruce Broom Rust – USDA Forest Health Protection Program

(D) Dead or dying spruce
South-central and interior Alaska is several years into a major outbreak of the native spruce bark beetle. The infestation is continuing northward and is currently active in Denali, especially south of the Alaska Range. Read an update on the outbreak specific to Denali. Another cause of spruce death in interior Alaska is the Northern Spruce Engraver. Generally, mortality of spruce caused by engraver beetles is not as widespread as it is during a bark beetle outbreak, but the two insects can be confused and do co-occur. For more information: Engraver Beetles - USDA Forest Health Protection Program.

(D) Brown zones, areas where shrubs are missing their leaves, appearing dead
Often constrained within particular elevations, dominant shrubs (particularly willows, and in some places dwarf birch) can lose all their leaves to an insect outbreak. For example, caterpillars were observed actively feeding on willow leaves between 3,000 and 3,500 feet along miles 9-16 of the Park Road in early July, 2020. Although temporarily striking, these types of insect outbreaks are generally cyclic, allowing shrubs to recover in subsequent years.

(E) An appearance of dead spruce tops - but look closer to see many, many cones
White Spruce produce their cones in cycles, meaning every few years there are many. Although not a ‘disturbance’, spruce cone production reflects many of the same climatic and weather-related patterns that biologic disturbances, such as insects or fungi, do. A year with very high spruce cone production is called a ‘mast’ year. Learn more about spruce reproduction cycles in Denali.

Blights to Other Species

"Symptoms" of disturbance in other species

(F) Photo courtesy of the City of Grand Prairie, Alberta, (G) NPS Photo/Sarah Stehn, (H) Photo courtesy of Brenda Mowery, (I) Photo by Jason Hollinger via CC BY 2.0.

(F) Brown blotches on willow leaves
These discolorations are caused by a native insect called the willow blotch leaf miner, or Micrurapteryx salicifoliella. The adults are small, gray moths that come out in May to mate and lay eggs on willow (Salix spp.) leaves. The yellow larvae burrow into the leaves to feed, producing distinctive brown blotches. An outbreak of these parasites can eat all the leaves produced by a willow that season, but the plants rarely suffer lasting harm.

(G) Brown zones - areas where shrubs are missing their leaves, appearing deadOften constrained within particular elevations, dominant shrubs (particularly willows, and in some places dwarf birch) can lose all their leaves to an insect outbreak. For example, caterpillars were observed actively feeding on willow leaves between 3,000 and 3,500 feet along miles 9-16 of the Park Road in early July 2020. Although temporarily striking, these types of insect outbreaks are generally cyclic, allowing shrubs to recover in subsequent years.

(H) Near complete lack of leaves, even though it is summer
Some species of insects that defoliate (including beetles, moths, and sawflies) are generalists, meaning they will eat whatever green photosynthetic tissue is available. The result is areas where several species of plants have been defoliated – alder, dwarf birch, willow, blueberry, and sometimes even spruce! These sites are dramatic to walk through and may seem like there has been a fuel spill or other major damaging event. However, it is important to realize that many of our subarctic plants are perennials, growing new leaves on existing stems each year, and have an inherent ability to endure such insect defoliation.

For more information: Forest Health Hot Topics – AK DNR Division of Forestry.

(I) Silvery aspen leaves with squiggly lines
The distinctive patterns pictured are caused by the aspen leaf miner, or Phyllocnistis populiella. This parasitic moth lays its eggs inside the leaves of aspen trees, where its larvae burrow long, winding tunnels inside the leaves called “mines.” As the larvae eat the leaf, the leaf loses much of its photosynthetic capacity and turns silver – that silvery hue can be seen from a distance in large infestations. While aspens can generally tolerate complete defoliation caused by leaf miners, they may reduce growth in severe cases.

For more information: Aspen Leaf Miner - USDA Forest Health Protection Program