Trees, Lichens and Exotic Pests

Have you noticed things growing on your trees? If you said yes, your next question may be “Will they hurt my trees?” That question was recently posed to DNR’s Forest Health program. A woman was concerned about the Douglas-fir trees in her neighborhood. She had noticed that more and more tree limbs seemed to be covered with some types of dry fungus or moss that almost crumbled when she touched it. “Has a new plant element invaded our country?” she asked. She sent some samples of lichen-covered tree bark (photo 1) and a piece of lichen she’d collected (photo 2). Forest Health Program Manager Karen Ripley answers the question.

Lichen's on a branch

Although some fungi cause disease, in general the moss and lichen (a cooperation of fungi and algae) growing on trees are considered neutral or beneficial. These organisms get most of their nutrients from rainfall and air. When they die and decompose, the nutrients they’ve collected become available to forest plants. In places like the Olympic rain forests, there are many, many pounds of nutrients pulled in from the atmosphere each year due to the biological activity of mosses and lichens.

Although there is a huge diversity of mosses and lichens with many different preferred habitats (on soil, on rocks, on vegetation), many of the tree-growing species require relatively high moisture and relatively cool temperatures, but not much light. They grow and accumulate mostly on the trunks and lower branches of trees where it is shady and most humid, nearest the ground. During dry, dormant periods when the mosses and lichens dry out, they are brittle and crumbly.

When the needles or leaves on these low, shaded tree branches aren’t getting enough light to photosynthesize efficiently, the tree naturally lets them die off, which is known as “self-pruning.” Over time the dead branches become brittle and break, but it’s just because the branches died from lack of light. The cause of death is not related to the presence of moss or lichens. Although the lichens will continue to grow and can completely cover the branches, they don’t harm or overwhelm or break those branches.

Lichens rely on the atmosphere for their water and nutrients, and are particularly sensitive to climate and air pollution. The presence or absence of certain lichen species can indicate weather conditions and air pollution levels. If air quality is poor, sensitive lichen species may disappear or may grow in shapes that indicate they have been exposed to and affected by air pollutants.

Sarah Jovan, a lichen specialist with the Forest Service, identified at least six different lichen species on the bark sample provided. They are not alien invaders. The pale green Usnea clump (photo 2) and another threadlike variety present in photo 1 are considered sensitive to air pollutants and appear small and somewhat stunted. However, according to Sarah, the air quality in this area is likely not that bad. If it were terrible, these lichens would not be present and, moreover, there would likely be other nitrogen-loving  “eutrophic” species present and flourishing. “Eutrophic” organisms are living things that flourish in areas with a lot of extra nitrogen in the air. Common eutrophic lichens are the yellow and orange crusty types you might see coating the trunks of street trees in metro areas like downtown Portland, Seattle or Tacoma. When eutrophic lichens grow on conifers, it usually means there’s quite a LOT of nitrogen air pollution. (Nitrogen, such as “nitrogen dioxide” (NO2), is a by-product of burning fossil fuels).

There are a couple of exotic aphid-like insects that produce a waxy, fuzzy surface that looks somewhat like a fine moss or mold on the twigs and trunks of eastern hemlock trees and certain fir trees. They cause the twigs to grow in an abnormal manner, stop producing new needles, die and dry out. These insects, the “hemlock woolly adelgid” and the “balsam woolly adelgid”, also occur in Washington State. Although our western and mountain hemlock species are not susceptible to damage, several types of fir trees like Subalpine fir, Pacific silver fir and Fraser fir are damaged here. Douglas-fir is not a host to these exotic pests.

It is good to keep an eye out for exotic (non-native) pests, since they can be a huge problem for forests and natural areas. If you have any forest health concerns, you can contact Karen at or 360-902-1691. Including a photograph or sample can greatly facilitate correct identification of the problem.

Editor’s note: Want to learn even more about lichens? Check out the photo gallery on the Ways of Enlichenment website.