Whenever the weather turns hot and humid, our office receives a few calls about a strange fungus growing on trees. Is this fungus killing my tree? Can it be removed? Fortunately, it is nothing to be concerned about and is actually a positive sign that air quality is improving. These growths are known as lichens. They are unusual because they are made of two completely different organisms. Most of the lichen is composed of a fungus, but living among the tightly packed threads of the fungus are cells of an alga. These two organisms live together and form the lichen structure, which is usually a bluish-green color.
The job of the alga is to provide food, as it’s able to use sunlight to make food for the fungus as well as itself. The fungus holds up its end of the arrangement by obtaining water and minerals for itself and the alga and by protecting the algal cells. Lichens are common on trees because the bark provides a nice place to gather sunlight and grow. They do not feed on the tree or harm it in any way. Lichens sometimes grow profusely on dead branches or trees, raising suspicion that they cause disease. The reason they grow so well on these leafless branches is because they are fully exposed to the sunlight. Many people say they have never noticed lichens before. That may be in part because lichens are extremely sensitive to air pollution. Back in the 1960’s, it was more difficult to find lichens. Since then, we have become more aware of environmental issues, and lichens are making a comeback. When there are too many harmful substances in the air, lichens die. Lichens will grow on almost any stable and sunny surface. Besides tree trunks, other common lichen habitats include rocks, tombstones, soil and on the tundra.
Lichens are tough organisms, able to survive hot or cold temperatures and able to survive with little moisture. They often grow in spots that are too harsh for most other organisms. Lichens grow very slowly, usually just a fraction of an inch per year. The lichens commonly found on trees tend to be circular and are scattered on the bark. Each lichen body is usually several inches or less in diameter, but as they grow together, large areas of a tree trunk can be covered. Scientists have identified as many as 20,000 different kinds of lichens. The color and growth habit are used to identify and classify lichens. Foliose lichens are gray-green and commonly found on trees. They also provide homes for a number of insects and are a part of the diet for animals such as caribou, mountain goats and deer. More than 50 species of birds use lichens in their nests. The moral of the story is that lichens are a good thing, not a sign that your tree is infected. Removing them will likely do more harm than good to the tree, so they are best left alone.