Portland Oregon Landscape and Tree Care

Lichens okay for trees: clean air indicators

Written by Mario Vaden - Oregon Arborist

The lichen in the image is in the Cladonia genus. Photo taken December 2006 at 2800 feet elevation; Rogue River National Forest. There are several Cladonia lichens. Ones like this may be called British Soldiers or Pixie Cup. These grow about 2 mm per year. This one in the image may be over 20 years old.

Forest Photos lichen on a stump

In most cases, lichens are not bad for trees. You may find lichens growing on old pruning cuts or bark. Either way, it is okay. Of course, some kinds are also found on stumps, rocks, old fences, etc. These are common in the forest, or in gardens.

Generally, lichens are an indictor of good air quality. Next time that you think about it, look at the trees near the downtown area of a big city and notice how sparse the lichens are on the trees. As you move out into the country, especially countryside areas with good air quality, the lichen populations will increase.

Lichens can grow almost anywhere, but rely on nutrients accumulated from the air. And they are sensitive to air pollution, making them valuable early warning indicators of reduced air quality. Scientists have used lichens as biomonitors for decades. The "bio-indicator" use of lichens has been written about in science magazines, and is researched by the USDA Forest Service.

When you have the opportunity, search the internet for "lichens" and "pollution". Many resources should be displayed.

One concern, is with trees like apple. Apple trees benefit from sanitation like early removal of leaves in autumn. This helps reduce the number of fungus spores near apple trees: spores that can generate fungal problems. This manicuring works well in conjunction with dormant spray which lands on the tree and the ground and spores on the ground. In similar fashion, I prefer that my apple trees don't have heavy accumulations of lichens which may harbor spores or hinder coverage of dormant spray.

For apple, I see no need to remove all the lichens. Wear leather gloves while pruning and rub your hands over any bulky lichens. Whatever falls off, falls, and whatever stays, stays. The concern is not about how much area of bark has lichens, but how thick the growth is.

Lichens are not plants. Lichens are a fungus and an algae together. Each part of lichens can help the other. The fungus provides the algae with a substance to live in, and the algae makes food for the fungus. Each organism can live separately, but they do well together. The main body of lichens is called a thallus.

There are many lichens, and three main types. Foliose lichens look like leaves. Crustose lichens look flat and crusty. Frutose lichens are upright or hang down. Frutose lichens tend to display some bright colors. The Cladonia in the image, is a frutose type.

Aside from trees, Lichens can break down old wood and put nutrients back in soil for plants. Lichens also gather nitrogen from air and distribute it into soil. If you enjoy small forest life, check out some images on my Oregon Mushrooms album.