Issue: Augus1 25, 2001

Lichens

I was wondering if you could look at this picture and tell me what the two things are that are growing on the pinyon pine tree. Are either harmful to the tree?

Your picture shows two forms of lichen, a symbiotic association of fungi and algae. The forms you show are a gray/silver foliose form and a duller silver/green foliose form. Foliose means that they look like they have leaves, when actually these are not true leaves. There are other forms that do not look like these, and there are many colors ranging from silvery, greenish, yellow, and even red-orange. In all cases they use trees, rocks, and in some instances even telephone lines as a substrate upon which to grow. They do not extract nutrition from the plant parasitically, thus they do not directly harm the tree. This is obvious if they can grow on rocks and telephone lines.

Lichen absorbs nutrients from the surface of the substrate and from the air (dust). Moisture is obtained from moisture in the air. All forms of lichen need sunlight but can grow in quite shady places.

Often it appears that they are damaging a tree because they grow better on a tree with declining health than on a healthy tree. However, they are not the cause of the tree's decline - they only benefit from the tree's bad fortune. An unhealthy tree allows more light to penetrate to the bark, and this is beneficial to the lichen. Sometimes a tree with declining health leaks nutrients and sugars which become available to the lichen once it is outside the tree. Consequently, the lichen on the unhealthy tree grows better and looks like the culprit.

This is a clue that you should investigate other factors that can cause tree decline. Insufficient water, disease, and insect attack are likely causes of poor tree health. Last summer was a dry summer, and many trees are exhibiting symptoms of drought injury this year. This may be the cause of excess lichen growth in some natural and home landscape settings.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.