P&PDL Picture of the Week for
January 12, 2004

Lichens - More Than Meets the Eye

Gail Ruhl, Senior Plant Disease Diagnostician, Interim P&PDL Director, Purdue University

As deciduous trees stand bare, the greenish "scaly" growth of lichens on tree trunks and branches becomes more visible. Questions often arise regarding the health of trees in relation to the presence of lichens. Many, unaware of the unique composition of lichens, wonder if the lichens will harm the trees.

Lichens are not harmful to trees. As per the following website, Lichens of North America at http://www.lichen.com/, lichens are composite, symbiotic organisms made up from members of as many as three kingdoms. The dominant partner is a fungus. Fungi are incapable of making their own food and thus usually provide for themselves as parasites or decomposers. The lichen fungi (kingdom Fungi) cultivate partners that manufacture food by photosynthesis. Sometimes the partners are algae (kingdom Protista), other times cyanobacteria (kingdom Monera), formerly called blue-green algae. Some enterprising fungi exploit both at once.

Lichens live as one organism, both inhabiting the same body and are merely using the trees as a place to anchor.

Lichens occur in a variety of habitats from the Arctic to the Antarctic and all regions in between. One may find them on exposed rocks in the deserts, on solidified lava flows in Hawaii, on frozen substrata in arctic regions, on the bark of trees, and on the leaves of plants. It has been reported that there are more than 3600 species of lichens in the United States and Canada. Some lichens are flat and appressed to the growing surface while others may appear leaf or shrub-like. http://www.lichen.com/vocabulary.html.

Lichens can serve as indicators of air pollution. In centers of heavy industrial pollution, it has been observed that few to no lichens can be found. Lichen populations have been shown to increase gradually with distance from these centers and are thus used in ecological studies as a measure of pollution intensity.

For more information on lichens you may want to check out the following web site: LichenLand http://mgd.nacse.org/hyperSQL/lichenland/.

Click on image to enlarge

Lichen on a tree trunk

Lichens collected from trees in North Carolina

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service