Lichens on trees
Lichens are organisms consisting of a fungus and a green or blue-green alga growing together in a mutually beneficial, symbiotic, relationship. The entire structure, called a thallus, is so different structurally from either of its partners that microscopic examination is necessary to distinguish the fungus and the alga.
The fungus obtains water and minerals from the air and the material it is growing on. The alga provides carbohydrates and vitamins. Some blue-green algae fix nitrogen that is used by both the alga and the fungus. Nitrogen is also obtained from bird excrement, organic debris, or plant leachate.
Lichens may be flat, leafy, or branched and hairlike. All three forms occur on tree bark as well as on rocks, soil, and other substrates. Colors may range from white to gray, red, green, yellow, and black.
Although lichens grow on tree bark, they are not parasitic (disease-causing organisms), and do not harm trees. The fact that lichens grow rapidly when exposed to full sunlight may explain their profusion on dead trees. The one conclusion that may be drawn with certainty from lichens on trees is that the air nearby is relatively pure. Most lichens will not grow in a smoky or polluted atmosphere.
Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 1987. Lichens, p. 506 In: Diseases of Trees and Shrubs. Comstock Publ. Assoc., Cornell U. Press, Ithaca, NY. 574 pp.
Webster, John. 1980. Lecanorales, pp. 367-368 In: Introduction to Fungi. Cambridge University Press, Cambridge. 669 pp.
Revised by Chad Behrendt and Crystal Floyd 1999