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LICHENS
ON TREES |
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Lichens are organisms consisting of a fungus and
a green or blue-green alga growing together in a
mutually beneficial, symbiotic, relationship.
Lichen is a combination of a fungus and a green or
blue-green alga enclosed by the fungal hyphae.
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 food water and minerals from
the air, the material it is growing on and from the
alga, which manufactures food through photosynthesis and
the alga receives some of its food and protection from
the fungus. 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.
Three forms of lichens exist, it may be flat, leafy, or
branched and hairlike.- crustose (flat type of growth),
foliose (leaf-like but with prostrate growth), and
fruticose (bush-like and erect or hanging growth). The
effect of lichens on trees is only slightly damaging.
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. Heavy lichen
growth indicates poor tree growth as a result of some
other cultural problem. Lichens can restrict gas
exchange from the limb or twig and can restrict the
amount of light received by a limb.
Materials
used in the control of ball moss will kill the lichens
for a short period. Regrowth usually occurs within the
same year after the tree was sprayed. Chemical control
of lichens is not currently recommended. This is due
to two reasons, one, chemicals are currently not
cleared by EPA and control has not been of long enough
duration to warrant spraying. Rather, trees should be
encouraged to develop a dense canopy which will shade
out the lichen growth.
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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.
References:
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. |
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