There are several things that may be on your tree trunk. They include moss, algae, and lichens. None of these are a problem.
Algae is the rarest of the three in Cabarrus County. Algae indicates there is moisture on the bark. This much moisture is not good. Above ground tree bark isn't designed to be wet, but if the plant has lived for a number of years it will probably continue to live. If the moisture developed in response to a brand new irrigation system then re-engineer the irrigation.
Moss grows on tree bark because of the shade. The old saying about more moss on the north side of the tree is true if you average hundreds of trees. If Native Americans used this method they did it by constant awareness of everything around them, tree after tree after tree. It is not a good method if you only check the next tree you come to.
Lichens increase over time. So older tree have more lichens. Often the lichens will multiply when the tree slows down it's growth and the canopy becomes less dense. Slow growth and few leaves are not good signs. But it doesn't mean the lichen is causing the death of the plant. The increase in lichens happens because the tree is dying. Sometimes a cherry tree, apple tree, or azalea becomes covered with lichens. When this happens the tree may be a few years away from death. Of course, every lichen doesn't mean the plant is declining. Lichens are common on the trunks of large healthy oaks and maples.
Lichens are a complex relation between a fungus and an algae. The algae provides food for the fungus and the fungus provides protection for the algae. Lichens have taken on a new status in recent years. Scientist laud them for fixing nitrogen, indicating air pollution, and for their aesthetic contribution to a plant community. Lichens are a part of the food chain in many ecosystems.
If you could increase the health of the plant you might reduce the number of lichens. Unfortunately, our NCSU specialist know of no way to reversing the process when a mature plants starts to die. You might try The Duke School of Divinity.
I have tried a couple of techniques to increase the life of plants. Fertilizing in the fall to stimulate root growth is worth trying. Mulching will improve moisture in the root zone and provide fresh organic matter for root growth. Mulching is a good thing to do but it may not make a difference to plants already in decline. Sometimes a plant is like a 120 year old person. Nature tells us that neither will be with us for many more years no matter what we do.
One reason to ignore the lichens is so you don't do more damage to the tree. Some people try to fight the lichens and wind up damaging the tree. One person swore up and down he was going to kill those lichens. He took diesel fuel to the trees. I reckon the lichens died when the trees died but I'm not sure the person was ahead.
Actually, lichens should be highly valued in the landscape. I don't know of any shortcuts to growing lichens other than waiting ten to twenty short years. Lichens on plants could be compared to beards on people. Sometimes it makes them look even more distinguished than they are.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.