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Does My Tree Have a Disease?

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I have received several pictures of trees with what looks like some type of weird growth on the bark. I have received these questions many times in the past and what people think is a tree disease  is actually an organism known as lichens. Lichens are a composite organisms consisting of a fungus and a photosynthetic partner growing together in a symbiotic relationship. They are known to grow in almost every climate found on earth. They can grow in deserts and in the arctic. They will grow on rocks, buildings, walls, roofs and trees and shrubs. They are long-living organisms that can survive droughts and floods. They can be used to make dyes and serve as food for many creatures, even humans. Lichens are very beneficial to nature's ecosystem. As lichens grow on rocks or inhabit barren areas, they trap dust, silt and clay, helping to create new soil. They have the ability to trap nitrogen from the atmosphere and release it to other plants when it rains. As they die, they contribute to the decayed organic matter in the area where they grew, allowing seeds from other plants to germinate. Many people become concerned when they see these weird looking growths on their trees or shrubs. They are worried that they may be parasitic and causing damage to the plant. I think this thought can come from the fact that lichens appear to grow more on dead branches than on live branches. This is not always the case, but lichens will often grow more on dead branches since those branches will often receive more sunlight, which favors the development of the lichens. Lichens will grow on healthy plants as well, but have little effect on the health of the plant. The best thing to do is enjoy the shape and color of the various lichens as they grow on your trees and shrubs. Realize that they are part of nature and its complex ecosystem. There are more important things to be concerned about than lichens growing on your trees.