Why Does Lichen Grow on Only One Side of the Tree?

Lichens are the moss-like organisms that grow on the bark of trees. They capitalize on the warm, sunny situation that a tree trunk provides. Contrary to popular belief, lichen neither harms its host plant, nor feeds on it. It is merely a squatter looking for a nice place to stay and grow. Another myth about lichen is that it only grows on one side of the tree.

Though lichen appears to be moss, it is actually a fungus and an algae existing together in a symbiotic relationship. Most lichen contain rhizoids, a fungus, which serve as the glue that attaches lichen to trees and rocks. Rhizoids are made up of organic material like bird excrement, rain water and wind-blown particles. The algae feeds on the fungus, which in turn feeds on the algae. The algae manufactures food for the fungus during photosythesis. This relationship is what makes lichen.

Lichens reproduce both sexually and asexually. In sexual reproduction tiny spores of the fungal partner are produced. These spores must find an algae partner to become a full lichen. In asexual reproduction, parts of the lichen are broken off and separated. This new piece will attach to a new plant or rock, or to another place on the same base. This effectively allows the formation of the new lichen.

In the northern hemisphere, lichen grows primarily on the north side of the tree, but not completely. Because direct sunlight does not hit the north side of the tree, the bark rarely dries out. This creates an environment where lichen can thrive. Usually, lichen grows around the entire tree. However, it is usually more dense on the north side of the tree, where there is more moisture.

Like all living things, lichen has its role in the circle of life. It is able to initiate soil formation when it adheres to a rock. When wet, the lichen spreads over a rocky surface, and as it dries it contracts, pulling tiny bits of the rock's surface with it. Lichen serves as food for a number of foraging animals. Because they are a common source of litmus, lichens are found in chemical laboratories, and they are used in dyeing and tanning processes.