lichen.
The growth of lichen on tree bark depends more on the physical surface of the bark than on the kind of trees. Young trees in general have a smoother bark that tends to attract crustose lichen species. As trees get older, the bark usually develops uneven broken surfaces that permit the foliose and fruticose lichens attach to the tree.
Lichens found on trees often are circular or oblong in shape with leaf-like lobes (foliose) that are parallel with or slightly higher than the bark. Each lichen body usually is limited in size to a few inches in diameter and initially may be dispersed along the bark. While they grow very slowly, i.e. one to a few mm per year, over time they tend to grow together and cover large sections of the bark.
Lichens tend to grow on the inner trunk and branches of deciduous trees and shrubs. While the tree’s leaves may shade the available sunlight for the lichens during the warm weather growing season, the lichens do not similarly affect the leaves. After the leaves fall from the deciduous trees in the autumn, the lichens receive a greater proportion of the available sunlight and continue to grow during the colder winter months.
Conifer tree bark differs in chemistry from deciduous tree bark as it is more acidic with organic resins and gums. Conifer canopies tend to be denser and allow little sunlight to fall on the bark. In the case of conifer or evergreen trees , some lichens can survive in the resulting year-round partial shade. To obtain sunlight they also tend to locate on branches that are stressed or dead, which may again give the erroneous impression that they have contributed to the plant’s health condition.
Lichens provide many benefits both in nature and in human culture. Lichens are eaten by many animals, such as deer, mountain goats and caribou. Many species of birds use lichen materials in constructing their nests. Certain lichens are used in producing antibiotics, while others provide the miniature plant landscape for model railroad tracks.
Lichens are numerous and important organisms in the natural environment that are generally beneficial in nature. The presence of lichens on healthy trees should be welcomed as likely positive indicators of lower levels of air pollution and a reasonably good quality of atmospheric conditions in the neighborhood. The homeowner with lichens residing on their damaged or diseased tree branches should concentrate upon identifying the real sources for the tree distress if any is seen. The lichens are not the cause for the condition of the stressed tree. However, lichens may co-exist on trees with other organisms that are causing disease or injury. Abundance lichen presence concentrated on damaged or dead wood may be a warning of present or impending invasive disease or decay caused by fungi, bacteria, viruses or insects and may require corrective action by homeowners or tree care professionals.
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