ation is usually required to identify crustose lichen species.
Lichens may reproduce in a sexual, asexual, or vegetative manner. Lichen sexual reproduction is quite complicated as two or more organisms are contained in the lichen. The algae or cyanobacteria do not have recognizable reproductive parts and do not reproduce sexually after they are in a lichen association. The sexual fruiting bodies of lichens are those of the fungi. Most fungi that form lichens are sac fungi (Ascomycetes) that produce microscopic spores in sacs. A fungus can produce millions of spores sexually. A new lichen association can be created only when fungal spores come in contact with the appropriate algae or cyanobacteria in the correct habitat.
Lichen reproductive parts containing both algal and fungal cells may occur asexually for dispersal. In vegetative reproduction, any fragment or shred of lichen containing both the algal and fungal components that breaks off the original can form a new lichen body. In some circumstances, the fragments must break down into undifferentiated fungal and algal cells before new lichens are created.
Different lichen species can grow on many types of surfaces, including tree bark, dead wood, bare rock, cleared soil, rusty metal, animal bones, glass, plastic and cloth. Some lichens can grow on many type surfaces while others are confined to specific types of trees or rocks. Many lichens commonly found on living tree bark are seldom discovered growing on rock or soil, as these different lichens need varying surfaces to become established.
Lichens of North America documents hundreds of species of lichens that reside on trees. It is the primary reference for lichen identification and related information in this fact sheet. Lichen species on bark do not follow the classic succession of crustose followed by foliose and then fruticose that is evident in the colonization of rock. The relatively faster growing foliose and fruticose lichens are often the first to appear on tree bark to be followed later by crustose 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 impor