 a lichen relationship can and do penetrate damaged or dead wood tissue and commence the decomposition of the tree. Fungi are one of the few living organisms that can break down all of the substances in wood tissue and are essential in nature in clearing away fallen tree trunks and in depositing the remaining material into the ground in the forest. A diseased or stressed tree can have both lichen and separate fungal organisms growing on the same dead branch or portion of the tree. In that case, the homeowner or tree expert may need to prune away the dead tissue for the benefit of the remaining plant. In making these types of decisions, homeowners need to understand the unique nature of lichen and the differences between those fungi within a lichen symbiotic relationship and other fungi operating separately.
Lichens do not conform precisely to the usual biological classification categories as they are composed of two or more types of organism living within a single body. Some previous classifications have been modified by recent DNA studies of lichens. The lichen’s appearance and structure are largely determined by the genetic makeup of the fungus that is generally considered the dominant organism. The lichen genus name is normally the same as the specific fungal name, while the species name is descriptive of the resulting dual organism in Latin.
Lichen identification most often is determined by the descriptive appearance, size, shape and color of the lichen body and the reproductive characteristics. Lichens are placed in groups based on their body forms and features. The three main body groupings are crustose (crust-like,), foliose (leaf-like, seen above), and fruticose (tube or beard-like strands).
The various chemicals produced by lichens are also identifying markers. Lichens also can be somewhat differentiated by the specific type of habitat where they live, such as rock, soil or trees, as well as their geographic distribution. While a 10X hand lens is essential in studying lichen specimens, microscopic examination 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 