Lichens are familiar sights in many landscape settings, even to those who don't know whether the colorful patchwork is animal, plant or even nonbiotic in nature. Actually partnerships between two organisms, lichens frequently grow on trees, almost always in a way that doesn't harm the host plant.
"Symbiosis" in common parlance is often equated with "mutualism," a mutually beneficial relationship between two organisms. Actually, mutualism is but one variety of symbiosis, which is an umbrella term encompassing all kinds of interactions between unrelated species. In most cases, the basic kind of symbiosis existing between trees and lichens is one of commensalism, in which one organism benefits from the association and the other is neither positively nor adversely affected.
Lichens mainly use trees as structural perches. They are not extracting nutrients or water from tree tissue itself, as they are capable of photosynthesizing on their own. Lofty trunk or branch sites can give lichens better access to sunlight for making their own food, as well as to moisture condensed on the tree through the phenomenon of fog drip.
Indirect Lichen Benefits
Thinking more broadly, though, trees can benefit somewhat from the presence of lichens in the vicinity. Those lichens that exist as a partnership between a fungus and a cyanobacterium can, by virtue of the latter's activities, "fix" -- or make available in a form usable to other organisms -- the essential nutrient of nitrogen from the air. Rain can leach nitrogen from lichens and thus distribute it in soil, where trees and other plants can tap into it.
The decomposition of lichens, as with all organic matter, also adds nutrients to the soil. Finally, lichens can function as notable agents of biological weathering: By expanding and contracting with moisture absorption and drying and to a lesser extent by actively extracting nutrients, lichen can pry apart cracks and crevices in bare rock. Such activity opens a foothold for the establishment of trees, as organic matter begins collecting in the fissure and proto-soil forms. Tree roots themselves can further rend open rock.
Adverse Tree Effects
Though the vast majority of lichen-tree relationships involve no harm to the tree, the host can be damaged is some situations. Some tree-infesting insects lay their eggs in arboreal lichens, thus proliferating a pest destructive to the tree. According to Ohio State University Extension, at least one species of North American algae that damages leafs and twigs of certain trees and shrubs partners with fungus to form lichen.
Outside the habitat relationship they sometimes engage in with trees, lichens themselves are among the most well-known and remarkable examples of symbiosis: They are actually biological collaborations between a fungus and a photosynthetic organism (a photobiont), usually a kind of algae or a cyanobacterium. The fungus certainly benefits from the relationship, gleaning the energy produced by its partner's photosynthesis operations. The photobiont may benefit by the moisture and shelter provided by the fungus. Ultimately, though, the relationship may be more akin to a farmer (the fungus) and its crop or livestock (the photobiont) -- or, as Daniel Mathews writes in "Cascade-Olympic Natural History," to a human being and its helpful intestinal microorganisms.