What are Lichens?
Lichens are not a single organism but a symbiotic relationship between a fungi and certain species of bacteria and blue-green algae. It is more than an inter-relationship between two species but more of a mutually beneficial dependency on one another. The fungus and the bacteria can live independently when separated in a laboratory but in nature the fungus often never exists on its own while some of the bacteria do and some don't. Some scientists now see the lichen as more of a controlled parasitism becuase it appears that the bacteria in a lichenized state grow much more slowly than their free living forms do, meaning that the fungal hoast benefits more from the relationship.
The fungus provides structural support, protection and a relatively stable environment for the bacteria and blue-green algae that are embedded in its hyphal strands (threadlike structures of all fungi). It also provides the algae with nutrients absorbed from its environment (ie: from the trees or rocks they live on as well as the air and water).
The bacteria/blue-green algae provides the fungus with carbohydrates produced during photosynthesis from carbon dioxide and water. Some lichens also contain wart-like structures called cephalodia which contain a second algal symbiont, usually a cyanobacterial species capable of fixing nitrogen from the atmosphere into a form that is usable by the fungal symbiont.
There are ~20 000 lichen species in the world, found from some of the hottest deserts to some of the coldest alpine habitats, beyond the reach of any tree. Some slow growing crustose species in these harsh environments can live to more than 1000 years old.
Structures of Lichens
A lichen is composed of a top surface known as the upper cortex, followed by an algal layer composed of fungal hyphae embedded with bacteria/algae, a medulla (composed only of fungal hyphae, thought to function in absorption of water and nutrients), and when present, a lower cortex which is often covered in short hairs called rhizines which serve as a form of root, attaching the lichen to its substrate (what it grows on usually rocks and trees). Lichens are divided into three categories based on their growth forms, Crustose, Foliose and Fruticose Lichens.
Crustose Lichens
Crustose Lichens are attached to the substrate over their entire thallus and lack a lower cortex, they are bilaterally symmetric. Many crustose lichens occur on rocks as well as the bark of trees and they contain a very thin medulla of fungal hyphae that penetrate the substrate. When the edges of the thallus are free of the substrate they are termed squamulose lichens (they look like little scale with raised edges).
Foliose Lichens
Foliose Lichens are bilaterally symmetric lichens that are leaf-like or have distinct lobes, hence the name foliose. They usually contain a lower cortex and are usually attached to the substrate by rhizines. Medulla thickness is variable and determines the thickness of the thallus. The lower cortex, when present is usually of a different colour than the upper cortex. Some foliose lichens are gelatinous when wet and may lack an upper cortex, with the fungal hyphae and algae intertwined throughout the thallus.
The Foliose Lichens are further divided into:
Fruticose Lichens
Fruticose Lichens are radially symmetric so that the thallus is more or less round. An outer layer of fungal cells surround the algal layer that is usually comprised of "clumps" of algal cells interspersed throughout the fungal hyphae. Next to the algal layer is a fungal hyphae layer. The center of the thallus may be hollow or filled with white, cottony, fungal hyphae. Filamentous fruticose lichens take the shape of their filamentous algal symbiont.
The Fruticose are further divided into:
Lichen Reproduction
Lichens reproduce through the physical fragmentation of the thallus body as well as through the production of soredia and isidia. Soredia are small granules originating in the algal layer from a crack or hole in or on the margins of the cortex. Each soredium contain components of the algal and fungal symbionts and is produced by cell division. Isidia are thumb-shaped growths out of the upper cortex anywhere on the thallus surface. They are protuberances of algal, cortical and medullary tissue that can be broken off and dispersed physically.
Sexual reproduction in lichens is confined only to the fungus when the fruiting body produces spores as do mushrooms; it must then be lichenized to produce an actual lichen. While this process is not entirely understood it does occur in nature and many crustose species rely on this method.
About Lichens & Old Growth Forests
Many lichens are very poor at dispersal as they may rely solely on fragmentation of the thallus. As such old growth forests are vital for the survival of lichen diversity, because most are extremely slow at dispersal. Some lichens require long periods without disturbance before successful colonization can take place, and certain species will not even appear in forests under 200 years of age, an important factor to take into consideration in conservation and forest management decisions.
Four lichen species have been recently listed as species at risk in BC; three of these are recognized as being old growth dependent species. For example the Cryptic Paw (Nephroma occultum) and the Old Growth Specklebelly (Pseudocyphellaria rainierensis) are both are found in old growth forests with moderate temperatures and high humidity, and both are unlikely to become established in second growth regenerating or plantation forests (Environment Canada, 2001). The same is likely true for many more species, many of which have barely been examined yet in BC, if at all.
Furthermore, lichens play important roles in the complex food webs of our forests. For example, certain species provide food for Flying Squirrels, which are the primary prey of the Spotted Owl, others are important winter food sources for browsing deer & elk and many others play an important ecological role in the fixation of nitrogen for the forest; as such the conservation of lichen diversity is crucial for a wide-ranging variety of species.
