Lichens

A lichen is not a thing. That is, it is not a single organism. It is a living partnership between two kinds of organisms: fungi and algae. Lichens grow on rocks, tree bark and other solid surfaces. They get the water and minerals they need from the atmosphere and make their own food using photosynthesis. They may appear plant-like, but they are not plants. They can have crusty, leafy, branching or other growth forms and come in various colors and sizes. 

Lichen and moss

The mutually beneficial relationship between the fungi and algae in lichens is called a symbiotic relationship. In fact, the word "symbiosis" was coined by scientists to describe this relationship in lichens. Filaments of the fungi provide the structure for the composite organisms. They also collect and store water and nutrients. The algae live between the filaments, near the surface where they can collect the sun’s energy for photosynthesis. These tiny solar cells convert the solar energy to sugars used by the fungus.

Common Lichen Growth Forms
There are over 15,000 species of lichen worldwide and 1000 in the Pacific Northwest, so learning to identify them may seem like a daunting challenge. A more realistic approach might be to distinguish these common growth forms: 

Crustose – A crust on a rock or other surface. 

Fruticose – A tuft of tiny, leafless branches.

Foliose – Flat leaf-like structures.

Leprose – Looks like a powder. 

Scientists once thought that a lichen was a single species of fungus living with a single species of alga. They recently discovered that there can be two or more species of each living together, forming a tiny ecosystem with different organisms performing different functions. 

As I was reading about lichens, it seemed to me that there is a basic problem with the classification of lichen species, even when it is a single fungus and alga living together. A lichen isn’t a single species. It is at least two species living together. Taxonomists have had neat little species boxes that they put all living things into, but, as it turns out, lichens don't fit in these boxes. It looks like we need a different way to classify lichens. Further reading revealed that by convention, lichens are classified as species using the species name of the fungus. However, this is not without problems. A fungus may pair with different algae, and the results can produce quite distinctive forms that would merit classification as different species. Furthermore, some lichens include two species of fungus. Oops! It appears that we need a different kind of box for lichen classification. This is a hot topic among lichenologists.

All this diversity of different fungi and algae living together is what makes lichens particularly hardy, enabling them to populate the entire planet, living where nothing else can, surviving icy cold mountains and hot, dry deserts. They are sometimes the only living thing that can survive in these extreme environments. They are often the first living things to grow after a disaster has destroyed other life forms.

Given a place to grow, sunlight, and water, lichens seemingly live independently in their own little world. However, they play many important roles in the larger ecosystems where they live. Some lichens are an important food source for animals, for example, reindeer. Northern flying squirrels eat lichen in winter and use it as a nesting material. Hummingbirds also use lichen in their nests. In the distant past, lichen has been an important human food in both Europe and North America.

I'm often asked whether or not lichen growing on trees harms the trees. I wasn't sure but I thought that it didn't. Understanding how lichens function in the ecosystem enables us to give a more definitive answer to that question. Lichens do not have roots. They are not parasitic like mistletoe, and do not rob nutrients from their host tree.

Lettuce Lichen (BLM photo)

There is at least one lichen that benefits the trees where it lives. One nutrient that all plants need to grow and thrive is nitrogen. In a young forest nitrogen is supplied by nitrogen fixing plants, for example, red alder trees, shrubs like red currant or deer brush, and other species in the Ribes and Ceanothus genera. These shrubs and trees have nodules of bacteria on their roots that convert the nitrogen in the air to a form that can be used by plants. However, the shrubs and alder are only present in a young forest. As the conifers grow and shade out the shrubbery and even the alders, these nitrogen fixers are unable to compete. For a long time scientists wondered where an old growth forest gets its nitrogen. Then in 1970, scientists at the Andrews Experimental Forest east of Eugene, Oregon, discovered that a lichen that grows in the canopy of an old growth forest is a rich source of nitrogen. This lichen is called lettuce lichen (Lobaria oregana). It grows only in the canopy of mature forests and fixes up to 22 pounds of nitrogen per acre in a year. Some of this nitrogen is used by bacteria and fungus-eating animals. Some of the lettuce lichen falls to the forest floor where the nitrogen is taken up by trees. The iconic Douglas firs of the Northwest could not grow to their immense size without the benefit of lettuce lichen.

Finally, another fascinating characteristic of lichens: Since they get their nourishment from the air, they also absorb any pollutants in the air. Some lichen species are sensitive to air pollutants and will be damaged or even die if air pollution levels are too high. It is possible to determine air quality by looking for the presence and at the quality of these sensitive species. Scientists also gather lichen and moss samples and use them to measure air pollutants. This is how they recently discovered high levels of cadmium and arsenic in the air near two artistic glass factories in Portland.

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More info

A Field Guide to the Lichen of Opal Creek

How a Guy From a Montana Trailer Park Overturned 150 Years of Biology

How Tree Moss Could Revolutionize What We Know About Air Pollution

Lichen (Wikipedia)

The Hidden Forest by Jon Luoma, Oregon State University Press, Corvallis, OR