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Lichens - mysterious and important

Posted: December 30, 2011 - 1:00am
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A tube lichen, with dark brown spore-producing bodies.   Bob Armstrong
Bob Armstrong
A tube lichen, with dark brown spore-producing bodies.

Our coastal rainforest is characterized by a rich assembly of lichens. Lichens colonize rocks — including the rocky intertidal zone, logs and fallen branches, and even bare sand. They also adorn the trunks and branches of trees. Some branches support numerous kinds of lichens in a sort of miniature garden, of varied colors and textures, often very beautiful.

What, then, are these often-ignored beings? The traditional view is that a lichen is a happy collaboration of a fungus with one or two kinds of alga: a green alga or what was called a blue-green alga, now known as cyanobacterium. Only certain kinds of algae associate in this way with fungi, and only some fungi make these associations with algae. Lichens have been used as classical examples of natural symbiosis (living together), in which the alga provides carbohydrates (by photosynthesis) and sometimes nitrogen (by the cyanobacteria) to the fungus and the fungus provides a protected habitat, water, and perhaps minerals to the alga.

The reality, however, may not always be quite so bland. When the fungus reproduces by spores, which germinate to form a new individual fungus, the new fungus eventually captures algal cells (thus becoming a lichen) and exploits them until they die. Although most lichenizing fungi can live for some time without algae, the algae apparently can live quite well on their own and getting captured by a fungus may not be entirely beneficial from the algal point of view. Thus, many lichens may be viewed as a sort of controlled parasitism of the alga by the fungus. One researcher even commented that sometimes the algae “escape” from the fungal grasp!

The relationship between fungus and alga may vary with circumstances, but the factors controlling that variation remain to be described, it seems. Whatever the precise relationship between the two (or three) different organisms that constitute a lichen, the ecological roles of lichens are important and widespread.

Lichens are important stabilizers of bare ground; in some situations, they help reduce erosion. Indeed, they are said to be the dominant form of ‘vegetation’ on 8 percent of the land surface (think especially of Arctic, Antarctic, and alpine areas). They also — very slowly — break down rocks, mostly by secreting acids that weaken the rock, and thus they contribute to the long-term weathering process that creates soil.

Ground-dwelling lichens (and mosses) trap wind-blown dust; the dust and associated nutrients, along with nutrients from rainwater, gradually build up soils. Then wind-borne seeds and spores have a good habitat for germination. Lichens containing cyanobacteria can extract nitrogen from the air and make it usable by plants; this is called nitrogen fixation. Foresters in some regions have even encouraged lichens to spread — for example, over a recent burn — in order to improve the soil and make new trees grow better.

Tree-dwelling lichens also extract lots of nutrients from rainwater and fog, and many of our common arboreal lichens are good nitrogen-fixers. Nutrients from these lichens eventually find their way into the soil. For example, we often see, on the ground, fragments of lichens that have been torn from the trees by a stiff wind. When these decompose, their nutrients are added to the soil.

Lichens provide shelter for numerous tiny animals, such as springtails and mites, which are food for spiders and insects, which in turn are food for shrews and birds. We often see warblers, chickadees, and woodpeckers searching among the lichens on branches for lurking insects or spiders.

Caribou have been called lichen specialists. Lichens comprise much of the caribou diet — as much as 90 percent in winter but even in summer about half the diet is lichens. The ground cover commonly called “reindeer moss” is really a lichen. Caribou were introduced to St Matthew Island in the Bering Sea some decades ago; the herd increased rapidly but eventually ate up most of the lichens, which grow too slowly to compensate for the rate of eating them, and thousands of caribou then died of starvation.

Arboreal lichens provide important food for woodland caribou, elk, black-tailed deer, and moose. Mountain goats in our area eat lichens all year long, including several types that grow on trees. Northern flying squirrels and boreal red-backed voles eat a lot of lichen; arboreal lichens may constitute as much as ninety percent of the winter diet of flying squirrels. The flying squirrels also use horsehair lichens to build their nests inside of tree cavities.

Dozens of species of birds incorporate lichens in their nests. I once found a crossbill nest that had blown out of a tree. The outside was made of lichen-covered twigs; in fact, an expert later identified many kinds of lichen in this nest. Hummingbirds commonly decorate the exterior of the nest with bits of leafy lichens, which help camouflage the tiny nest.

Humans in many cultures have found uses for lichens — as food, fiber, medicine and poison, dyes, decoration, and, in some parts of the world, as perfume. Many kinds of lichen can be eaten; only a few are poisonous. However, they are said to be not very flavorful, at best and are better eaten after boiling in several waters to reduce bitterness or with soda to reduce extreme acidity. The lichen known as rock tripe is famous as a survival food for Arctic explorers. Arctic Natives ate the partially digested lichens from caribou stomachs; this stuff is apparently more readily digested than fresh lichen. Mixed with raw fish eggs, it reportedly formed a favorite concoction (an acquired taste, no doubt!).

Over the centuries, lichens have been used as medicines in various ways, both internally and externally. Some of the rationales for medicinal applications seem ludicrous to us now. For example, lungwort was used to treat lung diseases because its lumpy surface reminded people of lung tissue. However, it turns out the lungwort actually does contain substances that work against Tuberculosis bacteria. The basis for using a yellow-orange rock lichen to treat jaundice, which turns the skin yellow, is still more tenuous.

Beard lichens contain usnic acid, which has some antibiotic properties. So beard lichens have often have been used in ointments to treat skin infections and sores. Indeed, they have been over-collected in some regions, to the point that they are scarce and hard to find. However, some people are very allergic to this acid, so medicines with this as an ingredient must be used with care.

Fibrous lichens were used in clothing by some Native cultures, mixed with bark fiber. Because such garments were not very durable, they were often restricted to ceremonial use. Branching, shrubby lichens are used in floral arrangements and architectural models to represent shrubs and trees.

Lichens have provided dyes for centuries: in our region they have been used to color mountain goat wool and porcupine quills. Prepared in boiling water, certain lichens yield russets, brown, and yellows. Others yield reds and purples when fermented with a source of ammonia (traditionally, this was stale urine—the aroma eventually dissipated).

An unusual use of lichens is directly applicable in our area. Crustose lichens generally grow slowly, less than a millimeter a year. By measuring lichens on dated (and undisturbed) gravestones, one can estimate the average rate of growth. Then if that estimate is applied to the same kind of lichen on rocks near a glacier or a rock slide, it is possible to estimate how long that rock was available for colonization by lichens; that is, how long it has been there.

Humans have a huge impact on lichen populations and communities. The most obvious is habitat destruction through increasing urbanization. Directly relevant to our area is the dramatic loss of tree habitat when old-growth forest is logged. Second-growth stands support only a much-reduced diversity of lichens.

More pervasive is the dramatic effect of air pollution on lichens, many of which cannot tolerate the pollutants poured forth from industrial processes. Sulfur dioxide is an example of a deadly pollutant, but excess amounts of lead or zinc, for example, could also be detrimental. Some of the most pollution-sensitive lichens include the beard lichens and those that contain cyanobacteria (for instance, lungwort and kidney lichens). The trunks of our red alder trees often look white or grayish, but this is actually a white crustose lichen; in areas with serious air pollution, this lichen dies and the trunks show their brown bark. Because of their sensitivity to air pollution, lichens are extremely useful as indicators of air quality.

• Mary F. Willson is a retired professor of ecology.

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