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Lichen: Team workers of the woods

Posted: Sunday, June 24, 2007

I am a slow learner. At the beginning of my career in biology, I found animals to be fascinating, and plants were just those green things that made perches for birds and whose leaves use up carbon dioxide and produce oxygen. Midway along this career, I belatedly discovered, rather suddenly, that plants are actually fascinating too. They do many more things than my ignorant brain had imagined - it's a matter of perspective. And just recently, I've sunk so low, so to speak, as to find fungi and lichens captivating.

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We are surrounded by lichens. They occur from pole to pole and from high elevations to the marine intertidal. They grow on rocks and tree bark, on soil surfaces (and my roof), on conifer needles and decaying plants, on the back of tortoises and some insects, on bone, on glass and metal and plastic, and on each other. We often see their multicolored crusts on rocks - yellow and orange, gray and green, bluish and black, sometimes making intricate and lovely patterns. In certain locations the trees are festooned with a drapery of yellow-green 'witches' hair' and 'old man's beard'. Some lichens are vaguely leaf-like in shape, often crispy when dry but succulent when wet.

Take any trail in Juneau, and start looking; you will find an astonishing array of these interesting and useful organisms.

Lichens are often used as examples of symbiosis - two different organisms living together in some intimate association, usually thought to be of mutual benefit. Lichens are formed by a close association of a fungus with green algae or sometimes with cyanobacteria (formerly called blue-green algae). A single type of fungus may associate with different algae in different areas, and the resulting lichens may look quite different.

Recent work has shown that the relationship is not symmetrical. The green algae are usually capable of living independently, but they are sometimes 'captured' by fungi. In most cases the fungus is a 'controlled parasite' on the alga, using the carbohydrates produced by the alga and usually providing little in return, except perhaps some water and a place to live. In fact, the fungus often cannot live without its associated alga and usually kills the algal cells eventually, so the fungus then must obtain new ones.

Lichens reproduce in three ways. Tiny spores, containing only fungal cells, are produced by sexual reproduction; when the spore germinates, the young fungus must acquire its own alga. Alternatively, asexual, vegetative 'buds' separate from the lichen surface; these contain both fungal and algal cells, so they can grow directly into a new lichen. And sometimes a lichen is broken into pieces that drift away and get re-established in a new location. Any of these reproductive units could be carried by wind, water, or animals to new sites.

Lichens have important ecological roles. The most far-reaching is probably their contribution to soil formation. Acids excreted by lichens help break down rock, both by eating away the minerals and by loosening the rock matrix and allowing freeze-thaw cycles to break off small chunks. Lichens catch floating dust particles (and obtain minerals therefrom), building up tiny pockets of soil that provide sites for plant seeds that waft in. Some lichens (the algal portion) can extract nitrogen from the air, using it for their own growth and adding it to the soil when parts of the lichen eventually die and decay. Lichens also stabilize soils by forming surface crusts that retard erosion. Some lichens are more reflective than soil itself, so they reflect solar radiation, keeping the surface from overheating and cooking plant seeds and seedlings. These effects are notably important on recently deglaciated lands, such as near our local glacier and all across the boreal forest in northern Canada and interior Alaska.

Lichens on tree bark are also important in capturing atmospheric nitrogen and adding it to the nutrient cycle of forests. This is reported to be especially critical in conifer forests of the Pacific coast of North America.

A surprising array of animals uses lichens. Best-known are caribou, which depend on lichen (including reindeer 'moss'-- not a moss at all) for food in both winter and summer. The lichens that caribou eat are low in protein, however, so the caribou have to eat other things as well. Deer, mountain goats, and moose also eat lichens in quantity. Other major lichen eaters include flying squirrels and voles, slugs and snails, and an assortment of invertebrates.

Many birds (reportedly over 50 species) incorporate lichens in their nests in some way. Hummingbirds and some flycatchers regularly camouflage their nests with a layer of lichens stuck around the outside. In eastern North America, the northern parula warbler commonly nests in hanging clumps of 'old-man's beard' lichen. Other animals, including some frogs and moths, have lichen-like patterns that camouflage the animals very well, as long as they are on a background of lichens. Several kinds of insects carry lichens on their backs as camouflage. For example, the larva of a species of lacewing excretes a sticky material onto its back, to which bits of lichen then adhere; the insect looks just like a patch of lichen on tree bark.

So-live and learn! Who know what bits of esoterica from the natural world will become interesting next?

Mary F. Willson is a retired professor of ecology and a Trail Mix board member.



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