Lichens: Fungi That Have Discovered Agriculture

The often misunderstood symbiote can poison wolves, break down rocks, and live for thousands of years.

By Gordon Grice|Wednesday, January 06, 2010
lichen
lichen

 

iStockphoto

When I moved to the Ozarks for graduate school in 1989, the place terrified me. First there was the Pig Trail, which is what the locals call the stretch of highway between Alma and Fayetteville, Arkansas. As I embarked on the steepest stretch of it, I saw a sign that read, “Caution: Eleven people killed on this road in the last two years.” The sign changed every year or so to update the count. Even though the Ozark mountains are more like hills, they were steep enough to make me wince; I’d come from the prairies to the west, so the precipitous roads and drenching humidity of this territory struck me as unnatural. It didn’t help that some of the locals pronounced “Fayetteville” as “fateful.”

My discomfort may help explain a sort of optical illusion I encountered shortly after moving into my home in the woods. One night I glanced out a window and was startled to see that the tree trunks looked completely different from how they had appeared the day before. Instead of dull brown, they were a ghostly gray verging on green and flecked with gold. The next day, exploring those woods, I found the trees still had the new colors of the night before. Their gold flecks were abundant, and I had a hard time distinguishing that gold from the glints of sunlight that filtered through the canopy. Then it struck me: The shimmering color I was seeing belonged not to the tree trunks themselves but to the lichens that covered the bark from top to bottom, made vibrant by the bright sunshine.

The trees never changed back; I soon learned that this was the way of the woods in the Ozarks. My eyes had opened to the life-form that dominated the look of my new surroundings. All around, the trees’ external surfaces were hidden. The forest was painted in the colors of lichens.

I was hardly the first to be baffled by lichens. For hundreds of years, naturalists didn’t quite comprehend what they were. Originally these odd forms were thought to be part of the plant kingdom, which is why we still see lichens collected by botanical gardens. Eventually, microscopy enabled scientists to identify lichens as composites of mutually beneficial fungi and algae. Because fungi take the more dominant role and cultivate photosynthesizing algae for food, in return providing them a shady, moist, vitamin-rich environment, scientists have classified lichens based on their fungi species. Their identity came into dispute again when blue-green algae, a frequent component of lichens, were reclassified as cyanobacteria, a kind of bacteria that obtain energy through photosynthesis. But as it turned out, whether the fungi were harvesting algae or cyanobacteria, the symbiotic modus operandi of the lichens proved to be the same. Perhaps Trevor Goward, the lichen curator at the University of British Columbia Herbarium, describes them best. “Lichens,” he says, “are fungi that have discovered agriculture.”

My walks in the Ozark woods impressed upon me the lichens’ diversity and the confusion they can create. Sometimes their identity was clear. A lichen species known as British soldiers sports distinctive, bright red caps atop green stalks. Old man’s beard can run more than three feet long and hangs from trees in the manner of Spanish moss. But without a microscope to see green clusters or strings of photosynthetic organisms running like arteries through the fungal flesh, you cannot always tell you are seeing a colony of lichens. A colony might look like a plant, an uncomplicated fungus, or even a patch of rust: here a fence speckled in autumnal reds and yellows, there some orange lace spread on a stone. Behind my house I found a fallen tree carved with a set of leathery lichenous steps.

There are an estimated 20,000 lichen species, living on every continent in practically every environment that supports life. In the Ozarks and elsewhere, a handful of biologists now see lichens as sources of unique chemical compounds and as sentinels of environmental change—and also as enduring biological puzzles.

“We’re still in our infancy in understanding the lichen biota of the world,” says botanist Doug Ladd, who has spent the last 15 years tramping through the woods with colleagues from the New York Botanical Garden and The Nature Conservancy to assess the array. Their area of interest includes my local hotbed of lichen activity: the Ozark Plateau, which covers most of Missouri, the northern third of Arkansas, the eastern edge of Oklahoma, a tiny corner of Kansas, and a nibble of Illinois.

Ladd and company have visited every county in that region. They have searched sunny patches of rock and shaded bits of forest, weathered fence posts and sheltered caves, grassy glades and the faces of cliffs. Some lichens they have uncovered are smaller than grains of sand. When they come across a crusty specimen, they use chisels or knives to cut a sample of the lichen and its substrate, sealing it all in a paper bag. Sometimes, Ladd says, a particularly interesting find—say, Phoebus hydrophobias, a bright orange lichen that he describes as a “mad-dog sunburst”—elicits a triumphant whoop from the team.

“For diversity, the Ozarks blow the rest of the United States away,” Ladd says. The region supports about 30 species of crayfish, including one that is blind and lives in caves; the Ozark hellbender, one of the world’s largest salamanders, which can attain a length of nearly two feet; the Ozark crocus, found mostly in the Current River Basin; and nearly 150 distinct species of tree. Within the dominant oak-hickory woodlands, one finds a diversity of habitats, from rocky glades inhabited by roadrunners and cacti to wetlands with highly mineralized groundwater—all welcoming spots for lichens to settle in myriad colors and forms. The local diversity and unique geologic history (covered by neither glaciers nor oceans for the past 225 million years, the Ozarks provided refuge for migrating species during the Ice Age) explain the richness of the lichens here: some 600 named species, along with 30 recently discovered ones awaiting their official designation.

Yet for all the diversity of the Ozarks, the region’s natural habitats pale in comparison with the abundance of the past, when more than 4 million acres were covered with old pines, some more than 100 feet tall and almost four feet thick. The forests vanished in the late 19th and early 20th centuries with the settlement of the relatively treeless Great Plains, where houses and fences were constructed largely from those once-grand woods. The Ozarks have continued to change as agriculture and grazing decimate stands of giant cane, impacting animals from swamp rabbits to warblers. Although past scientific ignorance of lichens makes it difficult to determine whether they have been impacted as well, at least one species, Umbilicaria mammulata, has not been seen in the region since the 1920s.

The story of vanishing biomes is now all too familiar, but lichens can issue a warning before destruction occurs; they can also signal an environment on the mend. Many lichen species are exquisitely sensitive to air pollution and other environmental changes, so their absence, presence, or health may be able to tell us what is happening to an ecosystem. In fact, lichens are enlisted in North America and Europe as one of the fastest and cheapest means by which to assess pollution levels. On the other hand, the hardy lichen partnership between fungi and photosynthetic microbes has withstood the vicissitudes of time. Scientists believe lichens were among the first living things to try their luck on dry land some 600 million years ago, 200 million years before the first plants arrived. Lichens are often among the first green things to colonize a harsh territory (as happens frequently after volcanic eruptions in dry climates), and hardy species pioneer damaged environments as soon as conditions allow. During certain seasons in the Arctic, caribou survive by eating reindeer lichens. Other varieties of lichen provide more than 90 percent of the winter and spring diet for northern flying squirrels in parts of Oregon and Idaho. The Northern Parula, a warbler common in the Ozarks, depends on old man’s beard to build its nests.

The significance of lichens probably runs even deeper than these scattered examples suggest. More than 600 organic compounds isolated in lichens have been found nowhere else in nature. Lichen-derived substances have been used as antibiotics and other drugs. Some lichens produce toxins that might serve as natural pesticides; one genus, Vulpicida, was once used in Scandinavia to poison wolves. Although they make up a small fraction of the Ozarks’ biomass, lichens hold a disproportionate number of its nutrients. That is partly explained by the cyanobacteria, which convert atmospheric nitrogen to the nitrate building blocks essential for plant growth and the overall food chain.

Some lichens release a chemical that breaks rock down, helping produce fresh soil. Some can survive drought for more than a year, siphoning moisture and nutrients from the air while clinging to bare rock, vertical expanses of tree bark, desert sand dunes, and even plastic, glass, and decaying cars. Vagrant lichens live loose on the ground, rolling freely with the wind. Other species adhere to specific rocks; Phoebus hydrophobias forms orange patches the size of silver dollars under ledges of dolomite. “You can often tell what kind of rock you’re looking at by the lichens,” Ladd says.

And lichens are forever, or just about. They can live hundreds, even thousands of years, and sometimes they grow so slowly they may advance only a few milli­meters a century—an inch or two per millennium. Scientists can therefore use lichens to date ancient rock slides and gauge the pace of retreating glaciers. These techniques form the basis of a little-known way of reckoning time called lichenometry.

It took a while for me to grasp the full significance of my surreal tour through the Ozark woods. Lichens are a part of almost every wilderness scene, hiding in plain sight. Because certain species favor the lee side of a tree, you can sometimes change the color of a forest just by shifting the angle of your gaze. If you look at a landscape dominated by a bluff, you may still be seeing the colors of lichens just as surely as you do in the glimmering trees. Lichens cover the land like paint.

I thought I was taking in the subtle hues of the cliffs along the White River, but I was really admiring the layer of life covering them. Lichens stipple landscapes of granite, blanket the ground like snow, and drip from tree branches as if they were tinsel. Chances are, wherever you look, lichens color your natural world too.

Next Page
1 of 2
Comment on this article
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT
ADVERTISEMENT
Collapse bottom bar
DSCOctCover
+

Log in to your account

X
Email address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »