Lichens

February 2015

by Linda Martinson

“There is a low mist in the woods—it is a good day to study lichens.” 

Henry David Thoreau, A Year in Thoreau’s Journal: 1851

At Richland Ridge, lichens stand out vividly on cold, wet, foggy winter days, clustered on the trunks and leafless branches of hardwood trees. Lichens are hardy, and they flourish in the cold winter months when, with the leaves gone, there is more available light. When it is rainy, drizzly, foggy, or misty, lichens break dormancy and quickly absorb 3 to 35 times their weight in water from the air. 

When lichens stand out clearly on the trees, they are busy photosynthesizing the light while they can absorb water from the air. When the sun comes out again, the lichens go back into dormancy, shrink, and become brittle and less conspicuous. 

Linda Martinson photo

There are so many diverse lichen species that there are individual exceptions for every general statement made about them. There are three or four basic lichen body types with many individual variations: crusty, leaflike, shrubby (erect or pendulous), and scalelike. Often, most or all of these body types and several different species of lichens occur on almost any given substrate, such as the tree in the photo above taken at Richland Ridge in late January. Note: lichens do not harm trees in any way because they have no roots to penetrate bark.

Lichens are neither plant nor animal, but instead a unique mutualistic symbiosis of two or three organisms; in a sense, they form rather than grow. They are a composite of organisms from two or three different Kingdoms: fungi, algae, and sometimes cyanobacterium. Lichens have been described as fungi that have discovered agriculture, because they capture algae and embed them almost like slaves, usually locating them just below their outer layer of cells where the light can still reach them. The algae cells produce carbohydrates through photosynthesis, while the fungi structures provides protection and a supply of moisture and minerals. If the lichen composite also includes a cyanobacterium (about 20% of the species in the Blue Ridge do), it can also fix nitrogen from the air, making it even more valuable in the soil production cycle and in other natural processes.

Once they combine, both the fungus and the alga give up their individual identities and change forever. Together, as lichens, they can go where neither organism can go alone. Algae alone can live only where it is moist, and fungi alone can grow only where there is decomposing matter for food, usually underground. The lichen composite of fungi and algae can live almost anywhere and on many different substrates. In a laboratory, the components of a lichen can be separated and grown individually, but they change to totally different forms and cannot be recombined. 

There are at least 20,000 species of lichens found on every continent on Earth, in virtually every ecosystem, in all climates, and at every altitude. Each species of lichen has its own niche and specific requirements, but the basic requirements for survival are undisturbed surfaces, time (they are exceptionally slow growing), and clean air. Lichens reproduce either by having tiny parts of the lichen break away or by the fungal partner producing spores that float away, land near the appropriate algae cells, and capture them immediately. Lichen growth is so slow that reliable methods of dating surfaces have been developed based on the lichens growing there. Some lichens have been found that are over 9,000 years old.

Lichens are also bio-indicators of air pollution since they absorb water, minerals, and other elements directly from the atmosphere and over their entire surface area, all year long. Because they are so sensitive to air pollution, especially sulfur dioxide pollution, very few lichens flourish in urban and industrial areas. The abundance of lichens on the trees at Richland Ridge indicates that our air quality is very good, making them a cheerful sight on a wet winter day and well worth studying.