Lichen on Tree Bark - indicates good air quality

Leafy-looking lichens on tree bark, photo from NZ.

Have you wondered about turquoise or pale gray-green, leafy circles on your tree trunks? These are lichens. 

CrabappleLandscapExperts know that lichens only grow where the air is relatively pollution-free, and their presence is a good sign—an indication that the air in your neighborhood is pretty clean! When an area is polluted, lichens will become scarce and eventually stop growing.

What are lichens?

Not a moss, not a mushroom, lichens are dual organisms, composed of two species living together in a mutually beneficial symbiosis.

One of the species is a fungus, which adheres to a substrate and absorbs water and mineral nutrient for the algae. The other species is an algae or a cyanobacterium (blue-green bacteria)  (or occasionally all three together) which uses water and carbon dioxide together with the energy of sunlight to photosynthesize and produce food for the dual organism. Quoting Trevor Goward, "Lichens are a case of “fungi that have discovered agriculture”.

Lichen circled in photo by PA-CES

What do Lichens look like?

Lichens are often turquoise or pale gray-green having a vegetative, leafy-like organ that grows outwards in a circle and is called a thallus. They attach themselves to undisturbed surfaces: Lichens often grow on unfinished wood surfaces, boulders or tree bark. Lichens are often viewed as making a tree more picturesque. Most people don't mind having lichens on their tree's bark because they can add color, contrast, texture, and natural beauty to the landscape. There is no reason to remove them, and the tree bark could be damaged in the attempt.

When the Japanese Garden of the Atlanta Botanical Garden was refurbished, disturbed lichens growing on the carefully-selected and -placed boulders were skillfully "touched up" with latex paint to preserve the aesthetics. 

Rugged bark with lichens, photo from WikiCommons

Lichens cause no damage

Because they make their own food with sunlight and water, lichens are not parasitic and pose zero threat to the health of the tree, or to you and your family.

Best Conditions for Lichens?

Lichens are found on every continent including the Arctic and Antarctica. Lichens on trees grow best in cool, partly sunny, moist locations. Thinning out tree branches overhead to allow more sun and air flow will help, as will occasionally “watering” lichens with a sprinkler system.

Ecological Roles and Economic Benefits of Lichens

• Lichens act as early colonizers that re-establish life on bare surfaces like rock and boulders 
• Lichens help regulate the composition of gasses in the earth’s atmosphere. Each lichen acts as a “sponge” to catch and store excess gasses like carbon dioxide, thus filtering the air we breathe. 
• Some lichens (those in symbiosis with cyanobacteria) have the ability to return valuable nitrogen to the soil, somewhat like legumes changing atmospheric nitrogen to plant-available nitrites and nitrates 
• Source of dye for wool, fabric, textiles,  particularly craft dying 
• Antibiotic properties act as a healing aid to superficial wounds 
• Used fresh or dried in floral designs 
• Used as a bitter flavoring for beer in Siberian Monestaries, in fermented corn beverages in Mexico; for flavoring by Swedish brandy distillers 
• Caribou / reindeer graze on lichens in northern tundra regions

Your LandscapExpert is happy to answer questions about the natural world.  

Digging Deeper

10 Things To Know About Lichens http://ohioline.osu.edu/sc195/029.html

Lichen website http://www.lichen.com/

Even more about lichens http://www.lichen.com/biology.html