From our Naturalist, Rosetta
Lichens can be found in every terrestrial habitat on virtually every substrate available: trees, moss, soil, rocks, and human-made substrates.
There are at least 18,000 identified species of lichen worldwide and about 3,600 in North America. The numbers continue to increase as new discoveries are made. Lichens are classified based on the fungus and fungal features.
How to Recognize Lichens
It is generally agreed that, based on their structure, there are three main types of lichens: foliose, fruticose, and crustose. A fourth type, squamulose, is not always recognized as a unique form.
Foliose lichens look like a leaf. They can be very flat, leafy like lettuce, or convoluted and full of ridges and bumps. Their top and bottom sides are easily distinguishable.
Fruticose lichens have a branched structure and look like a little bush. They can upright and shrubby or cup-like.
Crustose lichens form a crust pressed over a surface, such as a rock, the soil, or even roof shingles. They can be brightly colored like yellow, orange, and red, or dull, such as gray and green.
Squamulose lichens are often described as an intermediate between crustose and foliose lichens. They have small tightly clustered scale-like lobes like roof shingles.
When lichens are wet, they "turn on" and start photosynthesizing and growing. When dry, they "turn off", become brittle and go dormant. The simplest way to tell if lichen is dormant or growing is by looking at its color. If lichen is a dark black or bright green, chances are that it is photosynthesizing. If it is wet and pliable, that is a good indication too. If the lichen looks pale and is dry and brittle, it is dormant and will not recommence photosynthesis until the next rain or fog event.
The Importance of Lichens
Lichens provide habitat, food, and shelter for other organisms, and nest materials for birds.
Many types of lichen are pioneer species on bare rock: the rock is broken down by acids that the lichen produces in a process chemical weathering. A soil base of the broken down rock and decomposing lichen is formed on the surface, providing a substrate for mosses and vascular plants to grow.
Lichens play an important in the nitrogen nutrient cycle: they take nitrogen from the air and convert it into a form that plants can use.
Lichens are useful in environmental monitoring. Because different lichens have varying levels of tolerance to air pollution, their presence or absence in an area are good indicators of air quality.
Humans use lichens for food, natural dyes, medicinal uses, and décor.
The next time you see lichens on trees, soil, rocks, or anywhere else they grow, don’t remove them. They are doing their jobs in our ecosystems!
Our coverage of lichens barely scratches the surface for more detailed information, a good place to start is the USDA Forest Service web page on Lichen Biology (https://www.fs.fed.us/wildflowers/beauty/lichens/biology.shtml) and the National Lichens and Air Quality Database and Clearinghouse (http://gis.nacse.org/lichenair)