Lichens: that greenish stuff on your tree bark

Laura Bailey
Natural Resources Educator & Northwest Regional Director of the Master Forest Owner (MFO) Program

Have you been working in your yard or woodlot more lately and noticed that many of your trees have a greenish, fungus-like “something” growing on their bark? And you’re wondering what is going on with your tree? Is it unhealthy, does it have a disease, or is that stuff a type of moss? Well chances are, that “green stuff” has been on the bark for a while, and you’ve just discovered the interesting world of lichens for the first time. So let me fill you in on the basics of what you’ve been missing out on!

Laura Bailey – Cornell Cooperative Extension of Yates County Natural Resources Educator & Northwest Regional Director of the Master Forest Owner (MFO) Program

Lichens are a complex life form that is a symbiotic partnership between two separate organisms: a fungus and an alga, which interact somewhere along the line between a mutualistic (both organisms benefit) and parasitic (one organism benefits and the other is harmed) relationship, tending more towards parasitism. Ecologically speaking, the dominant partner in this relationship is the fungus, giving the lichen its most notable appearance characteristics. The alga species in this partnership can be either a green alga or a blue-green alga, also known as cyanobacteria. Many lichens have both types of algae present and based on the color of the lichen, you can usually tell which is prominent. Identifying them at the species level is a bit more complex yet, as there are at least 13,000 species of lichens throughout the world: 3,600 of which are in North America, and there are likely many species still undiscovered. 

When a lichen is dry, its color is usually gray but when it is wet, algal cells show their colors. Green algae generally give the lichen a bright green color and cyanobacteria display a dark green, brown-to-orange, or black color. Are you seeing something similar to this but more gelatinous? That’s known as a jelly lichen, where there are no distinct layers of the fungus and alga, everything is mixed together into a uniform layer the results in a gelatinous growth form. Some combinations of lichens are more commonly found on tree bark, whereas others you might encounter more often on rocks or other surfaces.

Lichens are a complex life form that is a symbiotic partnership between two separate organisms: a fungus and an alga.

So, now that you are a bit more familiar with what a lichen is, I’m sure you’re still wondering if it is harmful to your tree or the other surfaces it is found on, or possibly even to yourself. Lichens are a very important component of the environment and the ecosystems they are a part of. They help create soil, provide food, act as indicators of air pollution, some are being researched for their antibiotic properties for use in pharmaceuticals. Lichens provide shelter, building materials, and forage for wildlife. Some bird species use lichens as nesting materials and some insects have adapted their appearance to camouflage with the lichens in their habitat. Humans use lichens for dyes, decoration, and an ingredient in products such as deodorant and toothpaste. Although some species of lichens are edible, others can be poisonous if ingested, and some individuals may have allergic reactions to lichens. 

Tree-dwelling lichens are often found on tree trunks, branches and twigs as the bark provides a stable residence and access to sunlight, rainwater and needed components from the air. They can be found growing on healthy trees, as well as unhealthy or stressed trees. Noticing lichens on the bark of trees or shrubs often causes concern that the tree might be experiencing harm due to their presence or that they indicate poor tree health. Although lichens can be more plentiful on stressed or older trees, the lichens are not a cause of tree stress or decline. One of the primary reasons you might see more lichens on a stressed tree, is due to more availability of sunlight as the trees canopy starts to thin. Additionally, the bark of older trees is not sloughed off with new growth as it is on many younger tree species and in general, older bark tends to have more furrows, cracks and uneven surfaces for lichens to attach to.  

If you are noticing that there seems to be an abundance of lichens present on your tree and overall its health appears poor or declining, lichens are not the cause, but they might prompt you to inquire more about the health of your tree. If you have additional questions about lichens or tree health, contact Laura Bailey, Natural Resources Educator for CCE Yates and NW Regional Director of the MFO Program at or call the office at (315) 536-5123. 

For a more in-depth look at lichens, the U.S. Forest Service has a very informative webpage: