Why We Like Lichens: Nature's Air Quality Monitors

What's Up, Forsyth? For this next post, we’re not just thinking about what's “up,” but what's all around us! Have you ever seen one of these before? 

…or one of these?

…or, perhaps, something like this?

These specimens may look a little like a plant (think moss) or a fungus (think mushroom), but they're something different altogether—they’re lichens!

A lichen is an organism that forms when a fungus, alga, and/or cyanobacterium enter into a special relationship called a symbiosis. The photosynthetic partner (photobiont) in the relationship—either a type of algae or cyanobacteria—produces food for the entire organism. The fungal partner (mycobiont) provides a structure/home for the photobiont (and sometimes microorganisms like tardigrades), which protects it from harsh conditions. Because the mycobiont relies on the photobiont for food, lichenologist Trevor Goward has described lichens as "fungi that have discovered agriculture."

Lichens can grow almost anywhere but are often spotted on trees, wood, rocks, or even manmade materials to anchor themselves. A lot of times, the discolorations or differently textured areas that we see on a tree’s bark are actually lichens (such as in the second photo). They do not have true roots, stems, or leaves, and they get all their nutrients and water intake from the air. They can survive in harsh environments, from the Antarctic, deserts, coastal waters, and even Outer Space! Still, the lichens we have in North Carolina generally prefer humid, damper, cooler environments. (They are often found on the north side of a tree because they prefer shadier, moister environments with less direct sunlight).  

It is estimated that around 20% of fungal species live as lichens, and the vast majority these species (~98%) are from the phylum Ascomycota (sac fungi). The others are from Basidiomycota (club fungi). With there being 18k+ species of lichen worldwide, many of which must be brought back to the lab to be properly identified, lichen identification is often simplified to recognizing the 3 main growth forms. In order of the photos shown above, the first type is fruticose or "bushy", the second is crustose or "crusty" and the third is foliose or "leafy".

Fruticose (Bushy)-bushy lichens are three-dimensional and have a hairy or bushy-like appearance; only a small part of the thallus (body) of the lichen is touching or is attached to a substrate, which can be easily separated or broken off.

Crustose (Crusty)- crusty lichens create crusts and are completely attached to the substrate (thallus cannot be separated).

Foliose (Leafy)- leafy lichens have two sides and are flat and leafy looking; sometimes they are full of ridges and bumps; the thallus is partially attached to the substrate but not as easily separated as a bushy lichen.

Lichens are somewhat paradoxical—hardy enough to survive in extreme climates, yet sensitive to air pollution. This is because they do not have roots or other barriers to pollution in their environment. Whatever is in the air is absorbed directly into the lichen and can affect it over short timeframes. This characteristic makes lichens excellent bioindicators or biomonitors of air quality. Lichens that are especially sensitive to air pollution (mainly nitrogen dioxide and sulfur dioxide) may experience reduced photosynthesis, bleaching, and even death. In North Carolina, roughly half of our nitrogen dioxide emissions are from on-road mobile sources (cars and trucks), and almost all sulfur dioxide emissions are from coal-fired power plants or other industrial (stationary) sources. While the sensitivity of lichens to air pollution is species-dependent, crusty lichens are usually more tolerant because less of their total surface area is exposed to the air. Leafy lichens are typically more sensitive to air pollution than crusty lichens, and bushy lichens (with the most surface area exposed) are often the most sensitive. If you live in North Carolina, you may have already noticed that bushy lichens are more common in forested areas that are further away from busy roads and industrial areas. 

We can monitor lichen populations and thus, air quality trends, over time by sampling the same trees year after year and calculating the ratios of bushy, leafy, and crusty lichens to moss and bare bark. Greater lichen diversity indicates that there is better air quality, since pollution-tolerant lichens will replace sensitive species overtime. Some nitrogen-loving species flourish in areas with higher nitrogen-based compounds (like nitrogen oxides and ammonia). If these species begin dominating an area, and the presence of nitrogen-sensitive lichens is diminishing, it could be an indication that air quality is worsening in that area. Additionally, studying lichen populations at varying intervals from a major source of air pollution could be helpful for understanding how far pollution has spread from its source.  One other way that lichens can be used as biomonitors is collecting and analyzing them in a lab for the presence of specific pollutants like mercury, lead, and sulfur. All these data can be compared with measurements from nearby air quality monitoring stations.

Aside from their role as air quality bioindicators, lichens have important ecosystem functions. They help clean the air that we breathe by trapping pollutants like particle pollution. They also can store up to 3360% of their dry weight in water, which reduces soil erosion and the sedimentation of waterways. Lichens are a source of food for deer, birds, and rodents, and bushy lichens make great nesting material for birds. You may have also noticed that decaying wood or logs can be lichen hotspots. This not because the lichens caused the decay or that they have detrimental effects on trees. On the contrary, lichens can actually help protect tree bark from harsh elements like wind, rain, and snow. The reason why you might see more lichens on logs or old wood is that as the material breaks down, it creates more cracks and crevices for the lichens to grow, especially for the bushy and leafy growth forms. Crusty lichens, on the other hand, grow well on smoother surfaces like rocks, concrete, and smooth-barked trees like beeches. 

Lichens growing alongside moss on a decaying log. All three growth forms, as well as moss, are shown. Image taken on December 14, 2024 at Haw River State Park by Sarah Coffey.

The NC Air Awareness Lichen or Not? page provides more information about lichens and how we can study them to learn more about air quality. This page includes access to an excellent citizen science lichen survey that can be completed by anyone and from anywhere in North Carolina. Submissions are checked by specialists with the North Carolina Department of Environmental Quality to learn more about lichen populations across our state.

If this blog post has piqued your interest and you’d like to learn more about these fascinating organisms, please contact Sarah Coffey with Triad Air Awareness at coffeyse@forsyth.cc. She can facilitate free programs on lichens and any other air quality topic for your class or education program in Forsyth County.

Sarah Coffey teaches about lichen during the Triad Air Awareness/City of Winston-Salem Recreation and Parks “Learn to Love Lichen” program at Gateway Nature Preserve on April 12, 2025. (Photo credit: Laura Tanyi with the City of Winston-Salem Recreation and Parks). 

 

Sarah Coffey showing participants a lichen sample during the Triad Air Awareness/City of Winston-Salem Recreation and Parks “Learn to Love Lichen” program at Gateway Nature Preserve on April 12, 2025. (Photo credit: Laura Tanyi with the City of Winston-Salem Recreation and Parks).