Lung lichen grows on trees and rocks around Maine. It's large and leaf-like, and appears brighter green when wet. Credit: Courtesy of Grace Bartlett

Lichens are hidden in plain sight, an overlooked and under-recognized organism. I used to miss them until I was introduced to them in the Maine Master Naturalist Program. Now I see them almost everywhere. They are on trees, rocks, along the edge of trails, on rotting logs — almost everywhere, except in water.

Lung lichen is one of my favorite lichens. It’s also known as lungwort or tree lungwort. When I go for a walk in the forest among mature broadleaf trees and conifers, I start looking for this large, leafy lichen, which grows on tree trunks and mossy rocks.

Its body or thallus can grow to about 11 inches in length. On an average day, the upper surface of this body, known as the cortex, is a pale green. During dry spells, it can become brown and brittle. In either case, unless you are intentionally looking for it, this lichen easily blends into the “woodwork.”

However, on a damp, foggy, rainy or snowy day with moisture in the air, lungworts really stand out. They become as bright green as new grass. This significant color shift is due to the cortex becoming translucent when wet. When that happens, it reveals the green chlorophyll of its alga partner, which is sandwiched between the upper and lower fungal cortex.

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In addition to its large size and distinct color on wet days, lung lichens are leafy looking. The lungwort looks almost like subdued curly kale or green leaf lettuce. Because of their leaf-like appearance, lung lichens are categorized as having a foliose form.

Lungwort have distinctive reticulate ridges, with hollows or depressions between the ridges. The lower surface or cortex is tan and smooth, except for the underside of the reticulate ridges which are matted with short wooly hairs. Also on the lower cortex are dark colored pockets of cyanobacteria. As you look closely at the leafy lobes, you will notice that toward the tips they are divided, much like tree twigs diverge in two or more directions.

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Lichens aren’t plants, nor are they animals. They are fungi that exist in a symbiotic relationship with algae or cyanobacteria, or both. Some scientists think these relationships are mutual partnerships, while others think the fungi are dominant and enslave the algal partners.

Each lichen symbiotic team consists of one or more mycobiont (fungus) and one or more photobiont (algae and/or cyanobacteria). The fungus produces the thallus (body) and protects the algal partner from sunburn and desiccation. But the fungus cannot make its own food. The photosynthetic partner(s) makes the food for the fungus and itself by using sunlight, water and carbon dioxide to create carbohydrates (sugars).

In the case of the lungwort, there are three teammates in this symbiotic relationship: a fungus, a green algae and cyanobacteria. The green algae and the cyanobacteria are both photobionts that make food through photosynthesis. The cyanobacteria also “fixes” atmospheric nitrogen so that unusable atmospheric nitrogen becomes usable to the lichen. Therefore, if the lichen is blown off the tree and decomposes, nitrogen will be added to the soil and promote plant growth. Also, the symbiotic team that makes up lungworts, like all lichens, releases oxygen as a waste product of photosynthesis, which is helpful to humans.

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Lungworts are food for deer, moose and insects, plus they provide nesting materials for chipmunks and birds. They have been recognized as a pollution indicator, as they do not grow in polluted areas. So, if you find one in a mature, old growth forest, then you know you can breathe easy. You are breathing clean air.

The scientific name for lungwort is Lobaria pulmonaria. Lobaria indicates that lungwort is structurally like other lichens in the Lobaria genus. Pulmonaria indicates that it is unique in the Lobaria genus. The word, pulmonaria comes from the Latin root “pulmo” for “lung,” hence the common name lungwort or lung lichen.

Even without being pollution indicators, lungwort are remarkable organisms. They are always in a symbiotic, team relationship, and they are standouts, especially on moist, damp days. They brighten my day in the old growth forests and hopefully they will do the same for you.

Grace Bartlett is a Maine Master Naturalist who volunteers with Bangor Land Trust, the Orono Bog Boardwalk and Hirundo Wildlife Refuge.