Common greenshield lichen is a medium to large, green foliose lichen. Common and widespread in eastern North America, it grows on the bark of hardwood trees.
Looking more closely at this species, you’ll notice fluffy or dusty-looking soredia in irregularly shaped patches (soralia) scattered on the surface. The top surface of the lichen is rather dull-looking, smooth on the outer, younger parts and wrinkled in the central, older parts.
Greenshield lichens (Flavoparmelia ssp.) are named for their yellowish-green color. The lobes are broad and rounded, the tips rather flat against the substrate. The undersurface is black, with a narrow zone of brown along the margins. Like other foliose lichens, greenshield lichens grow outward from a central point and have a branching, lobed structure.
A lichen is an organism that results when a fungus species and an algae species join together. Although the relationship between the fungi and algae is quite intimate and integrated, the lichen that is formed does not much resemble either of the components. Learn more about lichens on their group page.
Similar species: Missouri has two other species of Flavoparmelia:
- The rock greenshield lichen (F. baltimorensis) looks very similar, but it grows on rocks (particularly sandstone), and only rarely on trees; also, instead of fluffy or dusty-looking soredia in irregularly shaped patches scattered on the surface, it bears coarse, isidia-like pustulae, and therefore it has an overall shinier, less-dull appearance.
- Another flavoparmelia lichen, F. rutidota, is characteristic of Missouri’s dry limestone or dolomite cliff faces; listed as a Missouri species of conservation concern, this lichen grows on ancient red cedar trees clinging to cliff faces at Vilander Bluff Natural Area in Crawford County. Missouri's populations represent the northeastern extent of its North American range. It does not form soralia, isidia, or pustulae, but it commonly forms apothecia cups.
For the record, there are plenty of other large, round foliose shield lichens in Missouri. We can't cover them all in this introductory field guide. But keep in mind there are many lookalikes. Flavoparmelia species were formerly considered to be part of genus Parmelia, but many in that genus have been split off into separate genera. Most of these new genera remain in the same family, the Parmeliaceae; most are called "shield lichens." Among others in this family, Missouri also has:
- 2 species in genus Parmelia,
- 3 in Canoparmelia,
- 1 in Flavopunctelia (F. soredica, the powder-edged speckled greenshield),
- 2 in Hypotrachyna,
- 2 in Parmelinopsis,
- 12 in Parmotrema (called ruffle lichens),
- 6 in Punctelia (the speckled shield lichens), and
- 12 in Xanthoparmelia (green rockshield lichens).
Oddly enough, the beard lichens (Usnea spp.) are in this same family, but with their shrubby-looking, fruticose growth habit, they look very different.
Diameter: typically to about 2 to 5 inches; 7 inches is about the maximum. Lichens begin as tiny spots, but groups of them may grow to cover several square inches.
Statewide.
Habitat and Conservation
Habitat can be very helpful for identifying lichens, and this is a great example. The common greenshield lichen looks very much like the closely related rock greenshield lichen, but the latter, true to its name, grows on rocks (only occasionally on tree bases), while the common greenshield lichen is almost always found on tree trunks and large tree limbs (almost never on rocks). You may sometimes find it growing on fence posts or other wooden substrates.
Status
Of Missouri’s 3 species of greenshield lichens, the common greenshield and rock greenshield lichens are both common. The third, F. rutidota, is listed as a Missouri species of conservation concern. Missouri’s lichen populations deserve broader and deeper study.
Even though lichens may cover large areas of trees, they are not parasites and do not damage trees.
Life Cycle
Lichens can reproduce sexually (via spores) or vegetatively (asexually).
Sexual reproduction, via cuplike structures called apothecia, which give off spores, is not very common in this genus, so you are not likely to see many cup or bowl-shaped apothecia in most greenshield lichens.
Vegetative reproduction, via tiny packets of the lichen that break away easily, is common in this genus. Some species produce soredia and some produce isidia (or isidia-like pustulae). A lichen species generally does not produce both types, so being able to distinguish between soredia and isidia helps with identification.
The common greenshield produces soredia, which are tiny packets of the fungus and algae that can begin a new lichen elsewhere. The soredia look fluffy or dusty. In this species, the soredia form initially in round structures (soralia), but these usually become so numerous that large patches of the lichen seem covered.
This lichen can also reproduce when portions of the lichen get broken off and are transported to a new location.
Human Connections
Lichens contribute to a beautiful landscape. When you start paying attention to them, you realize they are all around us, and that the woods would look barren without lichens decorating the tree trunks. Something would seem awfully wrong if lichens were to disappear.
When lichens do disappear, it’s a sign something is indeed wrong. Because lichens, with their soft, absorbent tissues, are extremely sensitive to air pollution, they are good indicators of air quality. In parts of the United States, many species started to disappear due to acid rain. Government regulations to protect the environmental helped the lichens to come back, signaling a healthier landscape for all of us. Scientists use lichens as sentinels of environmental health.
The genus name Flavoparmelia means greenshield. “Parma” was the name of a type of small shield used by Roman soldiers (they named it after the Italian city of Parma, well-known for its cheese and ham, both called Parmesan). The Romans had special words for their various types of shields, and many of these words endure in scientific terms. For fun, here are a few more:
- One shield used by the Romans was the large, round clipeus, which is now the term for a certain plate on the face of insects (male carpenter bees, for example, have a white clypeus, and the clypeus of a cicada looks like the grille of a 1940 Chevy truck).
- The Romans’ large shield, called a scutum, has come to us in many ways, such as in the word scute (one of the plates that make up the shell of a turtle) and scutellum, a certain plate on the backs of true bugs, flies, and other insects (in stink bugs, it’s the big triangular plate in the middle of the back).
Ecosystem Connections
Many animals have markings and coloration that helps them blend in with the mottled background of lichen-covered bark or rock. Examples include the backs of birds such as yellow-bellied sapsuckers and brown creepers, and gray treefrogs. Lichen moth caterpillars (such as the painted lichen moth and the black-and-yellow lichen moth) feed on lichens, and they are camouflaged to blend in perfectly with lichen-covered bark.
Foliose lichens in the genera Flavoparmelia and Parmelia are some of the favorite lichens used by birds to camouflage their nests. Among the birds that use lichens in nest-building are ruby-throated hummingbirds, eastern wood-pewees, and many types of warblers and vireos. Once decorated with lichens, the nest, perched on a tree limb, looks just like a lumpy old knot on the limb.
In some places, after population declines, the northern parula warbler’s numbers increased dramatically in the decades after the Clean Air Act was established in the 1970s: Many lichens are sensitive to air pollution and disappear when air quality is diminished. Apparently, the cleaner air allowed lichen populations to rebound. The parula relies on abundant lichens with which to camouflage its nests, so the rebound of lichens apparently led to the rebound of the sweet-singing little yellow-breasted warbler.
Mosses, liverworts, hornworts, and lichens seem rather similar, but these organisms are in very different groups. Mosses, liverworts, and hornworts are small, low plants usually found in damp habitats. Unlike more familiar plants, they lack veinlike structures and do not produce flowers or seeds — instead, they produce spores. Meanwhile, lichens are not plants at all: they are a collection of different fungi that have photosynthetic algae living within their tissues.