Greenshield Lichens

Rock Greenshield Lichens adorn the winter snowscape

Common Name: Rock Greenshield Lichen – The rosette shape is like a rounded shield and is greenish gray in color ― a green shield found almost exclusively on rocks. Lichen has an obscure etymology but may derive from the Greek word leichein which means “to lick” just as it sounds. There is no extant clue for this association as very few lichens are eaten (and thus licked). Some, like this species, have small lobes that could be a metaphor of sorts for little (leichein) tongues. The Common Greenshield Lichen is found mostly on trees.

Scientific Name: Flavoparmelia baltimorensisParmelia is Latin for shield,  the genus that was used broadly for all lichens that were shield shape until 1974 when it was subdivided. Flavo as a prefix means yellow, distinguishing these lichens from the blue tint of other shield lichens… yellow hues combine with blue so that the overall effect is green. This species was first classified from a Baltimore specimen giving rise to the familiar nomenclature.

Potpourri: The rock greenshield lichen and its virtually indistinguishable  cousin the common greenshield lichen (F. caperata) are encountered clinging to a substrate of  rock or wood while traipsing along almost any trail. In the winter months when  deciduous trees are devoid of greenery and mostly annual undergrowth has died back, only the grays and browns of rocks, dirt, leaf litter, and boles remain. The exceptions are the greenshield lichens that spread their leaflike (and tongue-like) lobes outward and onward, oblivious to the reduced light and frigid temperatures by which the rest of the forest is constrained. Their persistence is testimony to the lichen lifestyle, one of the natural world’s wonders. Comprised of a fungus that has partnered  with one or more organisms from a different kingdom, 14,000 identified lichens have mastered the art of survival in the most inhospitable of habitats from hot, dry desert to frozen tundra. They are even found on Mount Everest at elevations exceeding seven kilometers. [1]

According to the International Association of Lichenology, a lichen is “an association of a fungus and a photosynthetic symbiont resulting in a stable vegetative body.” The fungal partner is called the mycobiont and constitutes about 95 percent of the lichen body structure or thallus. Since fungi are heterotrophs and therefore cannot make their own food, they must rely on autotrophs that photosynthesize the sun’s energy to produce nutrients necessary for growth and reproduction. Some fungi consume dead plants as saprotrophs, some parasitize living organisms, and some connect to living plant roots in a mutually beneficial association called mycorrhizal (fungus root). Lichenized fungi evolved a relationship to photosynthesizing organisms that falls into the category of symbiosis, which is defined as an intimate relationship between two living things. The photosynthetic partner of the lichenized fungus is called the photobiont and can be either green, brown, golden algae or cyanobacteria, a type of bacteria that contains chlorophyll formerly called blue-green algae. Algae is now a broad non-technical name for several types of polyphyletic eukaryotes that photosynthesize, which is all that matters to the fungal partner. The photobiont for greenshield lichens is a green alga species in the genus Trebouxia, which is the most common photobiont for all lichens. [2]

The relationship between the fungus and the algae in a lichen is complex. Traditionally the symbiosis of lichens has been characterized as mutualism in which both partners benefit equally. In reality, the relationship frequently ranges from commensalism, where the fungus benefits but the algae do not, to outright parasitism, where the algae are harmed for the benefit of the fungus. Some insight into the living arrangements is afforded by the observation that the lichen’s fungi need the algae but not vice versa. That is to say that none of the lichen forming fungi, comprising almost half of ascomycetes, the largest division of the Fungi Kingdom (mushroom are in the other large division – the basidiomycetes), exist in nature without algae, whereas the algae can and do lead independent lives on their own. However, having a place to live with enough water and air for photosynthesis to make carbohydrates and respiration to oxidize them for energy (both plants and fungi need to breathe) is certainly an algal advantage. It is at the cellular level that the controlling dominance of the fungus can become sinister. The root-like tendrils of the fungus called hyphae surround and penetrate the algal cells, releasing chemicals that weaken the surrounding membrane so that the carbohydrates leak out, feeding the fungus. Weaker algal cells thus violated die, and were it not for periodic reproduction, so too would the lichen. [3] A lichen has been described as a fungus that discovered agriculture, an apt aphorism. The fungus uses the algae for subsistence in like manner to a farmer tending fields to extract their bounty ― it would be nonsensical to assert that farmers and soybeans therefore benefit mutually in symbiosis.

Lichen reproduction is also complicated, as it involves two different species that must reproduce independently and then come into close contact to form a union. While this union must have occurred at least once for any lichen to exist, a singular rare event in the millions of years of geologic time is not unusual. The mycobiont, in this case Flavoparmelia baltimorensis, produces reproductive spores in a fruiting body called an apothecia in a manner analogous to the gills of mushroom fruiting bodies. The photobiont, in this case Trebouxia, also reproduces using spores when it is independent of the fungus, but only reproduces asexually once lichenized. Apothecia are very rarely seen on greenshield lichens, direct evidence that, like most lichens, they have no pressing need for reproductive spores.  Since they are abundantly distributed and can on occasion cover vast swaths of boulder fields (F. baltimorensis) and exposed wood surfaces (F. caperata), it is evident that there is a successful reproductive workaround. In general, this consists of a lichen forming a detachable unit that includes both the fungus and its algal partner for windborne distribution to new locations. These “lichen seed packets” take various forms including soredia that are miniscule balls of fungal hyphae surrounding a few algal cells and schizidia, which are simply flakes of the upper layer of the fungal thallus which also contains the algal layer. One of the ways to tell rock and common greenshield lichens apart is that F. baltimorensis has schizidia and F. caperata has soredia. However, identifying small irregular components on the gnarled surface of a lichen is a challenge even for a lichenologist with a lens. It is much easier to identify a rock or on a tree and look for lichens.

Greenshield lichens often cover broad expanses of rock and tree surfaces to the extent that long term effects come into question. Do lichen covered rocks disintegrate at an accelerated rate? Do trees weaken due to the amount of bark covered by lichens? For the most part, lichens are self-sustaining in the sense that the heterotrophic fungus is supplied nutrients from autotrophic algae. While sunlight and water are the essential ingredients for photosynthesis, nitrogen, phosphorous and potassium are also required for plant growth (the three numbers on a fertilizer bag refer to these elements).  It is less well known that fungi need these same nutrients for the same metabolic reasons. [5] In many cases, lichens are able to get all of the nutrients they need from minute amounts dissolved in water. The quality of precipitated rainwater is why lichens are useful for environmental monitoring as their growth correlates to air quality. The two main substrate characteristics associated with lichen growth are moisture retention and exposure to sunlight. For lichens growing on exposed tree bark, the degree to which moisture is retained as it flows down the tree is the key factor. While it is true that the lichen will “rob” some of the nutrients that would otherwise go to the tree roots, the amount is negligible. Deciduous trees have more lichens than conifers because their leafless trunks are sunlit for six months of the year whereas evergreens are ever shaded. Rocks are not good at retaining moisture. Consequently, lichen hyphae penetrate rock surfaces to depths of several millimeters seeking water, and, depending on the type of rock, minerals as well. This contributes to the long-term weathering of rocks for soil formation, and more broadly to the million-year geologic cycle of mountain building and erosion. The answers to the two questions are yes, lichens do disintegrate rocks at a geologic rate, and no, lichens do not harm trees ― they are sometimes called epiphytes for this reason.

Common Greenshield Lichens do not harm the trees on which they grow

Chemistry is another important aspect of lichen physiology. More than 600 unique compounds are concocted by lichens in surprisingly large quantities … up to five percent of total bodyweight. It is instructive to note that when lichenized fungi are artificially grown without algae in a laboratory, chemical output is negligible. This can only mean that specific chemicals promote the associative nature of the individual lichen species.  There are any number of hypotheses that might explain this. Bitterness as deterrence to animal browse is certainly one possibility, as lichens grow quite slowly on exposed surfaces and are easy to spot. However, some lichens, notably reindeer moss (Cladinia rangiferina), are a major food source for animals and are quite likely propagated in their droppings. It is also believed that some chemicals act to coat sections of hyphae to provide air pockets necessary for photosynthesis by the algae. The chemical footprint of a lichen species is one of the main diagnostic tools used in field identification. Lye, bleach and several other reagents are dripped onto the surface; a change in color indicates the presence of a specific chemical that is related to a specific lichen. [4] There are many unknown aspects of lichen physiology. This was made manifest recently when it was discovered that many lichens contain a type of basidiomycete yeast (also a fungus), which is embedded in the body of the ascomycete fungus in varying concentrations that correlate to anatomical differences. Some if not all lichens may actually consist of two fungi and an alga or two, a far cry from simple symbiosis. [6] The function of yeast fungi is not yet known.

The Flavoparmelia genus was separated from the other Parmelia (shield) lichens in 1986 in part due to their production of the chemical compound usnic acid. [7] It is a large molecule with the formula C18H16O7 which simplifies the recondite but recognized international IUPAC standard 2,6-Diacetyl-7,9-dihydroxy-8,9b-dimethyldibenzo[b,d]furan-1,3(2H,9bH)-dione. Usnic acid is found primarily in the top layer of the fungus along with another chemical called altranorin just above the area where the algal bodies are concentrated. It is surmised that they contribute to shielding  green algae from  excessive sunlight exposure since bright sun is inimical to photosynthesis, the source of all lichen energy. Usnic acid is also a potent antibiotic, collected primarily from Usnea or beard lichens due to higher concentration for use as an additive in commercial creams and ointments. Flavoparmelia caperata is one of several lichens that have historically been used by indigenous peoples as a tonic taken internally or as a poultice applied to a wound. [8] The medicinal uses of lichen fungi should come as no surprise, as many polypore type fungi growing as brackets on tree trunks have been used medicinally for millennia. The abundance of rock and common greenshield lichens is evidence of successful adaptation. In addition to thriving on bountiful rock and wood surfaces, the chemical shield screens sunlight to protect the green algal energy source and guard against assault by microbes and mammals.  In other words, they are literally green shields.  

Carl Linnaeus assigned lichens to the class Cryptogamia meaning “secret life” along with everything else that created spores and not seeds. [9]  One of the more enduring lichen secrets is how and when the coalition between fungi and algae began. It is widely accepted that simple replicating organisms started out in aqueous habitats, as water affords bodily support and nutrient transport. The transition from sea to shore would have been nearly impossible for an alga with no structure or a fungus with no food. There is good reason to suppose that some form of union like a lichen may have come about by chance and was then promoted by survival.  Scientific research over the last several decades has cast some light into the dark shadows of this distant past. What look like lichen hyphae embedded in the soil around fossils from the pre-Cambrian or Ediacaran Period (635-541 million years ago) suggest that lichens may have been the first pioneers on dry land. [10] This is supported by the finding that marine sediments from this same period contain not only the root-like hyphae of fungi but also the rounded shapes of blue green algae or cyanobacteria. This suggests that something lichen-like started out in the water was left high and dry in a tidal flat to make the critical transition. [11] However, recent DNA analysis of primitive ferns and lichenized fungi revealed that the lichens evolved 100 million years after vascular plants.[12]  Lichenology, like all science, is a continuum that never ceases in its quest for knowledge. Future field tests and experiments are certain to clarify the origin story.


1. Kendrick, B. The Fifth Kingdom, 3rd edition, Focus Publishing, Newburyport, Massachusetts, 2000, pp 118-125.

2. Brodo, I., Sharnoff, Steven and Sylvia. Lichens of North America Yale University Press, New Haven, Connecticut, 2001. pp 1-112, 316-317, 479-484.

3. Wilson, C. and Loomis, W. Botany, 4th edition, Holt, Rinehart, and Winston, New York, 1967, pp 451-453.

4. Brodo, op. cit.

5. Kendrick, op. cit. pp 142-158.

6. Spribille, T. et al “Basidiomycete yeasts in the cortex of ascomycete macrolichens” Science Volume 353 Issue 6298, 21 July 2016, pp 488-492.

7. Hale, M.  1986. “Flavoparmelia, a new genus in the lichen family Parmeliaceae (Ascomycotina)”. Mycotaxon. 25 (2): April-June 1986.pp 603–605

8. Brodo, op. cit.

9. Linnaeus, C. Species Plantarum. Vol. 2. Stockholm: Impensis Laurentii Salvii.1753. p. 1142

10. Frasier, J. “Were Weirdo Ediacarans Really Lichens, Fungi, and Slime Molds?” Scientific American 13 December 2012.

11. Yuan, X. et al “Lichen-Like Symbiosis 600 Million Years Ago” Science  Volume 308, Issue 5724, 13 May 2005, pp 1017-1020

12. Frederick, E. “Hardy lichens don’t actually predate plants” Science 20 November 2019