by Luisa Black
February 1, 2021
In the late winter, when the beauty of the land tends towards subtle, sparing gestures, the cling and spackle of lichens sing out to me as vivid as bells in the biting wind. I am always drawn to ancient and obscure organisms, but lichens, perhaps the most cryptic of the cryptogams, are of special interest to me because they so profoundly subvert the conventions through which we understand species, taxonomy, and even the boundaries between individuals and collectives. This is because lichens do not descend from a common ancestor and thus do not constitute a single taxonomic unit – kingdom, family, or species. Depending on your perspective, you could describe a lichen as a “fungus that has discovered agriculture,” a mutualistic symbiosis, a mild parasitism, or a miniature, semi-permeable ecosystem.
The relationships between the component species of lichens are famously obscure, and there is probably a kernel of truth in each interpretation. What we do know is that lichens are always comprised of one or more fungi (a mycobiont) and one or more species of algae and/or cyanobacteria (a photobiont). In the simplest terms, the photobiont’s key role in the lichen is to convert sunlight into carbohydrates, which the partnered fungi rely on for their energy. In turn, the mycobionts protect their photobiont partner(s) from direct sunlight and provide a protective tissue (thallus), housing the algae or cyanobacteria in environments where it could never otherwise survive.
Indeed, lichens thrive in environments where few organisms can survive: tundras, heaps of toxic slag, crusts over desert soil, and thin air. Lichens are a pioneer species, always among the first wave to colonize newly disturbed surfaces, creating pockets of viability for organisms inside and outside the fuzzy borders of their strange holobiomes.
Though lichens are beautifully resilient, they have, like all of us, a dimension of intense sensitivity. Lichens gather their nutrients directly from the air around them, which makes many of them fatally susceptible to atmospheric contamination, especially nitrous oxides, sulphur dioxides, and heavy metals. Incidentally, the human species shares an acute sensitivity to these same pollutants. By developing literacy in just a handful of lichens, you can learn how to quickly assess the air quality of a landscape and thus, its hospitability to human life.
So, how do you identify a lichen? As you might guess, they are not the most readily legible organisms, but a good starting point is to learn the broadest categories of lichen morphology: foliose, fruticose, squamulous, and crustose. These are all descriptors of the shape of the lichen thallus, the cortex of fungal tissue that comprises the majority of the lichen.
Foliose lichens resemble leaves laid loosely over their substrate. The foliose lichens Flavoparmelia caperata (greenshield lichen) and Parmotrema hypotropum (powdered ruffle lichen) are the most common lichens of VA, so they’re good to know – if you find a greenish or bluish foliose lichen anywhere in VA, the odds are good that it is one of these two widespread species! F. caperata’s color can be variable, but you can recognize it most reliably by its lime green undertones and smooth, ruffled-ribbon margins. By contrast, P. hypotropum tends much more towards gray-blue, and its margins are characteristically fringed with fine black hairs called cilia. Another notable foliose lichen, Punctelia bolliana (eastern speckled shield lichen), can be spotted by its large apothecia, which are the reproductive structures of the mycobiont and another helpful identifying feature of lichen morphology.
Fruticose lichens can be upright/shrubby or pendant. Usnea strigosa (bushy beard lichen) is a member of the fruticose genus well-beloved for its medicinal properties, and Cladonia rangiferina (often misleadingly referred to as “reindeer moss) is a fruticose lichen prized by reindeer as a source of vitamin D in the winter. Many lichens, like Cladonia cristatella (British soldier lichen) and Cladonia pyxidata (pebbled pixie-cup lichen), are combination squamulous/fruticose – that is, they have both squamules (small, leafy growths closely appressed to their substrate) and erect fruticose growths called podetia.
Crustose lichens form thin crusts over their substrates. When discussing how to identify lichens to assess air quality, many resources suggest that crusts tend to be hardier than foliose and fruticose lichens to environmental contamination. And indeed, many crustose lichens like Buellia stillingiana (common button lichen) are widespread and resistant to poor air quality. Graphis scripta (common script lichen), on the other hand, is a crustose lichen that only grows in excellent air quality conditions, and the foliose lichens F. caperata and P. hypotropum are tolerant to nitrous oxides; each challenges attempts at quick rules in sorting lichen resistance.
The more we understand about lichens, the more they invite further questions, closer study, and deeper subtlety. Lichens challenge and complicate our understanding of the natural world. Their ecological impact colors and softens every corner of our landscapes, tilling stone to soil and conjuring nitrogen from the air. Their work is quiet, cryptic, crucial. I take inspiration from their ancient labor and try to meet them in turn with reciprocal patience and attunement to the indefinite dance of ecological relationship.
Brodo, Irwin M, Sylvia D. Sharnoff, and Stephen Sharnoff. Lichens of North America. New Haven: Yale University Press, 2001. Print.
Rose, Anita K. “Distribution and occurrence of lichen species across Virginia.” American Botanical Society. Chico, CA. Aug 2006. Poster. http://www.2006.botanyconference.org/.