Visit a forested area that’s not near a city, and you’ll see all sorts of lichens decorating trees and rocks. But if you visit an urban area or walk alongside a highway roaring with traffic, you won’t find very many lichens. As a matter of fact, Downtown Boston is a lichen desert.
Lichens get nutrients from the atmosphere. Particularly in urban areas, nitrogen dioxide, a pollutant from automotive exhaust and fertilizers is prevalent. Our factories also release sulfur dioxide (such an ugly word!). Lichens absorb such toxic gases either directly or when those pollutants are dissolved in rainwater. Unlike plants, they can’t filter out something nasty. As a result, the delicate balance between a lichen’s alga and its fungal partner is upset. Cells stop functioning normally. Certain species can’t create their spore-bearing apothecia, so it’s goodbye to the future. Both gases disrupt the alga’s ability to engage in photosynthesis, so it’s goodbye to the present.
Here I should mention that lichens with a high surface-to-volume ratio, such as fruticose species (example: Usnea strigosa known as bushy beard lichen) or the fruticose species (example: Cladonia arbuscula known as the reindeer lichen), find it almost impossible to deal with pollution. Crustose species (example: Xanthoria parietina known as the common sunburst lichen), which typically grow flat against a rock or on a tree, have a low surface-to-volume ratio so they’re not uncommon in an urban area. Like everything else in nature, there are exceptions to the rule: the foliose species, Flavoparmelia caperata, otherwise known as the common green shield lichen, is quite tolerant of pollution, so you’ll often find it in cities.
Lichens are the proverbial canary in the coal mine. By documenting which species occur in an area, it’s possible to determine the level of pollution in that area. Having read this brief essay, perhaps its readers will decide to start a lichen inventory at any of the EwA sites, including in the Fells, Waltham, or Woburn, and by doing so, we’ll learn just how clean the air in that place is…
by Lawrence Millman | July 19th, 2022 | Photos © Claire O’Neill.
Lawrence Millman is a mycologist, author, and explorer. He co-leads biodiversity documentation sessions for the EwA Biodiversity and Climate Participatory Science program.
EwA encourages nature enthusiasts to follow proper field ethics as highlighted in the EwA Wildness Etiquette. Also check the EwA provides a field guide The EwA Guide to Getting Good Pictures for Identification.
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