When I graduated from Cornell in 1991, I left Ithaca knowing only one lichen: Flavoparmelia caperata (Common Greenshield Lichen), a dead-common species that I’d collected from a tree near my dormitory. I’ve since become a fully-fledged Ph.D. Lichenologist, and have had the privilege of working in some of the biggest and best herbaria in the world. I now know nearly all of our northeastern North American lichens on sight.

The other day, while walking along the main east-west route on central campus (Tower Road), I was pleased to be able to name no fewer than eleven different lichen species on the oak trees that line the north side of the road— ten more than I was able to 20 years ago! Here is my preliminary species list:

My surveying strategy was casual, but similar to the standard survey methods used by lichenologists who use lichen species diversity as a measure of air quality: I assessed the diversity of lichens on various trees of the same species (in this case, red oak: Quercus rubra) and of roughly the same size (dbh, or diameter at breast height). Using lichen species diversity as a measure of air quality is common in Europe (especially the UK), because European lichenologists have developed lists of lichen “indicator species” —i.e., lists of lichen species that are especially sensitive to air pollution vs. lists of lichen species that are especially tolerant. In general, the further the distance from a “point source”of pollution (like a paper mill, or a smelting plant), the higher the lichen diversity, when measured on trees of the same species of tree and roughly the same dbh. Lists of indicator species, of course, are entirely dependent on geography and latitude. In other words, the same lichen species will not be found here in North America, so lists of European lichen indicator species are of no use here. Once somebody on this side of the Atlantic does the necessary work to develop North American lichen bioindicators, we’ll be able to perform the same sorts of air quality assessments here.

Even without well-developed lists of indicator lichen species, however, I can make two general conclusions about the air quality in the vicinity of Tower Road based on my species list:

All the lichen species that I found are common street-tree lichens in many cities in the northeast, and some (F. caperata, P. sulcata, P. millegrana, P. rudecta) are known to be pollution-tolerant in Europe. In addition, all of the species are either foliose (flat; leafy) or crustose (crusty; immersed in their substrate)–none are fruticose (shrubby). [Fruticose lichens are, in general, more sensitive to air pollution than foliose and crustose species.] I would conclude, therefore, that the air quality on Tower Road is not very good. Two of the lichens I found, Candelaria concolor and Physcia millegrana, prefer high nitrogen environments, at least in Europe. Both species, in both Europe and North America, are commonly found on roadsides (where they enjoy high emissions of NOx compounds from vehicles) and near agricultural areas (fields, pastures, and barnyards). So I would conclude from this that the air in the vicinity of Tower Road has an above-average concentration of nitrogen-containing compounds.

My conclusions may not be entirely accurate; the picture may not be so grim! After all, the Air Quality Act was passed in the United States in 1967, and studies have shown that our air quality in the northeast has increased steadily since that time. Modern emissions controls on motor vehicles have further reduced atmospheric pollutants. It takes many years, however, for precipitated atmospheric pollutants to wash away from tree bark. In other words, it may take a while for the lichen flora to recover, even though air quality has increased dramatically. To be truly sure, we’d need to precisely measure air quality, with a machine, to determine exactly what’s happening with regards to air quality vs. lichen diversity along Tower Road.

Another interesting observation–one not having to do with air quality–is the apparent, gradual replacement of Flavoparmelia caperata by the very similar-looking Flavopunctelia soredica on the Tower Road oak trees (the latter is the main lichen flowing down the trunk in our photo). Former CUP curator Bob Dirig has been tracking this phenomenon throughout the Finger Lakes, and other parts of New York. It’s not clear what may be causing this— but it certainly merits closer inspection.

I look forward to doing more field work, and discovering more about Ithaca’s lichens, once the weather warms up again. Meanwhile, you can find me indoors, in the safety and warmth of my microscope lamp!