• Science
What Lichen Can Tell Us
Andrew Deaett, Science Teacher

Several years ago, I had the chance to learn from CS12 students as they photographed rare American marten on campus, wrote poems under night skies, and identified curious organisms while visiting their phenology spots. Lucas Stanley, a CS12 (Spring 2016) student with a particular interest in lichen, often shared his learning with me. He told me of the three different growth forms of lichen, their use as a colorful dye for fabrics, and their potential as a food source for many animals in the Northwoods. His curiosity was inspiring to me and motivated me to learn more about the world of lichen. Several years later, and with some trial and error, we have now developed a research project in AP Environmental Science studying lichen diversity as a bioindicator of air pollution on campus.

With names like smooth horn, crazy scale, zoned dust, easter foam, and mealy pixie cup, it is hard not to be intrigued by the curious organisms we refer to as lichen. Lichen, often confused with mosses, are, in reality, two organisms living together in symbiosis. In this partnership, a fungus (mycobiont) and either algae or cyanobacteria (photobiont) have evolved to form a partnership in which both benefit, called mutualism. The mycobiont provides the organism's structure and essential minerals, and the photobiont offers energy in the form of sugars produced during photosynthesis. This partnership has been so successful that there are an estimated 14,000 to 20,000 species of lichen globally  (Walewski, 2007, p. 4-5), and lichen are estimated to cover 8% of Earth’s terrestrial surface (Brodo, Sharnoff, & Sharnoff, 2001, p 54).

When peering closer into this curious world, a skilled observer can begin to differentiate between the different growth forms of lichen: crustose, foliose, and fruticose. Crustose lichen appears to be painted on to the growing substrate (rocks, trees, etc…) and have no distinguishable top or bottom. Foliose lichen resembles leaves and have an easily distinguishable top and bottom. Fruticose lichen are often described as being shrubby or bushy and does not have an easily identifiable top or bottom, tending to grow in branchlike patterns.

Lichen are considered epiphytes, meaning they don’t root in the soil and acquire their water and nutrients directly from the air around them.  Unlike organisms in the plant kingdom, lichen lack a waxy cuticle, the protective barrier forming the surface of a leaf that separates the leaves' internal structures from their surrounding environment.  Because of this unique biology, lichen are highly susceptible to any atmospheric pollutants in their environment and can be used as bioindicators of air quality. Of particular interest are two air pollutants, sulfur dioxide (SO2) and nitrogen oxides (NOx).  Both SO2 and NOx are released from the combustion of fossil fuels such as coal, diesel, and gasoline.  When SO2 is deposited directly on a lichen, it inhibits the photobionts ability to photosynthesize, limiting the lichen’s ability to thrive.  When SO2 and NOx react with water in the atmosphere, they create sulfuric and nitric acids, both of which can acidify the growing substrate of the lichen (Grant & Littlejohn, 2009, p. 59-60).

The three different growth forms of lichen have different tolerances to the presence of these pollutants, with crustose being the least sensitive, then foliose, then finally fruticose being the most sensitive (Hutchinson, Maynard, & Geiser, 1996). Because of this, when high levels of SO2 and NOx are present in an ecosystem, we expect that lichen diversity will decrease. With this knowledge, CS19 (Fall 2019) engaged in fieldwork to answer the research question: Is traffic from Conservation Trail a significant source of pollution in the form of SO2 and NOx? To answer this question, we calculated Simpson’s Index of Diversity (SID) of lichen at two different sites with varying proximity to Conservation Trail. SID is a measure of diversity represented by a value between 0 and 1. SID approaching 1 represents infinite diversity, and SID approaching 0 represents no diversity. Site A was approximately 10m removed from Conservation Trail, and Site B was approximately 400m removed from Conservation Trail. 

We hypothesized that because Conservation Trail supports relatively low levels of traffic emitting SO2 and NOx, we would not see a significant difference in the SID of lichen between Site A and Site B.  

SID values at Site A ranged from .49 - .57, and SID values at Site B ranged from .49-.58. SID at both sites averaged .53. We did not detect any significant difference in SID values between Site A and Site B; after analyzing our results, our hypothesis has is supported. Our results indicate that Conservation Trail is not creating significant levels of air pollution in the form of SO2 and NOx.  

This information is helpful for our community as we consider our impact on our environment. To further explore our relationship between campus operations and effects on the environment, we can consider monitoring the volume of traffic on campus or measure ambient concentrations of air pollutants such as SO2, NOx, particulate matter, carbon monoxide, lead, and ground-level ozone. We can also broaden our ecological research to examine the health of our forests, lakes, and wildlife on campus. We might consider questions such as: are aquatic invasive species detectable in campus lakes? How are forest communities changing over time as our climate warms? How does our use of hiking trails on campus impact wildlife behavior?

While exploring lichen diversity on campus, students learned about a unique organism, air pollution in the form of SO2 and NOx, and a research protocol to study air pollutants. Students also practiced field research techniques such as using a compass to shoot a bearing, using a meter tape to establish a sampling transect, and proper data collection practices. With our newly found knowledge of lichen, the world will never look the same.


Works cited
Brodo, I. M., Sharnoff, S.D., Sharnoff, S. 2001. Lichens of North America. New Haven: Yale University Press.

Grant, T., Littlejohn, G. 2009. Teaching Green: The High School Years. Gabriola Island: New Society Publishers.

Hutchinson, J., Maynard, D., Geiser, L. (1996). United States Forest Service: Air Quality and Lichens - A Literature Review. Retrieved from http://gis.nacse.org/lichenair/index.php?page=literature

Walewski, J. (2007). Lichens of the North Woods: A field guide to 111 northern lichens. Duluth, MN: Kollath+Stensaas Publishing.