Lichens: they’re very common, but more often than not go unnoticed by the casual observer. What you miss seeing in these unobtrusive composite organisms is their sensitivity to air pollutants that makes them valuable bioindicators.
To begin with, lichens are formed through the symbiotic association of a photosynthetic algae and a fungus (Richardson, 1992). The fungus provides structure and protection for the algae, while the algae provides the fungus with nutrients and carbohydrates (USDA).
As early as 1839, naturalists in Europe observed that lichens were much less common in industrialized areas than in areas without industry (Richardson, 1992). In 1859, British biologist Leo Grindon was the first to attribute the decline of many lichen species to poor air quality (Lepp, 2012). Grindon’s attribution proved to be correct.
Figure 1. Leafy lichens are indicators of good air quality. We found this one at the Morgan Arboretum as part of our comparative study between the Arboretum and Macdonald Campus.
Unlike plants, lichens lack an outer protective film known as a cuticle, meaning liquid and airborne molecules can easily enter cells where biological processes are performed (Richardson, 1992). In industrialized areas, sulfur dioxide (SO2), a product of fossil fuel combustion, is often found in high levels of concentration in the air. This can lead to the formation of acid rain and sulfuric acid (EPA). SO2 and sulfuric acid are reactive, and can disrupt metabolic pathways in lichens (Richardson, 1992). In areas where precipitation is too acidic, or SO2 concentrations too high, this can lead to a loss of SO2-intolerant lichen species (Richardson, 1992). In addition to SO2, some lichen species have been shown to be sensitive to heavy metals, nitrogen-based fertilizers (Franti et. al.2006), and ozone formed from interactions between products of hydrocarbon combustion (Richardson, 1992). Furthermore, it has been demonstrated that sulfur and nitrogen oxides can disrupt chlorophyll ratios in lichens (Conti and Cecchetti, 2001) and that lichen susceptibility to pollutants can vary depending upon the secondary metabolites produced during photosynthesis (Hauck and Jürgens, 2008). Thus, hairy, leafy and shrubby lichens seem to be more sensible to the air pollution than crusty lichens (Agna, 2014).
Figure 2. Bright orange lichen at Macdonald Campus.
It is well documented that as air quality increases, pollution intolerant species become more abundant, and overall lichen diversity increases. The research question we are trying to answer is: How does the diversity and abundance of lichens differ between the Morgan Arboretum and Macdonald Campus, and what do the differences indicate about differences in air quality?
The object of our project is to compare the lichen populations found in the Morgan Arboretum and at the Macdonald Campus. Macdonald Campus’s proximity to Highway 20 and a large number of cars may result in higher localized levels of SO2, CO2 and other air pollutants, which would lead to lower species diversity and a lower density of pollution-intolerant lichens. We expect that the lichens found at the Morgan Arboretum will cover the surface area of the trees more densely, and with higher rates of species diversity.
For our experiment, we sampled a minimum of 50 trees in each location. We standardized our sample-taking by always sampling on the North side of the tree by using a compass, 1.3 meters above the ground in a 30 by 30 centimeter frame. For each tree, we took a picture of the frame and then identified what we saw to the lowest taxonomic level possible using lichen guides. Owing to the dominant forest type in the region, most of the trees samples were maples, oaks, hickories and ashes as well as a few coniferous trees.
Figure 3. The team in action: John Jansen (middle) holding the frame after Mercy Harris (left) has measured the correct height with the tape.
There are several possible confounding variables to take into account in our study. The first is the different type of environment that the trees are growing in. Even though the tree species are similar, the Arboretum is more biologically diverse, and has a greater number of introduced species. The tree density is also different. The Morgan Arboretum is more like a forest, while on Macdonald Campus, the trees are further apart and closer to buildings in greatly human-altered environment.
To date, not enough data analysis has been conducted to conclude whether a measurable difference in species diversity and abundance between the two locations. Initial observations suggest that similar species are found in both locations. However, observations also suggest that there is a discernable difference in abundances of certain species between the locations. Once the results have been fully analyzed, we will be able to draw a more precise conclusion based upon scientific data.
Agna, Y. 2014. Les lichens: témoins de la pollution atmosphérique. [cited 24 Oct. 2016].
Conti, M.E., and G. Cecchetti. 2001. Biological Monitoring: Lichens as Bioindicators of Air Pollution Assessment– A Review. Environmental Pollution 114.3 (2001): 471-92. ScienceDirect [Internet]. [cited 23 Oct. 2016]. Available from: http://dx.doi.org/10.1016/S0269-7491(00)00224-4
EPA: What Causes Acid Rain? United States Environmental Protection Agency. [cited 23 Oct. 2016]. Available from: https://www3.epa.gov/acidrain/education/site_students/whatcauses.html
Frati, L. et. al. 2006. Effects of NO2 and NH3 from Road Traffic on Epiphytic Lichens. Environmental Pollution 142.1 (2006): 58-64. ScienceDirect [Internet]. [cited 23 Oct. 2016]. Available from: http://dx.doi.org/10.1016/j.envpol.2005.09.020
Hauck, Markus, and Sascha-René Jürgens. 2008. Usnic Acid Controls the Acidity Tolerance of Lichens. Environmental Pollution 156.1 (2008): 115-22. ScienceDirect [Internet]. [cited 23 Oct. 2016]. Available from: http://dx.doi.org/10.1016/j.envpol.2007.12.033
Lepp, Heino. 2012. Pollution and Lichens. Australian Lichens. Australian National Botanic Gardens and Australian National Herbarium. [cited 23 Oct. 2016]. Available from: http://www.cpbr.gov.au/lichen/ecology-polution.html
Richardson, D. H. S. Pollution Monitoring with Lichens. Slough, England: Richmond Pub., 1992. p. 2-37.
USDA: What Are Lichens? United States Department of Agriculture. [cited 24 Oct. 2016]. Available from: http://www.fs.fed.us/wildflowers/beauty/lichens/whatare.shtml