A lichen is two organisms, a fungus and an alga, growing symbiotically. The alga produces carbohydrates and vitamins for itself and the fungus. The fungus provides physical protection for the alga and absorbs water. Because both organisms in this interaction benefit from it, the relationship is a mutualism. Lichens can grow in harsh environments, including bare rock, deserts, and tundra. They also grow on the bark of trees, although lichens neither help nor harm the trees. Lichens are slow-growing and long-lived.

Lichens can be used as bio-indicators of air quality because they are sensitive to atmospheric pollution, including heavy metals, radiation, and ozone. The component of air pollution responsible for the greatest damage to lichens is sulfur dioxide (SO2) released by coal-burning power plants (PathFinder Science 2002). SO2 combines with water in the atmosphere to produce sulfurous acid (H2SO3) or sulfuric acid (H2SO4) which lowers the pH of precipitation. Lichens may be directly harmed by the acidity of precipitation, and they also accumulate sulfur compounds in their thalli. Lichens growing on acidic substrates accumulate sulfur as toxic bisulfite ion (HSO3-) and H2SO3, but lichens growing on basic substrates accumulate sulfur as relatively harmless sulfite ion (SO3-2) (WHO 2000).

Lichens vary in their sensitivity to SO2 pollution; in general, crustose and squamulose lichens are least sensitive, foliose lichens are more sensitive, and fruticose lichens are most sensitive (The Georgia Conservancy 2001).  Sensitive lichens growing on trees such as birches (Betula spp.) and conifers that have acidic bark and low buffering capacity are affected by even low levels of SO2 pollution. Higher levels of SO2 pollution will also cause loss of sensitive lichens on oaks (Quercus spp.) and sycamore (Platanus occidentalis) with less acidic bark and greater buffering capacity. Extreme SO2 pollution will eventually cause loss of sensitive lichens even on elms (Ulmus spp.) which have basic bark and high buffering capacity (PathFinder Science 2002).

In today’s lab, we will examine lichen growth on two species of trees, Norway spruce (Picea abies) and Norway maple (Acer platanoides), on campus. As the common names of these trees suggest, these species are native to Europe, but both are widely planted in the US. Norway spruce has a bark pH = 4.8, and Norway maple has a bark pH = 7.1.



            You will work in groups of three. 

            Data collection.— There is a grove of conifers above Lightstreet Road next to the McCormick parking lot. Most of the trees are Norway spruce, but there are a few scattered white pines (Pinus strobus) and eastern hemlocks (Tsuga canadensis). The spruces have stiff, square needles 1-2.5 cm long and attached singly to the twig. On the other hand, white pines have thin needles 6-10 cm long and attached in bundles of five to the twig, and hemlocks have flat needles 1-1.5 cm long and attached singly to the twig.

            Norway maples are widely planted on campus, including a row along the sidewalk in front of Sutliff. All maples have opposite branching, and mature Norway maples have bark with vertical furrows that frequently crisscross, forming Xs.

            Locate five spruces and five maples. For each tree, tie a string around the trunk at a height of 1.5 m. Place the 100-circle grid just above the string, and record the number of circles that cover crustose lichen, squamulose lichen, foliose lichen, fruticose lichen, moss, bare bark, and other. Repeat the coverage count one-quarter, half, and three-quarters of the way around the tree trunk.

            Statistical analysis.— For each tree, find the mean coverage of crustose lichen, squamulose lichen, foliose lichen, fruticose lichen, moss, bare bark, and other based on your four coverage counts. Also sum the means for the four types of lichen to get a lichen cover percentage for each tree.

We will use Wilcoxon rank sum test to test for differences in the lichen coverage of Norway spruces and Norway maples. The Wilcoxon rank sum test is a non-parametric test; these tests have fewer assumptions than traditional parametric tests and are actually more powerful with small sample sizes and with non-normal data (Neave and Worthington 1988). To do the rank sum test, assign all ten lichen cover percentages their ranks (smallest value of the ten = 1, highest value = 10). If two or more lichen cover percentages are equal, give each of them their mean rank. For example, if two lichen cover percentages were the third and fourth values, you would give each of them the rank 3.5, the mean of ranks 3 and 4. To calculate the test statistic R, sum the ranks for Norway spruce and sum the ranks for Norway maple and select the smaller value. Compare R to Table 1 below to obtain a p-value. If p < 0.1, reject the null hypothesis.


Table 1. Rank sum and corresponding p values for a two-sided Wilcoxon rank sum test for nA = 5 and  nB = 5.














≥ 20

> 0.1




            Your lab report should include four parts: a descriptive title, an short introduction that states the purpose of the lab exercise, the results (a bar graph of the coverages for the spruces and maples, a bar graph of the different types of lichen growing on each species, and a sentence or two that describe trends in the data; the statistics should be incorporated parenthetically at the end of these sentences), and the discussion of the results.  The discussion should conclude with ideas for additional experiments that would build on what we have done in this lab.

            The lab report is due at the beginning of the next lab meeting time.  Deadlines are real; 10% of the possible points will be deducted for each day the report is late.


Literature Cited

Neave, H. R., and P. L. Worthington. 1988. Distribution-free tests. Unwin Hyman, London, UK.

PathFinder Science | Lichens and SO2. http://pathfinderscience.net/so2/ last updated 2004; accessed 5 January 2004.

The Georgia Conservancy. 2001. Not “lichen” air pollution. Teaching Conservation—Winter 2001:3-4. available at http://www.gaconservancy.org/Education/TC_Winter2001.pdf

WHO Regional Office for Europe, Copenhagen. 2000. Chapter 10—effects of sulfur dioxide on vegetation: critical levels in Air quality standards for Europe, 2nd ed. WHO Regional Publications, European Series no. 91, Geneva, Switzerland. available at www.who.dk/document/aiq/10effso2.pdf