50.351
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
Methods
You will work in groups of three.
Data collection.— There is a grove of
conifers above
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
Table 1. Rank sum and corresponding p values for a two-sided Wilcoxon rank sum test for nA = 5 and nB = 5.
R |
p |
15 |
0.008 |
16 |
0.016 |
17 |
0.032 |
18 |
0.056 |
19 |
0.095 |
≥ 20 |
> 0.1 |
Assignment
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,
PathFinder
Science | Lichens and SO2. http://pathfinderscience.net/so2/
last updated 2004; accessed
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