Sampling and Estimating Proportions of Pollutant Tolerant and Pollutant Sensitive Lichens

  1. Background

                You can assess air quality from the general morphology (shape and appearance) and colors of lichens on trees. Air-pollutant tolerant lichens (see Figure 1) are small (us. < 2.5 cm in diameter), bright orange and yellow lichens and small, rosette-forming (e.g. flat, appressed, circular shaped) whitish-gray macrolichens. Air-pollutant sensitive lichens (see Figure 2) are, for this exercise, all other macrolichens, here defined as the large (>2.5 cm in diameter) leafy, tubular, shrubby, and pendant macrolichens. Your task will be to estimate the general proportion of air -pollution tolerant to air-pollution sensitive lichens.

Figure 1:  Air Pollutant Tolerant Lichens

  1. Instructions

  1. Choose your area.
  1. Number and type of trees. Pick an area that has at least 10-20 trees. The more trees the better, but aim for at least ten trees that have lichens on them. Decide if you will sample hardwoods or conifers. If you have a choice, choose to sample hardwoods.
  2. Size: The area can be any size, but generally speaking about 35 m radius is a good standard.
  3. Light: For optimum results choose a stand that has a high to moderate amount of light coming through the canopy (lots of moss and no understory plants is a sign the site is too dark, lots of flowering plants or grasses is a sign of plentiful light; you want the canopy to block no more than 60-80% of the light).
  4. Age: Trees can be any age, but if they are less than 20-40 years old, there may be little to no lichen cover. Usually the wider the trunk, the older the tree. A good minimum trunk width would be > 15 cm—which is a little less than the width of a piece of loose leaf paper.

Figure 2 : Air Pollutant Sensitive Lichens

  1. Inspect your trees.
  1. What to inspect: You will be walking around to your trees, inspecting all the trunks and branches that you can see without climbing the trees, starting from about ½ m up the trunk from the forest floor. If most branches are out of reach and there are fallen branches, you could inspect these instead. Its also OK to include shrubs. Exclude lichens on stumps, rotting logs (recent tree fall ok), soil and rocks.
  2. Noticing and understanding variability in lichen cover: One thing you will likely notice is that lichen cover is very variable from tree to tree. This is normal as lichens respond to many other factors besides pollution such as the amount of light coming through the canopy (they like light), the roughness of the bark (rough bark is preferred to smooth bark as long as it is not peeling), tree species (hardwood barks tend to be more alkaline, conifer barks more acidic and there are species that prefer each), age of the stand (the older the stand, generally the more lichens), the microclimate within the stand (different lichens require different levels of moisture and temperature) and the distance to the next tree with lichens (dispersal distance).
  3. How to inspect:  Walk around your site inspecting your trees. Take at least 15 minutes, more is preferable. After you practice a few times you probably won’t need more 30-60 minutes, unless it is very difficult to walk around your stand due to steepness or thick brush, etc.. Think of yourself as a detective: on every tree look to see whether or not it has lichens. Look at the lichens closely enough to determine for yourself the relative proportion of pollution tolerant to pollution sensitive lichens. Enter your data one tree at a time. Remember it doesn’t matter if some trees have very little lichen cover and others have a lot, or if the trunk is bare but the branches of the same tree are well covered, you are estimating the proportion that are pollution sensitive vs pollution tolerant only among the lichens that you encounter. Exclude crustose lichens from your survey—crustose lichens have no distinct lower surface, some look like they have been painted on the trees, you cannot slide a fingernail under them to lift up and see the lower surface (SHOW CRUSTOSE PHOTOS). Also exclude mosses and liverworts (SHOW MOSS&LIVERWORT PHOTOS) --these have small green leaves that remain soft when dry, and exclude fungi (SHOW FUNGI PHOTOS) that are not lichens, i.e. that do not contain algae or cyanobacteria—break open the specimen in question and check for a grass-green to blue-green layer with your hand lens. Also exclude free-living green and red algae.
  4. How to estimate proportion. There are many ways this can be done. One way is to imagine a pie or rectangle laid over a trunk or branch that is mostly filled by lichens, and then divided into even pieces. How many pieces would the pollution sensitive lichens fill compared to the tolerant ones? If only a quarter of the pie pieces are filled by pollution sensitive lichens, then the proportion will be 25%; if it’s about half and half, then the proportion will be 50%.
  1. Slide the slider bar to the appropriate proportion.
  1. The slider bar indicates the proportion of pollution tolerant vs sensitive macrolichens. In other words, 0% on the slider bar indicates 0% pollution tolerant macrolichens and 100% pollution sensitive macrolichens. Or 40% on the slider bar indicates that 40% of the lichens are pollution tolerant and 60% are pollution sensitive. Remember, pollution tolerant lichens are the small (< 2.5 cm diam) red, orange, and rosette forming grayish-white lichens. Pollution sensitive lichens are the large leafy, shrubby, tubular, and pendant lichens. You must enter at least 7 tree scores.
  1. Press done:  The program averages the values for each tree, looks up the air quality in Table 1 and presents the air quality to the user.

Table 1. Calculating Air Quality Scores

% Pollution-tolerant lichens (small red, orange and yellow and whitish gray rosette lichens under 2.5 cm diam)

% Pollution-Sensitive lichens (large >2.5 cm leafy, shrubby, tubular, and pendant lichens)

Hardwood Rating

Conifer Rating

0 to 14

86 to 100


Good to Excellent

15 to 26

74 to 85



27 to 72

28 to 73


Poor-not acidified


0 to 27

Poor-not acidified

Poor-not acidified

No macrolichens are present on the site or > 7 trees have few to no lichens



Interpreting  Air Quality Scores



What it means


Poor, acidified

High nitrogen deposition; precipitation is likely acidified.

Pollution is high enough to reduce lichen diversity and eliminate most sensitive lichen species; other sensitive vegetation likely to be adversely affected.

Other reasons for lack of macrolichens include young stand age or too little light, or trees sampled had very smooth or peeling bark (eg. madrone, aspen)

Poor, not acidified

High nutrient nitrogen deposition

Pollution is high enough to reduce lichen diversity and eliminate most sensitive lichen species; other sensitive vegetation likely to be adversely affected.


Moderate nutrient nitrogen deposition

Pollution is affecting the lichens that are present; some to many ecologically important species are absent. Lichen response is a warning that other sensitive vegetation may be affected if pollution worsens.


Low nutrient nitrogen deposition

Subtle shifts in lichen communities may be occurring.

See box below


Nitrogen deposition is at or close to natural background levels.

No detectable adverse effects due to air pollution or acidic precipitation.

If total lichen cover is very low, i.e. there are very few individuals in the stand, and there was plenty of light and the trees were mature, then acid deposition may be occurring. Use poor, acidified rating.