TREE LICHENS AND AIR POLLUTION SURVEY
Background Notes for Teachers and Sixth Form or College Students
Lichens are highly sensitive to subtle changes in environmental conditions, especially
air pollution, so they are natural indicators of the quality of air in our
environment. In the past sulphur dioxide (SO2) from coal burning and industry was
the main pollutant leading to a loss of lichens (lichen deserts) in industrial areas.
With the introduction of clean air laws lichens are coming back to urban areas,
although today's atmospheric conditions are different. Following a reduction in SO 2,
nitrogen compounds from traffic on roads and from intensive farming have become
the major pollutants, affecting both town and country areas.
Every spring trees produce new twigs from the winter buds. The oldest part of a tree
is the trunk and the youngest part is this year’s twigs. Older trunks may carry a long
established community of lichens, mosses and algae whereas twigs provide a new
bark surface every year. In good conditions lichens are rapid colonisers of new
surfaces and 5-year-old twigs may be covered with many lichens. The mixture of
species that you find will depend on the kind of tree, the available colonisers and the
surrounding environmental conditions. Some lichens grow very slowly and may
themselves have begun their lives on a tree many years ago. If environmental
conditions have not changed very much these long-established lichens may still be
present. However, in areas where lichens have been eliminated by pollution or other
forms of disturbance, the conditions on the newly developed twigs may be more
suitable for lichens than on the old bark of the trunk, where they have to compete
with other organisms such as algae and mosses.
For the trunk survey, each site should have three trees (not conifers) of the same
tree species, preferably with acid bark (see table below). Trunks must be accessible,
preferably of girth between 1 and 2.5 m. Avoid shaded or ivy-covered trees or those
in a hedge surrounded by undergrowth. Remember to ask permission if you go on
private land. Use the table below as a guide to tree selection, preferably choosing a
tree with bark of low pH (acid).
Bark pH Tree species (not conifers)
Low (acid bark) Oak, Birch, Cherry, Alder, Sweet Chestnut, Rowan,
Medium (neutral bark) Hazel, Ash, Sycamore, Willow, Beech*
High (basic bark) Elm, Lime, Elder
* Beech twigs are not suitable due to heavy shading of lower branches in summer.
For the twig survey, avoid heavily shaded branches and sample along a canopy
edge on one or more trees of the same species where you can reach 10 twigs safely,
avoiding damage to the tree and yourself! You can use the same trees as for the
trunks - or different trees - but the twigs must all be from the same species of tree
and preferably from the same kind of tree as used for the trunk survey. If necessary
pull the branches down to eye level while you are doing the survey, taking care not to
break the branches. Start along the main branch of the twig where it is about 3 cm
thick (generally about 10 - 15 years old; age can be determined by counting the
girdle scars). Mature specimens are easier to identify, so start recording on the older
part of the twig to become familiar with the species. You will have 10 observations,
one for each twig.
Try to survey both trunks and twigs at two sites in your area, one of which is near
a source of nitrogen. Download as many recording forms as you need for your
surveys. (If you have time, you can choose yet more sites.)
3 Questions for guidance in making your discussion and
When you have results from 2 or more sites you can ask some questions of your
1 Where do you find the highest values for nitrogen-loving species?
What form of land management is associated with a higher nitrogen index?
2 Have you got any sulphur-tolerant species?
If only sulphur-tolerant species are present, you are probably in an area
where acid rain is or has been a problem. In many urban areas that were
formerly 'lichen deserts' due to acid rain, lichens are now returning, and many
of these are nitrogen-loving species. Why do you think this is? Where does
the nitrogen come from?
3 Have you got a mixture of nitrogen-loving and sulphur-tolerant species?
Where there are sulphur-tolerant and nitrogen-loving lichens at the same site,
you are probably in an area where change is occurring in atmospheric
conditions. In recent years there has been a tendency for the replacement of
sulphur-tolerant species with nitrogen-loving species across the country.
4 Do you have any clean-air species on your trunks or twigs? Check your site
records to see what form of land management these are associated with. In
former clean-air areas many lichens are being replaced by nitrophytes.
5 If you have completed both trunk and twig surveys on the same tree species,
is there a difference in the nitrogen indices?
You may have a difference between the index on the trunk and on the twigs,
especially in areas of the west where rainfall is high and there are long-
established lichens on the trunk. If the nitrogen index on the twigs is higher
than on the trunk the twigs are reflecting a recent increase in atmospheric