Lichen: the natural air quality monitor

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 Lichen on a wall

If you were asked to describe how air quality is monitored, you would perhaps conjure up an image of a very technical piece of equipment, attached to a laptop with graphs showing peaks and troughs as levels vary throughout the day. But our scientists are looking at how lichen can give us a picture of air quality over a longer time period, and it’s something anyone can get involved in.

Many people probably don’t give a second glance to lichen growing on trees in their area, or think about what information they can provide. However, this algae and fungus combination can provide a good indication what the air quality has been like in the past, and whether it is improving.

Why lichen?
Lichens are ideally placed to help us understand the long term impact of air quality on a particular site because they have no roots, so take all their nutrients from the air. This means they are particularly susceptible to changes in substances in the atmosphere. Some species are very sensitive to nitrogen while others tolerate higher concentrations, so studying which lichen appear in a certain area can show whether pollution levels are increasing or decreasing.

Claire Campbell, one of our scientists using the new technique, explained:

“Biomonitoring methods use the presence or absence of specific living organisms as an indicator of pollution on the ground.

“The advantage of lichen is that it provides an observable measure of the impacts of a pollutant, in this case nitrogen. Not only are they affected by the pollutants in the air around them, but they are also affected by changes in the acidity of the bark of the tree they live on. One of the problems that air pollution has caused for sensitive lichens has been to cause their tree’s bark to become more acidic.

“In some areas, although pollution levels have fallen, the bark of older trees is too acidic for recolonisation, and new growth develops on twigs and younger trees. So comparing those lichen found on trunks to those found on the branches, which have grown more recently, can show us how the situation has changed over time.”

Why is this information important?
Of course lichen cannot tell us the whole story, but when put together with plant species indicators and soil indicators, they are an important part of our understanding. Lichen monitoring provides a clearly observable way to see whether the pressure on the environment due to the nitrogen load in the atmosphere is reducing, or not.
Can I get involved?
The Open Air Laboratories (OPAL) network is running an air quality survey based on lichens, in addition to lots of other projects that provide great opportunities for anyone with an interest in nature and our environment to get involved.

OPAL has recently announced that it has received funding to extend its work into Scotland, Wales and Northern Ireland. By studying what lichen are found on trees in your area, and entering your results online, you can help increase everyone’s understanding of air quality in the UK. Visit OPAL’s website to find lots of information on how to get involved, including a workbook, the field guide and videos.

You can also find out more about more citizen science opportunities through the Scotland Counts projects, supported by ourselves, Scottish Natural Heritage and the Scottish Government

How will we use this information?
We have a legal duty under the Habitats Directive to assess the impact of licensable activities on Special Areas of Conservation and Special Protection Areas (also known as Natura 2000 sites) and to ensure that adverse impacts on site integrity are avoided. Information on the level of risk to a sensitive habitat from airborne nitrogen pollution will be valuable when assessing the likely impact of a new or revised Pollution Prevention and Control (PPC) permit.

Currently, the risk is estimated based on habitat-specific “critical loads”, which are thresholds of nutrient nitrogen deposition from the atmosphere above which damage is expected to occur. However, the critical loads approach is considered highly precautionary as it applies a critical threshold value to a habitat type, irrespective of their existing condition.

Claire explained:

“Understanding the baseline condition of the habitat in relation to the historical and current background atmospheric nitrogen deposition level will allow us to make better informed decisions as to whether a proposed activity will lead to adverse impact on the integrity of designated nature conservation sites.

“By using a biomonitoring approach to complement the critical loads approach, a more accurate, evidence-based conclusion should be reached, which should result in a more proportionate approach. This approach may reduce the number of PPC permit applications receiving conditions due to impacts on sensitive habitats and designated nature conservation sites, which will allow us to target conditions to where they may generate the greatest benefit.”

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