Lichens are sensitive to air pollution and can be damaged by polluting gases. Their presence or absence can therefore indicate the level of air pollution in a given environment.
Lichens are organisms that are very sensitive to air quality, making them effective bio-indicators for monitoring air pollution. Since they have no roots, they absorb elements directly from the surrounding air, including pollutants. As a result, as soon as the air becomes too polluted, their growth slows or they completely disappear. Given their rapid reactivity to environmental changes, their presence or absence on trees, walls, or rocks provides a clear and easily observable visual sign of the cleanliness of the air in a given location. Better than a sophisticated machine, these small organisms directly reveal the actual state of atmospheric quality.
Lichens are highly sensitive to atmospheric pollutants such as sulfur dioxide, nitrogen oxides, or ozone. Since they absorb their nutrients directly from the surrounding air, they have no protection to defend against aggressive chemicals. When pollutants are present, lichens begin to decline: their growth slows down or completely stops, their color changes, often shifting from green to grayish or brown, and they can even end up completely disappearing. In contrast, an abundance of varied and clearly visible lichens typically indicates rather clean and healthy air. Some lichens tolerate moderate pollution better than others, so depending on the type of lichen found in a location, one can also get a small idea of the pollution level.
When lichens are numerous, varied, and healthy, it's generally a good sign: the air is likely clean and low in pollutants. Conversely, if you observe less diversity or only a few resistant species, be cautious: this indicates that the air probably contains pollutants, such as sulfur dioxide, nitrogen oxides, or other harmful compounds. Some very sensitive lichen species even completely disappear as soon as a little pollution appears, while others manage to cope with these conditions quite well. Observing which types of lichens grow on trees or rocks allows, therefore, with the naked eye and without complicated devices, a good estimation of the local air quality.
In the Paris region, researchers regularly use lichens to assess pollution levels. A study conducted near heavily trafficked highways observed a marked decrease in some sensitive species like Usnea when the concentration of nitrogen dioxide increased. In Northern Italy, in the 2000s, scientists monitored lichens around industrial sites: the hardiest species dominated near chimneys, while sensitive lichens reappeared noticeably further away. In Scandinavia, particularly in Sweden, lichens are also closely monitored to detect radioactive fallout after events like Chernobyl. They easily accumulate certain radioactive pollutants, allowing for precise tracking of the dispersion of these contaminants.
Lichens are widely used to quickly and cost-effectively identify areas where air quality is deteriorating. Some municipalities even place these plants directly on urban structures or trees to monitor changes in pollution. Convenient, right? But be careful, this tool also has its limitations. The slow growth of lichens means they do not react instantly to short-term fluctuations in pollution. They instead provide a general idea over a longer period. Moreover, lichens cannot specify which specific pollutants are involved; they simply indicate an overall poor air quality without precise distinction. Finally, there are also external factors such as climate or humidity that can influence their presence, which sometimes complicates their interpretation.
Some lichens can live for several hundred, or even thousands of years, allowing them to serve as historical indicators of air quality over very long periods.
There are about 20,000 different species of lichens in the world, and each has a specific tolerance to certain pollutants, making their observation particularly useful for refining environmental diagnostics.
Lichens have no roots and absorb pollutants directly from the air, which explains their extreme sensitivity to air pollution.
In Scandinavia, the indigenous Sami populations traditionally used certain lichens to feed their reindeer and noticed as early as the 19th century that a local decline in these lichens indicated a deterioration in air quality.
Yes, there are other bioindicators such as certain mosses, conifers (for example, the condition of pine needles), insects, and some plant species sensitive to atmospheric pollutants, which can be used in addition to or instead of lichens depending on specific needs.
No, lichens are generally not harmful to trees. They are not parasites but epiphytes, meaning they live on the tree without directly extracting nutrients from it or causing damage. They simply use the support provided by the bark.
Yes, regular observation of the lichens present in a given area is an effective method for detecting local variations in air quality. The appearance or disappearance of sensitive species particularly provides valuable clues about the evolution of ambient pollution.
No, different types of lichens have varying levels of sensitivity. Some species can only survive in very clean air, while others, which are more resilient, can tolerate moderate pollution. It is primarily the diversity and presence of sensitive species that indicate better air quality.
In general, foliose lichens (which resemble leaves) or fruticose lichens (shrub-like or filamentous) are more sensitive to pollution. Conversely, crustose lichens (firmly attached to the substrate) often withstand pollution better. An environment rich in fruticose or foliose lichens typically indicates cleaner air.
The reappearance of lichens after a decrease in pollution depends on the species and environmental conditions. This process can take anywhere from a few months to several years. That is why long-term monitoring is often recommended to assess the lasting improvement of air quality.
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