Unlike plants, lichens do not have roots. They uptake all the nutrients from the atmosphere, which makes them sensitive to air pollution. However, not all lichens are affected in the same way by pollution: the same pollutant can be harmful for one species and beneficial for another.
By studying both diversity and abundance of lichens that grow on trees, it is possible to assess air quality without employing chemical or physical sensors. Due to their slow growth, lichens integrate pollution over several years, making them good indicators of air quality.