Lichens are organisms that are formed by the symbiotic relationship between fungi and algae. Symbiosis is a mutually beneficial association that occurs between two different species.  In the case of lichens, the alga, which contains chlorophyll, provides the fungus with sugars produced by photosynthesis, while the fungus releases water and mineral nutrients to the alga, at the same time protecting it from too intense light, drought and excessive heat.  Together they can survive in extreme environments (high altitude rocks, deserts, etc) where they would not be able to live on their own.  Depending on their shapes, lichens are divided into crustose, foliose and fruticose.

Within the forest (particularly in coniferous forests, where microorganisms find it hard to attack and decompose the acidic litter that is produced therein) they play a very important ecological function since the rapid proliferation of lichens in a moist environment provides the soil with a continuous supply of herbaceous organic material which is crucial to its fertilization.

Lichens are amongst the first organisms to    colonize bare surfaces and are able to thrive also in extreme temperatures. However, since they lack a secretion system, lichens are very sensitive to air pollution. This characteristic makes them excellent biological indicators, which are increasingly used in biomonitoring of air quality.