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A lichen growing on a tree branch

A lichen growing on a tree branch. Photo courtesy of Andrea Nick.

Lichens are composite life-forms that include two to three different organisms. The majority of the lichen thallus (body) consists of a fungus (known as the mycobiont). The other organism is either a green algae or blue green bacterium (known as the photobiont). Some lichen species are comprised of all three organisms.

The diversity of lichens in North America is estimated to be around 3,600 species. In many forests, lichens play important roles in nutrient cycling, food webs, and providing habitat for insects and microorganisms along with nesting materials for birds and squirrels.

Lichens also have a long history of use as indicators of air pollution with records dating back from the mid 1800ís. Epiphytic lichens (lichens found growing off the ground in trees and shrubs) absorb the bulk of their nutrients from the air, lack a waxy cuticle and stomata (which, in plants, helps slow the absorption of pollutants), and have no roots (in which elemental transfer can take place). Because of these characteristics, lichens are highly sensitive to changes in habitat structure, climate, and air pollution, especially fertilizing and acidifying nitrogen and sulfur containing pollutants.

Monitoring Lichens

Monitoring current conditions, tracking changes, and predicting ecosystem responses to changes in climate and air pollution can be an expensive and challenging task through instrumentation alone. Biological indicators such as lichens can provide an economical and practical means to maximize monitoring resolution, especially in remote areas. For more information on lichens and air quality monitoring visit the Lichen Data Clearing House.

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