Lichens are long-lived and can be monitored, field conditions permitting, in any season. Many lichens have extensive geographical ranges, allowing study of pollution gradients over large areas. These properties make them useful for spatial and temporal evaluation of pollutant accumulation in the environment. Epiphytic lichens (those that grow on trees or plants) are often best suited to the study of air pollution effects on lichen communities, lichen growth or physiology, and to the study of pollutant loading and distribution. Because they lack roots and are located above the ground, epiphytic lichens usually receive greater exposure to.
