Lichens have been studied as bioindicators of air quality for more than 100 years. A terrific presentation called Urban Lichens: Environmental Indicators from NC State researchers dives deep into worldwide studies of lichens as bioindicators of pollutants.
More recently, researchers are finding that when toxic emissions are under control in urban areas, other factors affecting lichen abundance and diversity come into play, like temperature and humidity level. What is the urban heat island-lichen interplay? The theory: lichen abundance and diversity on urban trees can be an indicator of the level of urban heat island effect but also an indicator of the success of urban heat island mitigation efforts, like tree planting. More research is underway.
In this fascinating article, Council member and horticulturist Laura Wyeth explores the truly enthralling biology of lichens—simultaneously vulnerable and cannily adaptive organisms. —Michelle Sutton, Ed.