Lichen abundance was selected as a measure of stress in forest ecosystems because lichens are well-established bioindicators and have been used for monitoring air pollution globally for many years. They are particularly sensitive to changes in their environment. Lichen species vary in their sensitivity to pollutants, and as a result by monitoring the assemblage and abundance of different species, we can obtain a measure of the prevailing air quality. This monitoring work is performed in Kejimkujik once every 5 years using field observations from trained Lichenologists.