mapping of lichens Maps of lichens distribution indicating air quality. Fruticose lichens (with branched structures well above the surface) are more susceptible to SO2 damage than foliose lichens (whose leaflike thallus lies nearly flat on surface) and both in turn are more susceptible than crustose lichens (which embed their tissue in the cracks of bark, soil, or rocks). The use of morphological lichen types as indicators of air pollution concentrations is well developed