Lichens of the Sierra Nevada have been little studied. A lichen is a composite organism consisting of a fungus (the mycobiont) and a photosynthetic partner (the photobiont or phycobiont) growing together in a symbiotic relationship. One classification of Sierra Nevada lichens is according to functional groups, by the National Park Service. These functional groups overlap with each other. These include forage lichens (eaten by animals), nitrogen fixers (can take nitrogen molecules from the air and attach them to other molecules), acidophiles (acid loving lichens), wolf lichens, crustose lichens on rock, crustose lichens on bark and wood, biotic soil crusts, aquatic lichens, other green algal macrolichens, and pin lichens (calicoids)”. These functional groups overlap. In this article, we include wolf lichens as a subsection of crustose lichens growing on wood. Forage lichens Forage lichens includes hanging, hairlike species that serve as food for animals and humans, including by Native Americans. Wila (Bryoria fremontii) is the most important species in this group. It is notable for its palatability because it lacks defensive chemicals commonly found in other lichens that protect them against being eaten (herbivory). It becomes more increasingly more rare moving south in the Sierras. It is one of the only food sources in the harsh winters of the Sierras, including for the northern flying squirrel (Glaucomys sabrinus) and Douglas squirrel (Tamiasciurus douglasii). Mule deer (Odocoileus hemionus) may also depend on it. Nitrogen fixers Lichens with cyanobacteria as a symbiotic partner (cyanolichens) convert atmospheric nitrogen into forms usable by plants and animals (nitrogen fixation). They are common in the relatively warm and dry Sierras, though less common than in cool oceanic climates. They are typically found on mossy boulders, rotting logs, trunks of hardwoods, and bases of trees in general. Common genera include Peltigera, Fuscopannaria, Collema, and Leptogium. Crustose lichens Crustose lichens grow flat against the surface that the colonies are growing on (substrate), typically rocks or wood. The plant grows tightly appressed to the substrate, and is very close to the substrates at all points, forming a biological layer. There is overlap between crustose lichens growing on rock with those growing on wood, as well as with lichens listed in other sections of this article. Among the most brightly colored Sierra Nevada crustose lichens are the bright yellow Pleopsidium flavum and Pleopsidium chlorophanum, and the orange Caloplaca trachyphylla. On rock Crustose lichen communities are part of the aesthetic appeal to