The inside of the Sierra de Guadarrama National Park forests host abundant lichens, upholstering the bark of the trees and certain shady enclaves. But it is in the rocks that form the mountains summits where we can enjoy the exceptional lichens diversity, being the lichens the dominant living organisms in these rupicolous communities. Their yellowish, greenish, gray, orange, brown or blackish petals, give color to the rocks, depending of the snow annual duration or the humidity level to which they are exposed.
The lichens are bodies constituted by a fungus and a photosynthetic organism, which is usually a green algae or cyanobacteria. They have a beneficial and complex mutual coexistence, the symbiosis, in which the fungus is responsible for the lichen shape and to create favorable conditions for the photosynthesis, and algae provide the carbohydrates needed for growth.
The lichens, as well as other groups of cryptogams (algae, fungi, and bryophytes), are living beings in general little known. They are present in almost all habitats in the National Park. According to their living environment, they can be divided into three large groups: the epiphytic lichens, which live in the bark of trees and shrubs, the earthlings, directly living on the ground, and the rock-dwelling ones.
The epiphytic lichens communities live in the Park distinct formations, and mainly in the forests of the mountain streams, the Pyrenean oak groves and in the pine forests of Scots pine. In the Pyrenean oak groves, the lichen Evernia prunastri dominates, while Pseudevernia furfuracea communities develop in the pine trees. In the shady areas of forests and next to the ground, live leafed lichen species which are very sensitive to pollution, as species of the genera Peltigera and lobaria.
Contrary to what we might think, on the summits rupicolous environments and their surrounding areas, takes place an astonishing lichens diversity. For example, in the Peñalara Massif, as a Sierra de Guadarrama summits representation, it has been noted the existence of about 200 different lichens species, which represents more than 50% of the cracked lichens flora classified in the Community of Madrid. This number is similar to the lichens flora from other European high mountain areas. Artict-alpine species -that is to say, the flora of the cold regions of the northern hemisphere- dominate with almost 40%. However, the under-represented group is the boreo-oceanic element, formed by species living in cold regions and North Atlantic Ocean. This reflects the Sierra de Guadarrama mountains continental character. Among the most abundant species that inhabit these summits we can highlight Rhizocarpon geograficum and Lecanora intricata, characteristic for its greenish-yellow color, and several species of the genus Umbilicaria, with a darker foliose upper cortex.
Due to its particular physiology, the lichens have a very slow growth, 0.1 to 0.5 mm per year. These low rates of growth and its pioneering nature, have allowed the use of some species as to evaluate, in cold and high mountain areas, colonization processes time indicators. Some of the Sierra de Guadarrama R. Geographicum bodies could easily exceed 200 years of age.