The most “primitive” pioneer cenosis (“species groupings”), composed almost exclusively of lichens and bryophytes, are confined to the most recent lavas; among the very frequent lichens present is Stereocaulon vesuvianum, with its typical filamentous and greyish appearance, which almost entirely covers the lavas, giving them a characteristic appearance. This species plays a decisive role in the early stages of disintegration of the most recent lava surfaces.
Lichens are plant groups, composed of a fungus and an algae that live in symbiosis generating a distinct morphological and physiological unity. In symbiosis, the fungus grows around or penetrates the algae. In general, the algae that become part of the lichens are unicellular or filamentous (green algae or Cyanophycee), while the fungi are mostly ascomycetes; they have very different forms that depend mostly on the structure of the fungus: are known lichens crusted, leafy, fruticose. The fungus provides water and mineral salts to the algae while the latter provides the fungus with the organic molecules it needs. Thanks to this advantageous symbiosis, the lichens are able to survive extreme living conditions, succeeding in colonizing even very different and inhospitable environments and allowing the subsequent settlement of other species. Because of their disintegrating action, they open the way to the erosive action of atmospheric agents and ice. Their sensitivity to pollutants and their ability to absorb substances from the atmosphere make them excellent indicators of environmental quality.
