Proceeding from the top to the bottom, the volcanic complex is home to pioneer cenosis (species grouping) that colonize the ash deposits and lava flows, starting from the expanses of Stereocaulon vesuvianum, a fruticose lichen typical of this area, which dominates the most recent lava flows unchallenged. The lichen entirely covers the Vesuvian lavas with a thick greyish mantle and colours them with a silvery grey that gives them silvery reflections on full moon nights. On the oldest flows, the Stereocaulon vesuvianum is flanked by vascular plants of pioneer vegetation, including red valerian (Centranthus ruber), helichrysum (Helichrysum litoreum), mugwort (Artemisia campestris).
The lava produced during the 1944 eruption, due to the phenomena of colonization and evolution of the vegetal cenosis ( groups), represent the most interesting biotope of the Vesuvius National Park. On these flows it is still possible to observe the most primitive pioneer cenotic species, composed exclusively of lichens and bryophytes. The resting period of volcanic activity, which has now lasted since 1944, has allowed the progressive settlement of more advanced phases.
The highly incoherent and coarse pyroclastic substrate, in continuous landslide movement, which characterizes all the slopes of the Gran Cono from about 900 m above sea level to the peak, determines drier environmental conditions.
The fumarolic activity, a sign on the surface of the vitality of the volcanic complex, manifests itself inside the Grand Cone formed by the last eruptive event of 1944. Inside the crater there is a subvertical sequence of lavas and scoriae that locally highlights the presence of fumaroles. These are characterized by temperatures of about 100 ° and emit mainly water vapor and carbon dioxide. Significant in these cases is the presence of the fern Pteris vicittata, a tertiary wreck rather rare, interesting because it is found in an environment not entirely favorable but where it is able to vegetate thanks to the heat released by the fumarolic activity.
The substratum allows the colonization only to a reduced number of species; besides the Stereocaulon vesuvianum, there are also species such as Artemisia variabilis, Centranthus ruber, Helicrisum itoreum, Rumex scutatus.
The high slopes of the Gran Cono, from the peak up to about 900 meters above sea level are covered with incoherent sands and lapilli, whose pioneer vegetation consists mostly of herbaceous perennial species in rapid evolution towards more complex stages with the presence of shrubby entities such as brooms Cytisus scoparius, Spartium junceum and Rumex scutatus.
