The nature of the soil limits the development of typical Maquis shrubland groups where altimetric and climatic conditions are appropriate. The populations are limited to scattered and circumscribed groups of the main species that characterize the maquis at these latitudes, consisting of evergreen sclerophyll shrubs such as the Phillyrea (Phillyrea latifolia), myrtle (Mirtus communis), arbutus (Arbutus unedo), heather (Erica arborea), laurel (Laurus nobilis), viburnum (Viburno tinus), alaterno (Rhamnus alaternus), lentisk (Pistacia lentiscus), cyst (Cistus spp.) and the brooms. The lowest layer of the Maquis shrubland is represented by perfumed plants such as rosemary (Rosmarinus officinalis), sage (Salvia officinalis) and oregano (Origanum vulgare). In most cases, however, the herbaceous vegetation and pioneer groups are followed by wide brooms, which take over as soon as the environmental conditions become suitable to host shrub species, and which give a characteristic appearance to the slopes of Vesuvius, especially in spring during flowering, there are several species of broom, including the broom of coalmen (Cytisus scoparius), the fragrant broom (Spartium junceum) and the broom of Etna (Genista aetnensis).
Holm oak forest are formations in which the holm oak (Quercus ilex L.) is the predominant element and which, both in terms of physiognomy and floral composition, enclose the main aspects of the evergreen Mediterranean broadleaf forest. They are distributed all over the Vesuvian area, but are mainly found on the southern slopes of Vesuvius, where the substrate, made up of coarse scoriaceous fragments, is very permeable and the holm oak forests are the potential vegetation. The typical structure of a holm oak forest has an arboreal layer, a shrubby one, a herbaceous one, a muscinal one (mosses and lichens) and a thick bedding in which mushrooms are widely represented. The arboreal plane, between 6 and 15 m high, is often monospecific and only occasionally other species of
deciduous type enter its composition, such as Fraxinus ornus (Orniello), Ostrya carpinifolia (Carpino nero), Quercus pubescens (Roverella). The shrubby layer is varied and diversified, composed mainly of brooms, lance (Ruscus aculeatus), viburnum (Viburnum tinus).
Particular and significant examples of this type of wood are constituted by rocky holm-oaks, present on the almost vertical walls that are exposed to the south of the caldera of Mount Somma. In these xerophilous formations there is a clear reduction in the height of the vegetation and an increase in the less exigent pioneer species such as Artemisia variabilis, Centranthus ruber, Scrophularia canina, Spartium junceum.
