Phytosociological and phytogeographical analysis of the fragmented Alnus glutinosa riparian forest in the Thyrrenian district (Latium, Central Italy)

Alnus glutinosa stands are common in riparian forests along the main Italian peninsular watercourses. In many sites of central and southern Italy, it is possible to observe interesting forest fragments not yet completely compromised by human alterations. The study pointed out the importance of the central Tyrrhenian sector of the Italian peninsula, both in floristic and vegetational terms, and highlighted the phytogeographical role of the deep valley as a significant habitat for the concentration of rare species. In the present paper, the structural, coenological and syntaxonomical features of the forest fragments occurring along the main and minor watercourses of northern volcanic Latium (Tolfa district) are described. Here, and similarly in the whole territory of the Tuscan and Latium Maremma, there are various populations of rare species on a regional and national level (Osmunda regalis, Blechnum spicant, Polystichum setiferum, Dryopteris filix-mas and Athyrium filix-foemina) that emphasize the gorge habitat with well-preserved riparian coenoses. The presence of many interesting species allowed us to assign the investigated communities to the Polysticho-Alnetum glutinosae (Osmundo-Alnion), a syntaxon rich in hygrophilous ferns that shows a strong resemblance with other alder woods with ferns described in various sites of the western Mediterranean area.