Check dam influence on vegetation species diversity in mountain torrents of the Mediterranean environment

Previous research has developed a method to assess the impact of check dams on riparian vegetation and related river bed sediment size with vegetation coverage and development. On the basis of this knowledge, this paper aims to investigate the effects on riparian species diversity induced by 32 check dams in four torrent headwaters of Southern Italy. Species richness and composition of riparian vegetation in the active channel were surveyed upstream and downstream of each check dam and in intermediate (less disturbed) sites. Furthermore, links between hydromorphological processes and vegetation diversity were analysed. The identified differences in species diversity seem to be related to the morphological adjustments of the channel after the installation of check dams. These fluvial morphological adjustments introduced variations to the flood depth and frequency of the riparian areas creating new riparian conditions. Higher species richness and evenness were observed upstream of check dams, where alien species prevail over native ones in new habitats on the sedimentation wedge behind the dam. Conversely, downstream of check dams, where the boundary of the inundated area moves down towards the thalweg (due to incision processes after check dam installation), riparian vegetation tends to assume a terrestrial character, losing some herbaceous species of the riparian environment. This research confirms that the discontinuity in the fluvial environment caused by check dams induces morphological adjustments that can be translated in new riparian conditions for the establishment of vegetation, affecting the ecological aspects and species diversity of riparian habitats.