Diachronic analysis of variations induced on the flora and vegetation of river ecosystems by actions taken to reduce the risk of flooding. Case study of the River Chienti (central Adriatic, Italy)

We present here a diachronic comparison of the ora and vegetation of a stretch of River Chienti, in central Marche (Italy). We assess the impact of mechanical cleaning of the river on its ecological structure and biodiversity. The rst survey was carried out in 1993, and then repeated in the same areas and with the same methodology in 2005, following heavy and repeated hydraulic maintenance of River Chienti. This process included excavation of gravel from the river bed and its accumulation on the river banks, with the objective to broaden and deepen the ow channel. In some places that were under strong anthropic in uence and subjected to continuous erosion, this process was accompanied by work on the protection of the river banks using various materials, to reduce the risk of ooding. In connection with this, it should be noted that the mouth of River Chienti has been de ned as an area at very high hydro-geological risk according to the Speci c Basin Plan (Piano Stralcio di Bacino) for the Hydrogeological Assets of the Basins of Regional Importance (PAI), as approved by Resolution N° 116 of the Regional Council, of 21 January, 2004. This work has resulted in the disappearance of lateral branches of the river that had still or slow- owing water, and mechanical removal of the vegetation of the river bed, and in some instances, also of the river banks. These actions have removed the wet ats at the sides of the main channel, which represen- ted favourable habitats for the establishment of different types of helophytic and hydrophytic plant communities according to the main ecological gradients. These actions that were designed to secure the river banks have led to an upheaval in the vegetation of the alluvial terraces closest to the water course, with profound alterations to the structure of the communities that had developed in the area, resulting in some cases in their disappe- arance. The oristic comparison shows the loss of 95 (69.34%) entities in 2005 in comparison to those surveyed in 1993. The oristic entities that have been lost in the area are mainly those related to helophytic and marsh environments, such as Zannichellia palustris, Nasturtium of cinale and Glyceria notata. On the other hand, particularly invasive exotic entities have appeared, such as Robinia pseudoacacia, Amorpha fruticosa, Arundo donax, Cuscuta scandens subsp cesatiana, Paspalum distichum and Helianthus tuberosus, along with species from the elds or from the surrounding marshy areas, such as Helianthus annuus, Brassica nigra, Rumex crispus and Tripleurospermum inodorum. Analysis of the vegetation has shown a decrease in the plant communities both of the river bed and the river banks, which have been reduced from 15 (in 1993) down to 10 (in 2005). The following associations have disappeared: Zanichellietum palustris, Nasturtietum of cinalis, Helosciadetum nodi ori, Bolboschoenetum maritimi, Glycerietum plicatae, Typhaetum latifoliae, Sparganietum erecti, Saponario-Artemisietum verlotorum, Ranunculetum repentis, Aro italici-Alnetum glutinosae and Salicetum incano-purpureae. The vegetational types that have disappeared can be attributed to the classes Potametea pectinati, Phragmito australis-Magnocaricetea elatae, Agrostietea stoloniferae, Alnetea glutinosae and Salicetea purpureae. The class that has suffered the greatest number of losses is Phragmito australis-Magnocaricetea elatae. Many of the communities that are no longer found have high ecological value, such as Zanichellietum palustris, Nasturtietum of cinalis, Helosciadetum nodi ori, Bolboschoenetum maritimi and Glycerietum plicate.