Damming effects on upstream riparian and aquatic vegetation: the case study of Nazzano (Tiber River, central Italy)

Abstract

Key-words:damming, man-made wetland, hydrogeomorphic change, diachronic analysis, river vegetationIn the Mediterranean region, the majority of watercourses have undergone hydrogeomorphic alterations, mainly due to flow regulation such as damming. Despite this, few investigations analyzing dam impact on upstream river vegetation exist. In this study, we analyzed the temporal vegetation changes occurring along the Tiber River in Nazzano (Rome), as a consequence of the construction of a dam in 1956. We examined which communities are more sensitive to this anthropic alteration of the river ecosystem. An evaluation of how and with what timing these communities varied their extension as a result of such alteration is provided. Moreover, the role of the hydrogeomorphic alterations on the evolution of the river vegetation, linked to the management of this dam as a run-of-the-river impoundment, was analyzed. Our results show the main vegetation changes occurred during the first decades after the dam construction. Indeed, the dam operation caused significant local expansion of the upstream river waters, causing the flooding of most of the surrounding lands, and the formation of new sub-lentic wetlands. The altered hydrogeomorphic conditions favored the development of natural riparian and macrophyte communities typical of lacustrine ecosystems and therefore mostly different from those expected for the analyzed river typology. The changes in extension over time of these vegetation communities highlight their close relation to the alteration of some hydrological parameters especially affecting the river water level. RÉSUMÉEffets d'un barrage sur la végétation riparienne et aquatique en amont : l'étude de cas de Nazzano (Tibre, Italie centrale) Mots-clés :barrage, zones humides artificielles, changement hydrogéomor-phologique, Dans la région méditerranéenne, la majorité des cours d'eau a subi des modifications hydrogéomorphologiques, principalement causées par la régulation de l'écoulement comme dans le cas de barrages. Malgré cela, peu d'analyses de l'impact du barrage sur la végétation de la rivière en amont existent. Dans cette étude, nous avons analysé les changements temporels de végétation se produisant le long du Tibre à Nazzano (Rome), comme conséquences de la construction d'un barrage en 1956. Nous avons recherché quelles communautés sont les plus sensibles à cette altération anthropique de l'écosystème de la rivière. Une éva-luation de comment et à quel moment ces communautés ont varié leur extension analyse diachronique, végétation lotique à la suite d'une telle modification est fournie. En outre, le rôle des altérations hydrogéomorphologiques sur l'évolution de la végétation de la rivière, lié à la gestion de ce barrage modifiant les écoulements, a été analysé. Nos résultats montrent les principaux changements de la végétation survenus au cours des premières décennies après la construction du barrage. En fait, le fonctionnement du barrage a provoqué l'expansion locale significative des eaux en amont de la...

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