How do geomorphic characteristics affect the source of tree water uptake in restored river floodplains?

Alpine rivers and their floodplains have been highly modified by human activities during the last decades. River restoration projects aim to counteract these negative impacts and to restore ecosystem services provided by riparian habitats. We studied two recently restored river sites in the Ahr/Aurino and Mareit/Mareta Rivers (Italian Alps) to investigate how geomorphic conditions, soil moisture, and groundwater level affect the source of water used by grey alder (Alnus incana (L.) Moench). We compared the isotopic composition (delta H-2) of tree sap at different locations (low terraces formed during bed incision and recent floodplains formed after restoration) with that of potential water sources, that is, groundwater, soil water, and rainfall. The monthly variation in the isotopic composition of rainfall was reflected in both shallow and deeper soil water, as well as in the isotopic composition of sap. The redistribution of precipitation and groundwater in the soil differed between the post-restoration floodplain sites and the post-incision terraces, leading to a different relation between the sap water, soil water, and groundwater isotopic composition. The results show that transpiration of A. incana trees growing on recent floodplains is mostly supported by stream-fed soil water, whereas trees growing on terraces mainly use precipitation-fed soil water. These marked, morphology-related differences in the source of transpiration water of grey alder highlight how channel degradation still affects the ecohydrological processes in Alpine fluvial corridors. Nonetheless, large restoration interventions-in terms of channel widening-can enable the self-formation of new floodplain areas characterized by stream water-fed riparian ecosystems.