Ecological implications of the replacement of native plant species in riparian systems: unexpected effects of Reynoutria japonica Houtt. leaf litter

In European rivers, one of the most invasive of terrestrial plant species, Reynoutria japonica Houtt. (Polygonaceae), is frequently found at the water-land interface. In this study, ecological effects of native species replacement by this invasive species was explored. In particular, we examined changes in leaf litter composition, and the subsequent impacts of their decomposition on riparian and in-stream plant community growth. We compared leaf litter from two riparian species, native Rubus fruticosus L. agg. (Rosaceae) and its spontaneous substitute along river corridors, R. japonica. Effects were assessed through phytotoxicity tests on (1) a terrestrial model plant species, the lettuce Lactuca sativa var. capitata L. (Asteraceae) and (2) a typical semi-aquatic riparian plant species growing in the same riparian zone, the watercress Nasturtium officinale R.Br. (Brassicaceae). Seedling root length measurement permitted calculation of a root length reduction index. The experiment was performed over a six-month decomposition period, with samples taken at five time intervals. Phytotoxic abilities of R. japonica were inconsistent over the decomposition period, leading to breaks in the regulatory processes of the structuring native species, R. fruticosus. Decomposition of R. japonica leaf litter led to the unexpected emergence of successive competition and facilitation interactions. This study showed distinct phytotoxic impacts from R. japonica leaf litter decomposition, causing ecological effects that differed from that of native species. In a context where R. japonica progressively replaces native riparian species, such impacts need to be considered further as R. japonica phytotoxicity does not steadily prevent other species with offset phenologies to establish, including invasive species. Graphic