Reciprocal interactions between the native Mentha aquatica and the invasive Ludwigia hexapetala in an outdoor experiment

Abstract The invasion of aquatic ecosystems by non-native species often leads to competitive interactions with native species. However, non-native and native species can co-occur. This study presents a comparison between two wetlands species differing in origin: the invasive plant Ludwigia hexapetala (Lh) and the native plant Mentha aquatica (Ma). Both species were grown for three months in monocultures and in mixtures at different planting ratios (6Ma/0Lh, 5Ma/1Lh, 3Ma/3Lh, 1Ma/5Lh; 0Ma/6Lh). We assessed species performance in an experimental, outdoor garden. The shoot length, and flower production were measured weekly. At the end of the experiment, root and shoot biomasses were determined, the number of lateral branches was counted, and shoot lengths were measured. Based on biomass, two competition indices were calculated: the Relative Yield Total (RYT ab ) index, and the Relative Competition Intensity (RCI) index. The RCI index suggested a facilitation effect for Mentha and for Ludwigia at the ratio of 1Ma/5Lh, whereas competition was established for both species at the ratio of 3Ma/3Lh. Both the total biomass of L. hexapetala and its production of lateral branches were significantly reduced when grown as a monoculture, suggesting intra-specific competition within L. hexapetala populations. The number of flowers produced by L. hexapetala was highest in the presence of a high ratio of M. aquatica individuals. However, when L. hexapetala outnumbered M. aquatica , its ratio of below/above-ground biomass decreased, indicating an inter-specific competition effect. The intra-specific competition for L. hexapetala was much stronger than interspecific competition.