Mind Even the First Steps: Colonization by an Alien Perennial Grass, Paspalum Distichum Poses a Threat to the Native Floodplain Vegetation in Hungary

At the advanced stages of invasion, the control of invasive alien species (IAS) often exceeds the capacity of the agencies responsible for it, so early detection is vital for success. This is particularly urgent when perennial grass species become established in wetlands. Here we present a case study on knotgrass (Paspalum distichum L.), an alien perennial grass species first detected in Hungary in 2020, in a floodplain of the Danube River at Dunavecse. The study aims to make predictions about the invasion behaviour of knotgrass by combining trait-based approaches and risk assessment tools. In the place of its first detection in Hungary, we compared the species richness, biomass, and seed production of the native and the invaded vegetation. We found that knotgrass formed monodominant stands characterized by extremely high above-ground living biomass with a mean dry weight of 3,670 g/ m 2 . The invaded stands were characterised by significantly lower plant diversity compared to the non-invaded controls. The above-ground biomass was almost three times higher in invaded plots than in native vegetation. There were no significant differences between the seed yield of knotgrass and the dominant species of native vegetation. All three risk assessment systems classified knotgrass into the highest risk category implying high possibility of its future invasive spreading and becoming a transformer species in Hungary. There are a lot of further potentially suitable habitats for the knotgrass connected with the already occupied site, and the high propagule pressure and strong competitive ability also suggest a high probability of future expansion.