Relations Between River Plant Richness in the Portuguese Floodplains and the Widespread Water Knotgrass (Paspalum Paspalodes)

The distribution and invasive status of the amphibious plant Paspalum paspalodes (water knotgrass) are observed. The P. paspalodes distribution clearly shows that it is an invasive plant established in Portugal, which has never before been recorded as invasive in Portugal. The purpose of our study is to determine the various stages of invasion of this plant on a large biogeographical scale, in five basins or groups of floodplain river basins belonging to five Portuguese regions covering the principal floodplains of the south and centre of the country. The composition of the river plant community in terms of structure and species richness was obtained from a data set of 401 sites. In some cases, water knotgrass could be a threat for particular river flora and consequently for the integrity of the river system. In other cases, it occurs in highly degraded places, where other taxa considered as invasive plants and/or indicators of degraded ecosystems are present. The pattern of invasion of P. paspalodes is discussed in relation to risk assessment for the other river plants and compared to the North European invasion processes of P. paspalodes. We conclude that the evolution of the distribution of this predominant Portuguese water plant has to be carefully followed in some of the studied floodplains, as well as its North European distribution limits. In-depth survey should be carried out to relate the current biogeographical changes to the hypothesis of indication in climate changes