Invasive alien plants in riparian ecosystems: a study from two lowland rivers in Eastern Poland

Riparian ecosystems are biodiversity hotspots but are also highly vulnerable to biological invasions due to strong hydrological connectivity, recurrent disturbance and exposure to nutrient inputs. This study examined patterns of invasive alien plant species (IAS) occurrence along two lowland rivers in eastern Poland (the Biala Lada and Czarna Lada). Vegetation was surveyed at ten sites representing different land-use contexts using 150 phytosociological plots. Floristic diversity metrics, Ellenberg indicator values (EIVs), and a vegetation-based Index of Biotic Integrity (IBI) were used to characterise vegetation composition and ecological conditions. Logistic regression based on species-level Ellenberg indicator classes showed that invasive species differed significantly from non-invasive taxa in their ecological preferences. Species belonging to higher nitrogen and temperature indicator classes were more likely to be classified as invasive, indicating a stronger representation of eutrophic and thermophilous taxa within the invasive species pool. In contrast, soil reaction showed only a marginal relationship with invasion status. Sites with greater IAS occurrence were characterised by reduced evenness, increased dominance of ruderal taxa and stronger floristic homogenisation. The vegetation-based Index of Biotic Integrity (IBI) reflected variation in ecological quality across the studied riparian sites. The results suggest that invasive species in the studied river valleys are associated with distinct ecological preference profiles, particularly with respect to nutrient availability and temperature requirements. Field-based Ellenberg indicators and the vegetation-based Index of Biotic Integrity (IBI) describe variation in habitat conditions among the studied sites, while the species-level Ellenberg classes describe the ecological attributes of the local invasive versus non-invasive species pool; together these indicators provide complementary tools for characterising riparian vegetation and invasion-related patterns.