Tracing plant invasions in freshwater habitats: spatial and temporal insights from herbarium metadata

Biological invasions represent a major threat to freshwater ecosystems, yet their long-term spatial and temporal dynamics are often poorly documented. Herbarium data represent a valuable resource to reconstruct invasion patterns. This study aims to use herbarium specimen metadata to reconstruct the invasion histories of six invasive alien aquatic and riparian plant species in Italy, detecting spatial and temporal invasion patterns. We analysed herbarium specimens collected over more than two centuries from national and international collections, using standardized georeferencing procedures and spatial aggregation to reconstruct distribution patterns. Temporal dynamics were assessed using cumulative invasion curves and generalized additive models; spatial patterns were examined using time-sliced distribution maps. We found pronounced heterogeneity among species, ranging from long-established invaders with extensive spread and recent stabilization, to species still in early invasion stages or characterized by fragmented introductions. In several cases, invasion trajectories showed distinct lag, expansion and plateau phases, whereas others displayed limited or discontinuous spread. Northern and central regions were affected earlier than southern areas, reflecting both ecological drivers and sampling biases. These findings demonstrate the value of herbarium data for reconstructing invasion dynamics and highlight their relevance for early detection, risk assessment and management of invasive alien plants in freshwater ecosystems.