Species-specific fragmentation rate and colonization potential partly explain the successful spread of aquatic plants in lowland streams

The vegetative spread potential of aquatic plant species is largely based on the quantity of dispersed plant fragments (propagule pressure) and their potential for regrowth and establishment, i.e., fragment regeneration and colonization. In streams, fragment dispersal is of particular significance as the exposure of plants to flow facilitates fragmentation and downstream drift of fragments. We conducted field investigations to quantify the relevance of fragment dispersal and the species-specific propagule pressure due to fragmentation in five small to medium-sized German streams. These field surveys were combined with determination of the potential for regeneration/colonization of fragments collected in the field indicated by relative root formation under standardized conditions. In general, the number of drifting fragments tended to increase with larger stream size. We documented species-specific differences in fragmentation rate, which contributed to weak correlations between the number of drift units and specific plant cover within four streams. The overall likelihood for root formation increased significantly with increasing fragment size and was highest for the invasive Elodea nuttallii (70% of fragments). We conclude that the fragment dispersal capacity in streams is highly species-specific and that propagule pressure alone cannot explain the successful spread of invasive species like Myriophyllum heterophyllum.