The effect of legacy sediment removal and floodplain reconnection on hydrochory in urban streams

Riparian plant communities are affected by regional and local processes. Regionally, urbanization and increased impervious surface cover (ISC) amplify propagule pressure from exotic plants and increase streamflow. Locally, increased runoff leads to stream downcutting, disconnecting streams from water tables, reducing overbank flooding, and limiting hydric vegetation on floodplains. Stream restoration efforts, like legacy sediment removal and floodplain reconnection, aim to lower floodplains, slow flows, and enhance nutrient cycling and sediment deposition, potentially boosting hydrochoric seed deposition. We assessed urbanization and restoration effects on hydrochoric seed dispersal in unrestored and restored streams in Baltimore County, Maryland. Restored streams had lower banks, more frequent inundation, and higher abundance and richness of water-dispersed seed than unrestored streams. At high levels of urbanization (> 20 % ISC), watershed ISC was not correlated with hydrochoric seed dispersal or the proportion of exotic seeds dispersed on either unrestored or restored streams, however, watershed ISC was positively correlated with flooding frequency in restored streams. These findings suggest that ISC levels in this study exceed a threshold beyond which their influence on hydrology and exotic seed dispersal diminishes, and that stream restoration alters local conditions but does not mitigate watershed ISC impacts on hydrology. Notably, inundation frequency and subsequent seed deposition were highest on restored streams with bank elevations below 0.5 m, emphasizing the importance of designing low, wide floodplains—an approach that may be challenging in heavily urbanized areas.