Seed deposition in drift lines: Opportunity or hazard for species establishment?

Abstract Arrival at suitable sites for germination and establishment is crucial for hydrochorous plants. Seed deposition in riparian landscapes occurs mainly within litter in drift lines. We investigated whether conditions within drift lines are suitable for seed survival and germination and whether drift lines act as seed sources or sinks for species diversity in riparian fen grasslands. We analysed drift litter in a small river valley in Northern Germany (Upper Eider valley) after typical late winter floods in 2000, 2002 and 2004. The content of viable seeds in drift lines was studied via a germination trial with drift litter samples taken from five fen grassland sites representative of the vegetation of floodplain. Further, seedling recruitment of both abandoned and moderately grazed fen grasslands affected by drift line deposition was compared with equivalent sites without drift lines. Additionally, the decline of viable seeds after the deposition of drift litter was studied. Finally, the relative importance of seed-containing drift litter and microsite availability for seedling recruitment was investigated in a field experiment with the factors disturbance (mowing, large gaps, small gaps, undisturbed control) and addition of (seed-containing) drift litter (yes, no) in an abandoned riparian fen. Seedlings which recruited from drift litter collected at different sites differed in species composition. A nearly 50% reduction of the number of viable seeds in drift litter was observed already during 2 weeks after deposition. Naturally deposited drift lines had a positive effect on the number of seedlings, although an increase of seedling recruitment in areas with drift litter was only observed at the grazed site. Adding drift litter reduced the number of seedlings in the field experiment, while creating gaps enhanced seedling recruitment. These results indicate that in riparian fen grasslands, drift lines can affect species diversity either negatively by hampering germination and/or establishment or positively by increasing seed availability. As the studied river system is comparatively species poor and flooding extension is limited, the role of drift lines as seed source might be of minor importance in the study area.