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Keywords:

  • colonization;
  • disturbance;
  • fens;
  • hydrodynamics;
  • restoration;
  • riparian vegetation;
  • waves;
  • wetlands

Abstract

Question

In Dutch fens, species that colonize open water and induce the formation of floating peat mats have become rare. Many such riparian pioneer species occur predominantly on shorelines sheltered from the wind, whereas the majority of seeds tend to be deposited on exposed shorelines, as seeds are dispersed via wind-driven waves and currents. Do differences in germination and seedling survival between sheltered and exposed shorelines explain this difference?

Location

The fen reserve ‘De Westbroekse zodden’ (52 °10′ N; 5 °07′ E)

Methods

With a sowing experiment, the germination, seedling survival and overall recruitment of Berula erecta, Calla palustris, Comarum palustre, Glyceria maxima and Mentha aquatica were studied on sheltered and wind-exposed banks in eight fen ponds. Temperature, light availability, water level, wave impact, litter and seed deposition and vegetation height were recorded over 16 wk. The probability of washing away was quantified with small seed mimics. With a greenhouse experiment, we separately examined the effects of environmental differences between sheltered and exposed banks.

Results

In the field, compared to sheltered shorelines, exposed shorelines had a higher wave impact, higher light availability in spring and more litter and seeds deposited on them. In the greenhouse experiment, only litter addition decreased germination. This effect was overridden in the field, where the higher light availability on exposed banks increased germination. In the field, the number of seedlings decreased strongly over time, and eventual recruitment was determined by the degree to which seeds and seedlings were washed away by wave action. The probability of being washed away was highest on exposed shorelines (where waves were larger), which resulted in higher recruitment on sheltered shorelines.

Conclusions

The recruitment of colonizing species to fen pond shorelines is limited by the probability that seeds and seedlings may be washed into the open water. This process can eventually cause more successful recruitment on upwind or lee-side shorelines despite lower seed inputs there.