From meadow to shallow lake: Monitoring secondary succession in a coastal fen after rewetting by flooding based on aerial imagery and plot data

Abstract

Year-round flooding can be a cost-effective measure for rewetting highly degraded fens, and is gaining popularity for lowland fen restoration in Europe. We investigated the short-term effects of such permanent inundation on species composition and spatial distribution of the vegetation of a formerly drained coastal fen, and addressed the question of whether re-establishment of peat-forming reed vegetation is foreseeable. For vegetation mapping and monitoring we combined permanent plot data acquired during four years following shallow flooding, high-resolution aerial imagery and an elevation model. Five vegetation types were distinguished, and we analysed their spatial distribution and succession patterns throughout the years. Pre-existing vegetation, its spatial arrangement and the water level played major roles in secondary succession. Existing patches of Phragmites australis showed high stability, but their growth was not consistent through the years and at all inundation depths. Existing stands of Bolboschoenus maritimus were outcompeted by Schoenoplectus tabernaemontani or vanished and formed relatively stable ponds of open water with hydrophytic species. We concluded that the expansion of reed as peat-forming vegetation is likely to proceed slowly, but fluctuations in water level and edge effects will probably maintain a persistent mosaic of vegetation and open water in the near future.

Keywords

• fen restoration
• image classification
• Phragmites australis
• vegetation mapping