Responses of protected groundwater‐dependent vegetation communities to prolonged flooding along a lowland stream

Many groundwater‐fed habitats, including alkaline rich fens, depend on finely balanced hydrological and geochemical conditions that are increasingly altered by climate extremes and catchment pressures. Understanding whether vegetation responses are driven by flooding itself or by the sediments transported during floods is critical for predicting habitat resilience and guiding conservation under the EU nature restoration regulation

We conducted a 6‐year field experiment in three alkaline rich fen sites, simulating prolonged flooding, flooding combined with deposition of natural sandy sediment and flooding combined with deposition of organic‐enriched sediment from an agricultural catchment. Vegetation was monitored over short‐, second‐year and long‐term phases to assess effects on community composition, trait‐based response variables and indicator species of alkaline rich fens.

Flooding alone had relatively weak and context‐dependent effects on the vegetation, with contrasting responses among fens. In contrast, flooding combined with sediment deposition was the primary driver of vegetation change. Natural sandy sediment induced strong but transient shifts, whereas organic‐enriched sediment produced the strongest and most persistent changes, including sustained declines in indicator species of alkaline rich fens.

Trait‐based responses revealed distinct underlying mechanisms. Flooding alone caused little evidence of directional functional change, whereas natural sandy sediment induced only transient shifts consistent with physical disturbance and organic‐enriched sediment increased Ellenberg N values, canopy height and competitive traits associated with eutrophic conditions.

Synthesis and application : Our results demonstrate that the ecological consequences of flooding in alkaline rich fens depend strongly on the quality of deposited sediments. Nutrient‐rich agricultural sediments can accelerate eutrophication‐driven reassembly of alkaline rich fen vegetation. Conservation efforts should therefore prioritize an integrated catchment‐to‐fen management approach that reduces nutrient‐enriched sediment delivery through soil‐conservation practices, vegetated riparian buffer zones and restoration of natural channel morphology, while safeguarding the groundwater‐fed hydrological conditions required to sustain species‐rich alkaline rich fens.