Question: In fen meadows with Junco-Molinion plant communities, falling groundwater levels may not lead to a boosted above-ground biomass production if limitation of nutrients persists. Instead, depending on drainage intensity and microtopography, acidification may trigger a shift into drier and more nutrient-poor plant communities.
Location: Nature reserve, central Netherlands, 5 m a.s.l.
Methods: Long-term study (1988–1997) in a fen meadow along a gradient in drainage intensity at different scales.
Results: Above-ground biomass increased only slightly over ten years, despite a lower summer groundwater table. The accountable factors were probably a limited availability of nutrients (K in the higher well-drained plots, P in the intermediate plots and N in the lower hardly drained plots), plus removal of hay.
Junco-Molinion species increased in dry sites and Parvocaricetea species increased in wet sites, presumably primarily because of soil acidification occurring when rainwater becomes more influential than base-rich groundwater. The extent of the shift in species composition depends primarily on the drainage intensity and secondarily on microtopography. Local hydrological measures have largely failed to restore wetter and more basic-rich conditions.
Conclusions: Acidification and nutrient removal, leaching and immobilization resulted in the succession towards Junco-Molinion at the cost of Calthion palustris elements. Lower in the gradient this change was reduced by the presence of buffered groundwater in slightly drained sites. To conserve the typical plant communities of the Junco-Molinion to Calthion gradient in the long term, further acidification must be prevented, for example by inundation with base-rich surface water.