Questions: Were continued groundwater discharge and mowing regimes sufficient for vegetation preservation from 1944 to 1993? Which has a stronger effect on vegetation development; groundwater discharge or mowing? What is the role of surface water eutrophication as driver of vegetation change?
Location: Het Hol, The Netherlands (ca. 92 ha, 52°13′ N, 5°05′ E).
Methods: Hydrology was simulated for the late 1940s, early 1960s and 1987. Vegetation maps (1944, 1960, 1975 and 1993) were compared for biotope cover. Vegetation recordings in 1944 and 1987 were compared. Surface water quality was compared between 1950 and 1987. Which sites were mown was reconstructed from an interview. Effects of periodic mowing and groundwater discharge on vegetation development were tested for correlation.
Results: Biotope diversity reduced significantly through decrease of semi-aquatic and tall-herb biotopes, and expansion of forest. The quagfen terrestrialization sere nearly disappeared from 1987 recordings, while the reed sere did well concerning abundance and species richness. Several typical (rich) fen species disappeared from recordings, while new species were mostly field margin species. Periodic mowing and discharge combined are correlated with increasing species numbers. The P-concentration in surface water increased while N-concentration decreased.
Conclusions: Preservation of the reed sere was successful, whereas preservation of the quagfen sere was not. Periodic mowing and discharge stimulate species richness, discharge more so than periodic mowing. But slight eutrophication likely induced a shift from P-limitation to N-limitation, which stimulated the reed sere at the expense of the quagfen sere.