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
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