Keywords:
dune slack
basiphilous vegetation communities
hydrology
calciumcarbonate
Contact: A.P. Grootjans, Univ. of Groningen, Lab.
of Plant Ecology, P.O. Box 14, 9750 AA Haren, NL, A.P.Grootjans@biol.rug.nl
./ F.P. Sival, Alterra, P.O. Box 47, 6700 AA Wageningen, NL, sivalfp@hetnet.nl
/ E.J. Lammerts, Mieddyk 2, 9036 LD Menaldum, NL, E.Lammerts@sbb.agro.nl.
Location: The Netherlands, Wadden Sea islands
Dune slacks on the Dutch Wadden Sea islands
One of the motives for ecological dune slack research can be found
in the great conservation value of dune slack pioneer vegetation.
Many rare and protected plant species occur in nutrient-poor,
moist to wet slacks, where the soil pH is buffered around neutral;
these species, among which are many orchids, rushes and sedges
are for example Schoenus nigricans, Epipactis palustris, Dactylorhiza
incarnata and Liparis loeselii. Wet dune slacks
are depressions in dune systems which are flooded during winter
and most of spring.
Almost all habitats of rare basiphilous plant species have changed
dramatically due to human interferences, especially in the western
part of the Netherlands. The main reason for this are: (i) fixation
of the dunes by pine plantations and decreased grazing by rabbits
or cattle, (ii) shifting of the coastline , (iii) drinking water
extraction and drainage, (iv) eutrophication by polluted infiltration
water used for drinking water extraction; (v) the influence of
acid rain. These impacts have resulted in dessiccation, acidification
and eutrophications of the dune slacks.
The aim of the studies were to identify the most important factors
controlling the occurrence of basiphilous pioneer vegetation in
dune slacks. The intention is, on the one hand, to specify some
key factors from the perspective of nature conservation and management
and, on the other hand, to gain insight into some ecological processes
concerning the pH and nutrient status of the habitat and the chances
of establishment of species on bare soil.
Observations
In the studies between 1980-1996 care has been taken to include
a large variety of slacks with respect to their position in the
dune area, their age, management and especially their geomorphological
origin. The two main types concerning the last aspect are primary
slacks, arising by the development of dune ridges which isolate
(partly or completely) beach plains from sea water influences,
and secondary slacks, hollows in the inner parts of a dune area
blown out to groundwater level. Both primary and secondary slacks
are well represented.
Results
Different pH buffering processes were found within the range preferred
by basiphilous species. In primairy slacks important were the
calcareous soils and inundation of brackish groundwater. In decalcified
secondary slacks inundation or capillary rise of calcareous groundwater
maintaned the pH due to specific hydrological conditions. In some
slacks even secondary precipitation of carbonates was an important
factor. The study showed that a high pH buffering capacity decreases
the accumulation of the organic matter content and prolonged
the longevity of basiphilous vegetation in dune slacks.
Dune management strategies to temper the negative effects can
be categorized as either measures affecting the above-ground biomass
only (mowing and grazing), or measures affecting both the above-ground
and below-ground parts of the vegetation including the soil profile
(sod removal and sand-blowing). Both measures contribute to the
removal of nutrients. Especially by sod removal the organically
enriched topsoil is removed, which reduces the nutrient pool considerably.
In addition, due to a lowering of the soil surface by sod removal,
inundation frequencies can increase, and this may stimulate the
occurrence of exfiltration of calcareous groundwater. Another
benefit of sod removal is a reduction in decalcification depth,
although in deeply decalcified soils, the pH buffering capacity
is not restored by this management practice. Sand-blowing reduces
the acidification rate by the input of calcareous sand. A stimulation
of natural processes such as the above, in addition to the formation
of new dune ridges enclosing primary slacks, may be a solution
to preserve endangered dune slack species for the future.
References
- Sival, F.P. 1997. Dune soil acidification threatening rare
plant species. Thesis Groningen University. (18.4.13).
- Lammerts, E.J.. 1999. Basiphilous pioneer vegetation in dune
slacks on the Dutch wadden Sea islands. Thesis Groningen University.
- Grootjans, A.P., Ernst, W.H.O., Stuyfzand, P.J. 1998.
European dune slacks: strong interactions of biology, pedogenesis
and hydrology. TREE13: 96-100.
