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Dune Slacks
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Most of the dune system is characterised by very dry conditions.
In large dune systems such as the Sefton Coast, however, there are
a number of dune slacks, wet and sometimes very large depressions
in the dunes. They are formed in two ways.
Primary dune slacks are formed on rapidly accreting coasts where
the top of the beach is cut off from the sea by a dune ridge forming
further out on the beach. This is occurring on the Sefton Coast
at Birkdale, a feature known locally as the green beach. Primary
dune slacks are long and narrow and parallel to the coast.
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The green beach at Birkdale.
A new dune ridge (left) is cutting off
a low section of the beach (right) that might eventually
form a primary dune slack.
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Secondary dune slacks result from a blowout, where erosion down
to the water table has occurred. If the erosion is extensive, a
large flat area of wet sand is exposed. Most of the dune slacks
on the Sefton Coast are secondary dune slacks and they are orientated
at right angles to the coast, parallel to the prevailing wind direction.
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Erosion to the water table to
form a
secondary dune slack
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The dune water table fluctuates annually
because of the differences in rainfall and evapo-transpiration between
the summer and winter months. Because of this, many dune slacks are
flooded during the winter period.
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Dune slack at Ainsdale during the winter
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The wet sand created by secondary dune slack formation
is colonised by plants and a succession occurs. This succession
on wet sand, however, is very different to that on the dry dunes.
A range of wetland plants are important and early vegetation can
be extremely species-rich with plants such as
- Sedges Carex spp
- Bog pimpernel Anagallis tenella
- Variegated horsetail Equisetum variegatum
- Early Marsh Orchid Dactylorhiza incarnata
- Marsh Pennywort Hydrocotyle vulgaris
- Water mint Mentha aquatica
together with a number of mosses and liverworts
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Dune slack vegetation at the early stages of succession
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Creeping Willow Salix repens,
is an important species of the dune slack habitat and begins to outcompete
these plants when it gets established in the later stages of dune
slack succession. In this situation the vegetation can consist of
only two major species, Creeping Willow and the moss, Calliergon
cuspidatum.
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Old, wet, dune slack vegetation
dominated by Creeping Willow
(here with no leaves in winter) and the moss Calliergon cuspidatum
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Older dune slacks may become drier
and the later stages of succession may lead scrub invasion and eventually
woodland. Grazing will maintain the diversity of more specialised
dune slack plants which are found at the earlier stages of succession.
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