When the vegetation has developed so that it forms a more or less
complete cover of the substrate, the dunes are said to be 'fixed'.
Although conditions for plant growth are considerably better than
at the start of the succession, the fixed dunes still represent
a stressful environment. The pH is still very high, drought is a
problem and nutrients may still be in very short supply. In addition
to these abiotic factors, the dunes may be affected by grazing or
trampling. A thin, brown, organic layer has, however started to
form at the surface of the soil. If the dunes are grazed, for example by rabbits or by the sheep
on Ainsdale Sand Dunes National Nature Reserve, a fixed dune grassland will develop. The most important grass is often Festuca rubra.
Together with a number of flowering plants, mosses and lichens,
fixed dune grassland can be a very species-rich vegetation type.
Species such as
- Ladies bedstraw Galium verum Wild thyme Thymus polytrichus
- Harebell Campanula rotundifolia
are typical of this vegetation. Marram still persists
in the grassland. Because of the lack of grazing, this vegetation
type has been quite uncommon on the Sefton Coast. In recent years
grazing by domestic stock has been re-introduced to the Ainsdale
area and rabbit populations are now more healthy again. Grazing
also encourages blowouts which are essential in maintaining the
dynamics of the dunes. Many of the specialist plants and animals
of the dune system require the bare sand environment provided by blowouts.
On the Sefton Coast, as with other dune systems
in North West Europe the rabbit population was severely affected
by a disease called myxomatosis which was introduced in the 1950s.
The absence of grazing pressure caused by the loss of rabbits allowed
a different type of fixed dune vegetation to develop. Large coarse
grasses and a woody plant
- Creeping Willow Salix repens
became dominant. Creeping Willow is able to outcompete the smaller,
slower growing plants that are specially adapted to survive the
grazing and environmental pressures of the species-rich fixed dune
grasslands. The resulting vegetation type has a higher biomass,
has less species than the dune grassland and contains species which
are more common and widespread elsewhere, for example
- False oats Arrhenatherum elatius
Under the Creeping Willow the surface organic layer of the soil
builds up more rapidly. In many places the lack of grazing pressure
allowed the establishment of tall woody plants to form scrub.
The high biomass of these fixed dune vegetation
types makes blowouts far less likely and the dunes start become
much more stable.