A clump of rotting seaweed can
start the whole process of dune buildings as sand forms around it, plants take
root and Marram dunes build up inland. Several gently sloping sandy shores are
backed by sand dunes. As the tide ebbs more and more sand is dried out by the
sun and wind. Whenever on-shore wind blows faster than 10mph sand is driven
inland. Usually it accumulates in dunes which run at right angles to the
direction of the prevailing wind.
Fore-Dunes
The land ward movement of sand is
obstructed by the seaweed and refuse that accumulates along the high tide line.
A small fore-dune begins to build up. The humus derived from the rotting
seaweed is just enough to allow a few flowering plants to survive. Sea rockets,
prickly saltwort, sea beet, and in particular the two grasses sea couch grass
and lyme grass, all have creeping root systems that enable them to hand on
during on shore gales. Even so, the fiercest of these gales often entirely
obliterate the beginnings of the dunes.
Nevertheless, the time comes when
the weather is calm over a long enough period to allow sufficient sand and
humus to build up to give the main colonizing plant the depth of sand it needs
for survival This plant is the marram grass. The increasing depth of sand
eventually smothers the original colonists. As the main dune extends towards
the sea, another strand line of drift builds up and a new fore-dune develops.
Marram Dunes
The root system of Marram
progressively penetrates many feet deep while its stiff leaves more and more
effectively obstruct the landward blowing sand. More leaves appears as the
original plant sends out lateral roots; when a shoot is buried by sand it
produces a bud which pushes up another vertical shoot. Marram can spread up to
9am (30ft) sideways and 1m (3ft) vertically in one year. In this way the dunes
stabilize, rapidly increasing in height and width. To begin with the bare sand
between the Marram tufts is a habitat of such extreme temperature and moisture
ranges that no secondary plant colonizer can grow until enough humus from dead
bits of Marram an wind borne plant refuse has accumulated, and a season of wet
weather has enabled chance wind borne seeds to germinate. At this stage the
original fore-dune colonists reappear, along with sand sedge which binds the
surface with long shallow runners, and the tight rosettes of Portland and sea
spurge. When these plants die they humus to the sand which helps to contain
moisture and provides nutrients for another generation of more tightly packed
plants. The establishment of the thick dune vegetation (turf) is underway.
Adapting to Extremes
Dune plants have several features
which allow them to survive the gale force winds the drying heat, the lack of
moisture and the wind borne salt. Several have deep and extensive root systems
to reach the water underground and to anchor them securely. The scarlet
pimpernel, for example which also grows inland on non-sandy soil, develops much
more extensive roots than its inland counterpart and grows almost horizontally
to minimize the buffeting from the wind. The rosette plants too are protected
from the wind by hugging the ground closely. Sea-holly and sea bindweed have stiff
varnished leaves which help to reduce water loss and give protection against
abrasion by blown sand. The hairiness, even downiness, of many other plants
such as common stork’s-bill traps dew on their stalks and leaves.
Winter annuals such as chickweeds
and early forget–me-nots, thicken the turf. Their seeds germinate in autumn and
survive the wet winter as seedlings, to flower and set seed again before the
drought and heat of summer threatens survival. At this stage of the vegetation
rabbits begin to browse the tur; their droppings add to the supply of plant
nutrients. By now the Marram has almost completely disappeared because it can
survive only where bare new sand is plentiful. Mosses and lichens eventually
cover these landward dunes which look more-grey than yellow due to the amount
of grey dog lichen growing on them.
Where dunes meet land
The type of vegetation in the
final stages of the dunes depends largely on the quality of the sand. Where the
sand consists mainly of mineral grains such as quartz (and not lime-rich
shells) the humus remains too acid to support any but a limited flora. In this
case heather or heath (and sometimes both) dominate the mature dunes and may
even grow so far towards the sea that they mingle with the last surviving Marram.
Where the sand contains a lot of
finely ground shells the lime-rich pieces neutralize the dominant acidity of
coastlines, providing the right conditions for thick scrub to dominate the
dunes. Sea-buckthorn is very prominent in parts of north Norfolk, Kent,
Somerset and Glamorgan. Once the buck-thorn is established elder, blackthorn
and privet begin to be common
Dune Slacks
The bottom of the hollows between
dunes is often wet and small pools of water stand in them. The flora of these
slacks contains a group of plants not found together anywhere else. The
creeping willow may make a hummocky carpet. Marsh helleborine, wintergreen, bog
pimpernel and several species of fen and marsh orchid may be found growing side
by side. The natterjack toad still
survives in some of these slacks. When disturbed it has the distinction of
being able to defend itself with a secretion from its skin “smelling” like
burnt gunpowder, sulphur and boiled India rubber a defence not to be sneezed
at.
Hardy survivors
A number of small creatures can
survive among the Marram tufts. The temperature inside each tussock is cooler
than on the surrounding sand and the air is slightly humid. Some species of
spider cling to the stems while others burrow into the sand below. They feed on
flies which breed in the rotting vegetation of the strand line and are blown
into the dunes. The spiders in turn form the food of the rare and beautiful
sand lizard.
The camouflage colors pale
ochres, sandy greys and reddish browns of the few dune moths make them hard to
spot. Only a minimal supply of nectar is available for the adults. The shore
wainscot is confined to elusively on Marram, while the sand dart feeds on sea
holly and sea rocket. The long legged and long bodied sand wasps are
comparatively conspicuous, occurring mostly along the southern coastline.
Birds and Mammals
While numbers of spiders and
insects manage to survive the hardships of living in the dunes the poor plant
cover restricts larger animals to a few kinds. Birds are probably the most
visible of these. Terns, ringed plover and oystercatchers all nest along
shingle and at the high tide mark. Black headed gulls build their nests in
colonies in the dune vegetation and shelduck use rabbit burrows. Skylarks and
meadow pipits lay their eggs on the ground in mature dunes, where occasionally
the short-eared owl nests too. Rabbits are the most common mammals; they tunnel
burrows in the sand of the mature dunes and eat the well-established
vegetation. Foxes sometimes prey on the rabbits, even using their abandoned
burrows for dens. Hedgehogs, stoats and weasels feed on small birds, “birds”
eggs and young rabbits.
Blow-outs
No matter what its plant
composition the turf of sand dunes is thin and fragile. The passage of too many
human feet can break up the vegetation as they pass from car park to shore.
Given half a chance the wind will exploit any break. In no time a small hole is
a large blow-out. Great widths of dune become unstable, sand drives inland, and
the whole process of stabilizing has to start again.
How Dunes Build up.
Grey Dunes
Rabbit droppings fertilize the
ground small plants invade, ousting the marram. Grey lichens give these stable
dunes their color.
Marram Dunes
Marram grass spreads quickly
blocking the blowing sand more efficiently. Buried Marram can push up through
sand. Only few plant pecies can survive
the wind-blown conditions of fore-dunes. As Marram takes root the dunes form
large obstacles for the wind.
Fore-dunes
Seaweed and drift block the
blowing sand and small fore-dunes build up. Rotting seaweed allows a few
species of plants to take root.
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