Transcript Sand Dune Succession

Geography
Sand Dune Succession
A “psammosere” is the technical term for this
environment .
We are focusing on the landform rather than the
biological make-up of the dunes but some
knowledge of plant types is needed to explain dune
succession.
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What is needed for sand dunes to be
Critical
factors
for
sand
dune
creation:
created?
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Source of abundant sand
Relatively consistent wind
Water level fluctuation
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dune growth is accelerated by high water levels
Vegetation to foster sand accumulation
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plants trap and stabilize the sand
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Sand on the move
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There are three ways wind can move sand:
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Suspension
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Impact creep
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Moves the finest grains
1%
Moves the heaviest grains
4%
Saltation
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Moves intermediate grains
95%
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Beaches
Foredunes
Interdunal wetlands
or troughs
Back Dunes or Dune Forests
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Sand dunes are divided into different
ecological zones
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Vegetation to foster sand accumulation
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Pioneer grasses’ fibrous root systems grow rapidly, binding
sand together and stabilizing the dune
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Dune formation
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The sand moves until it meets a slight obstruction, such as a
clump of grass, which deflects the wind and allows the sand
grains to drop.
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Thus a slight mound or hummock is created.
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Wind moves sand up to the top of the pile until the pile is so
steep it collapses under its own weight and falls down the
leeward side called the slip face.
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Migrating dunes
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The collapsing sand comes to rest when it reaches just the right
steepness to keep the dune stable. This angle is called the angle of
repose and is usually 30-34 degrees .
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The repeating cycle of sand inching up the windward side to the
dune crest and falling down the leeward side allows the dune to inch
forward in the direction the wind blows.
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Aerial view of a sand dune system
youngest
dunes
oldest
dunes
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A transect across a sand dune system
X
Reveals variations in relief and vegetation ………..
Y
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A transect across a sand dune system
X
Y
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Sand dunes follow a set layout.
Copy this diagram to act as a key for your
notes.
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The beach ends at the strand line. This is the line
highest up from the water where the material
deposited by the sea is stranded.
The plants here are
tolerant of the harsh
conditions- strong winds,
sand constantly shifting,
salty spray, high
alkalinity. There is little
evidence of any soil.
Plants here are
pioneers;sea sandwort
sea rocket
saltwort.
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2
Embryo dunes are the smallest hummocks of
sand. They create a tiny area of shelter from
the on-shore wind. It is here that the first
pioneer plant may grow. They are very easily
destroyed.
The plants here are only
slightly better off than at
the strand. They have
still to put up with very
poor conditions. Soil can
only be distinguished using
chemical analysis.
Plants here are;sand couch grass
sea rocket
Lyme grass.
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Embryo and Fore Dunes: the plants
The plants which grow here have
adaptations which allow them to
grow in a difficult environment :
• waxy leaves to retain moisture
and withstand winds
• prostrate (low) habit
to avoid strong winds
• deep tap roots to obtain
available moisture
Sandwort
Frosted orache
• high salt tolerance
Saltwort
Sea rocket
Sea couch
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3
Fore dunes are starting to look like proper dunes,
though they are still very small. There is an
increase in the amount of humus in the sand, and
a little less saltiness in the environment.
The plants here are
modifying their environment
and a wider variety of
plants can be found here as
a result. Their roots are
helping to bind the sand
grains together and this
allows them to get more
water.
Plants here are;sand sedge grass
Sea holly
Marram grass.
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Yellow or main dunes are substantial dunes. They
can be up to 10m tall. There is a much higher
amount of humus, nutrients and it is more acidic.
The younger dunes create much more shelter.
The plants here are less
tough in many respects,
although it is still not a kind
environment. Marram grass
is the dominant plant, and
its long tap roots bind soil
and find water deep down.
Plants here are;sand fescue grass
Ragwort
Marram grass
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Yellow Dunes: the environment
Click on photo to reveal annotations
above the
level of high
tides
reduced
wind speeds
pH 6.5
%OM 0.23
‘Soil’ slightly
less alkaline and
more water
retentive
Some
humus
forming
Surface continually
blown away and
replenished with
fresh sand
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Yellow Dunes: the plants
The dominant plant species is
Marram grass:
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Salt tolerant
Thrives on being buried by sand
Inrolled leaves to reduce moisture
loss
Long tap roots
Underground rhizomes stabilise the
sand
Other plants such as Ragwort, Red
fescue and Sand sedge begin to appear
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Fixed dunes are often called grey dunes. They
are lower and more consolidated than the yellow
dunes. They have troughs between the ridges of
sandy soil- called slacks.
This area supports plants
that require more shelter
and more acidic conditions.
Most need more moisture,
and find it in the slacks;
more specialist plants
appear, like reeds,rushes
and creeping willow.
Plants here are;sand sedge grass
trefoil
buckthorn
heather
lichen
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Grey Dunes: the environment
Click on photo to reveal annotations
sheltered by higher,
seaward dunes
lower pH
little mobile
sand
sand no longer
accumulating
pH 5 - 6
higher humus
content
a more closed vegetation
community in which
marram grass is no longer
able to compete
%OM 1.0
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Dune Slacks: the environment
Click on photo to reveal annotations
occur in low lying hollows
between dune ridges
soil acidic
pH 4 - 5
relief
intersects the
water table
water table high –
especially in winter
%OM 8.5
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This is the last sere of the plant succession. The
land here is damp, sheltered, acidic, rich in
nutrients and humus.
The plants here are as
evolved as they are going to
get- unless man or storms
modify their environment
again
CLIMAX has been reached !
Plants here are
true pioneers;heathland plants
gorse
Ash, rowan,
alder, birch and
spruce trees
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Dune Heath/Woodland: the environment
Click on photo to reveal annotations
well sheltered
from winds
maritime influence
is minimal
acidic soil
nutrient rich
pH 4
soil has high organic
matter content
% OM 12.1
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BIOSPHERE
SUCCESSION
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Management of sand dunes
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Sand Dune Succession
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