Transcript Sand dunes

Biosphere
Sand dunes
evolution of vegetation communities as
ecosystems to climax stages as demonstrated
by vegetation succession on coastal dune
belts, ie colonisation, the role of pioneer
species and a knowledge of plant names at
various stages in the succession
SQA
Biosphere Sand dunes
Biosphere Sand dunes Strand line/Embryo dunes
Plants growing at the back of the beach, together with flotsam and jetsam
dropped at the high tide mark eg seaweed, dead seabirds and driftwood,
cause the wind speed to drop locally. Sand being carried by the wind can
then fall at these points and begins to
build small sand piles. This now forms
another more substantial wind break,
which encourages more and more sand
to be deposited. The sand dune is
beginning to build.
Depending on wind speed, the dune
will either gradually move inland (high
wind speeds), or it will remain static. If
there is a good supply of sand, new
dunes may form to seaward (between
the dune and the sea).
Biosphere Sand dunes Strand line/Embryo dunes
These embryo dunes may disappear as quickly as they form but some may
eventually be colonised by plants which stabilise the sand. Conditions here
are very extreme:.
>highly alkaline pH values (over 8) from the calcium carbonate of the
shell fragments;
> rapid drainage (because sand does not hold water well) and the
desiccating effect of on-shore winds ;
> high salinity from frequent immersion in sea water;
> no humus for nutrients;
> high wind speed and lots of salt spray.
Consequently dunes may not grow higher than 1m
Biosphere Sand dunes Strand line/Embryo dunes
Species which are dominant in this
hostile environment are Sea twitch
grass and Sea rocket.
Sea rocket.
Sea couch grass can also survive
regular immersion in sea water.
Sea couch grass.
These early colonisers are very tolerant of salt and can therefore grow much
closer to the sea than most other grasses, and help to stabilise the embryo
dune and decay to provide humus for the plants of the next stage.
Biosphere Sand dunes Fore dunes
If the embryo dunes persist they can
be colonised by lyme grass, sea
couch grass and sea holly,
developing them into fore dunes. The
pioneers from the previous stage
have added humus to the sand,
Marram grass
making conditions more favourable.
These plants are drought-resistant (sand does not hold water) and capable
of withstanding burial by the shifting sand, as well as still significant
salinity levels. Falling levels of calcium carbonate (shells) reduce levels of
alkalinity. As they grow up through the sand, they help trap more sand and
so the dunes increase in height. Here the dunes may reach up to 5 metres.
Importantly, these improvements lead to the marram grass taking over
from sea couch grass. It thrives on the fresh supplies of sand and its long
root system (rhizomes) helps to stabilise the dune still further encouraging it
to build up further .
Biosphere Sand dunes Fore dunes
Lyme grass
Marram grass
Biosphere Sand dunes Yellow dunes
As the fore dunes continue to build up we now call them yellow dunes due to
the still large amount of sand content visible. These yellow dunes begin to
show a greater diversity of plants as conditions become more favourable.
As plants die and decay, a humus layer builds up and this traps both
fresh water and nutrients. The pH is now only slightly alkaline (about
7.5), there is more shelter and less salt spray. Salinity also drops with
distance from the sea. Marram usually still dominates the vegetation but
other plants are taking hold on the stabilised surface. Plants may include
creeping fescue, sand sedge, mosses, lichens, sea holly and sea
spurge.The dunes by this stage may well have reached between 5-10 metres
high. As much as 80% of the sand surface may now be vegetated.
Additionally, rabbits and other mammals may add their droppings to help
enrich the developing soil.
Biosphere Sand dunes Yellow dunes
Biosphere Sand dunes Grey dunes
The grey dunes are much more stable and mosses and lichens fill the few
remaining spaces between plants so that vegetation cover may reach 100%.
Marram grass becomes less common and appears now in isolated patches
due to less sand being blown onto the dune and other plants out-competing
it for water and nutrients. Small shrubs (brambles, gorse, buckthorn)
appear for the first time. There is shelter from the harshest winds, humus is
beginning to darken the surface layers and a true soil begins to form.
Soil pH is increasingly acid and heathers may take advantage of the acid
conditions. Water content is still low and plants have to search for water with
their spreading root systems. These large dunes are commonly 10 metres in
height and wider than those dunes nearer the shore.
Biosphere Sand dunes Grey dunes
Biosphere Sand dunes Dune slacks
The dune slacks are found in
between the grey dunes and
the more mature dunes,
where
the
water
table
reaches the surface causing
seasonal (and sometimes)
permanent waterlogging and
surface water. Plants which
are well adapted to these
damp,
sheltered
hollows
include rushes, sedges,
cotton grass and creeping
willow. If decay is slow, a
peaty soil may develop.
Biosphere Sand dunes Mature dunes
The most mature dunes are found several hundred metres from the shore.
High humus and water content allow these plants to succeed previous
vegetation. Left undisturbed these dunes develop a soil which can support
shrubs and trees including hawthorn, ash and birch. Eventually an oak
climax vegetation may develop.
Biosphere Sand dunes Mature dunes
Biosphere Sand dunes Pioneer stage
Both species have a waxy coating on the leaves which enables them to retain water.
The leaves are often rolled, which is another adaptation which reduces the leaf surface
area exposed to the atmosphere. This decreases the plant transpiration rates.
These will be some of the first plants to begin to stabilize new dunes with their networks
of root systems. By helping to bind the sand in one place, they produce a more stable
wind break, encouraging yet more sand to be deposited and building the dune still
higher.
However, as the amount of sand being deposited increases, these pioneer plants cannot
grow quickly enough to avoid being smothered. Where the rate of sand deposition is
greater than 30cm per year, they will be outcompeted by Marram Grass.
Marram Grass then takes over the process of binding the dunes.
This species is less tolerant of exposure to sea water and therefore occurs slightly
further away from the sea than the first pioneer species.
Its growth can keep pace with sand being deposited at a rate of up to 1 metre per year.
Each plant can also spread sideways by up to 3 metres a year, helping to trap the sand
and keep it in one place.
Biosphere Sand dunes White dune stage
Marram Grass stabilizes the sand some distance below the surface because of its deep
vertical root system and extensive horizontal root network.
The clumps of Marram Grass are continually buried, only to regrow up through the newly
deposited sand.
Under dry atmospheric conditions, the leaves of Marram Grass will roll up tightly,
exposing only the tough outer waxy cuticle of the leaf. This reduces transpiration and
water loss.
These later plant pioneers are more efficient at binding the surface sand, both because
of the cover they provide and because of their near surface horizontal network of roots.

Salinity decreases with increasing distance from the sea

Humus content

Soil acidity
increases slightly from colonisers
increases slightly due to less calcium carbonate

Plants e.g. Sea Holly continue to stabilise the dune, allowing the
key establishment of marram grass

sand and
Marram grass succeeds Couch grass. It thrives on fresh supplies of
it’s long roots stabilise the dune further and help the dune build up
Biosphere Sand dunes Gray dune stage
The dunes by now have a more or less continuous plant cover, effectively anchoring the
dune sand in one place. Where a series of dunes has formed at the back of the beach,
this stage will be on the dunes further inland.
In colder autumn and winter months much of the available water will come from dew
being deposited at night. Developing humus layers from decaying vegetation will help to
retain some of the moisture. Water will still be scarce in hot summer months.
As sand stops being deposited, Marram Grass will die out. Grey-green lichens
(Peltigera, Cladonia) will colonize the dune extensively, combining with accumulating
humus to give the dune sand a characteristic dirty grey colour.
Mosses such as Bryum and Sand-dune Screw-moss, Tortula ruraliformis, also colonize
the dunes.
The lichens and mosses become dry, hard and dormant when water is in short supply,
but as soon as it becomes available, they start to grow again. In high summer, the dried
lichens and mosses give the dunes a very crunchy texture, which is almost like walking
on crisps.
Where there are large quantities of pulverized sea shells included in the dune sand,
plants which thrive in lime-rich areas will become established.
Biosphere Sand dunes Gray dune stage
SEMI-FIXED DUNE

Once yellow dune over 10m high, less sand builds up behind it.
Marram grass dies out and forms thin humus layer.

Other plants able to grow here e.g. dandelions
Biosphere Sand dunes Gray dune stage
GREY DUNE / FIXED DUNE

Damper soils with higher humus content

More acidic due to leaching of carbonates by rain

Shelter increases,
so sand and salinity decreases
Wider variety of plants develop e.g. heather, grasses, lichens
Biosphere Sand dunes Gray dune stage
The dunes by now have a more or less continuous plant cover, effectively anchoring the
dune sand in one place. Where a series of dunes has formed at the back of the beach,
this stage will be on the dunes further inland.
In colder autumn and winter months much of the available water will come from dew
being deposited at night. Developing humus layers from decaying vegetation will help to
retain some of the moisture. Water will still be scarce in hot summer months.
As sand stops being deposited, Marram Grass will die out. Grey-green lichens
(Peltigera, Cladonia) will colonize the dune extensively, combining with accumulating
humus to give the dune sand a characteristic dirty grey colour.
Mosses such as Bryum and Sand-dune Screw-moss, Tortula ruraliformis, also colonize
the dunes.
The lichens and mosses become dry, hard and dormant when water is in short supply,
but as soon as it becomes available, they start to grow again. In high summer, the dried
lichens and mosses give the dunes a very crunchy texture, which is almost like walking
on crisps.
Where there are large quantities of pulverized sea shells included in the dune sand,
plants which thrive in lime-rich areas will become established.