1. BiosphereBiosphere
Sand dunesSand 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
3. Sand dunesBiosphere 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).
4. Sand dunesBiosphere 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 ;
> no humus for nutrients;
> high wind speed and lots of salt spray.
Consequently dunes may not grow higher than 1m
> high salinity from frequent immersion in sea water;
5. Sand dunesBiosphere Strand line/Embryo dunes
Species which are dominant in this
hostile environment are Sea twitchSea twitch
grassgrass and Sea rocketSea rocket.
Sea couchSea couch grass can also survive
regular immersion in sea water.
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.
Sea rocket.
Sea couch grass.
6. Sand dunesBiosphere Fore dunes
If the embryo dunes persist they can
be colonised by lyme grasslyme grass, sea
couch grass and sea hollysea holly,
developing them into fore dunes. The
pioneers from the previous stage
have added humus to the sand,
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 grassmarram 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 .
Marram grass
8. Sand dunesBiosphere 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 fescuecreeping fescue, sand sedgesand 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.
10. Sand dunesBiosphere 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, buckthornbrambles, 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.
12. Sand dunesBiosphere 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,rushes, sedges,
cotton grass and creepingcotton grass and creeping
willowwillow. If decay is slow, a
peaty soil may develop.
13. Sand dunesBiosphere 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 hawthornhawthorn, ashash and birchbirch. Eventually an oakoak
climax vegetation may develop.
