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Summary Sand Dune Geo Factsheet
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Sand Dune Geo Factsheet based on the AQA a level environmental science specification in the biosphere module. Helpful for a level environmental science students
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G Feo actsheetSeptember 2001 Number 119
The Geography of Coastal Sand Dunes
Introduction
Sand dunes are simply accumulations of sand grains, shaped into Key:
mounds or ridges by the wind under the influence of gravity. Sand
dunes are found wherever loose sand is windblown: in deserts, on Sand dune
coasts, and even some eroded and abandoned farm fields in parts of
north west India and the south western United States. They tend to be
less well developed in tropical and subtropical zones due to lower wind
velocities and damper sand.
Until thickly vegetated, dunes are a very fragile environment, and heavy
use - through recreation, for example - may cause the break-up of the
roots, ultimately causing the destruction of the dune system. They are,
therefore, a very dynamic system. Dunes can be classified as true dunes
formed without vegetation, or dunes such as coastal dunes which are
formed in conjunction with vegetation. Ainsdale Sand
Dunes,
Formation of coastal sand dunes Sefton Coast
Coastal dunes are found where beach sands have been reworked by the (Case Study2)
wind. The predominant source of material for dune creation is the sea
bed and rivers which flow into the sea or through coastal areas. Macro-
tidal environments (large tidal range) combined with onshore winds
and winddriven currents provide the ideal conditions for dune
evolution. These are likely to form in zones of net sediment Braunton Burrows
accumulation. In some areas, such as East Anglia, complex sediment
(Case Study1)
transport systems can cause material which is eroded in one part of the
coast to be transported, and eventually deposited in another location,
sometimes fuelling the development of new dunes. Fig. 1 shows the Coastal sand dunes are diverse ecosystems, involving a complex
distribution of sand dunes in the UK. Note that sand dunes are found on interaction between plant communities (biotic) and environmental
all British coasts, but they are more sparsely distributed in the south and (abiotic) conditions. Sand dunes should be regarded as dynamic
south east. entities, with stages and interplays between building phases and erosion.
Their appearance is very variable depending on climate, underlying
Fig. 1 Distribution of UK sand dunes. relief and past and present management.
Succession and Zonation
The structure of the sand dune ecosystems is constantly changing. All
plant communities and environments are subject to periodic
disturbances, ranging from events that have only localised effects, such
as the fall of a tree, to those which have catastrophic effects, such as
huge storms which pound the coastline. Each new disturbance creates
an opportunity for a new species to colonise the area. New ‘invaders’
will also modify the character of the environment, creating an
environment that is suitable to other new species. By this process,
known as succession, the structure of the community evolves over time.
1
, The geography of coastal sand dunes Geo Factsheet
There are two different types of succession: primary and secondary. only a small number of species from surrounding habitats are capable of
Primary succession occurs in essentially lifeless areas - places in which thriving in a disturbed habitat and harsh environment. As new plant
the soil is incapable of sustaining life such as recently glaciated areas species take hold, they modify the habitat by altering such things as the
and newly formed sand dunes. Secondary succession occurs in areas mineral composition of the soil. These changes allow other species that
where an ecosystem that previously existed has been removed. This are better suited to this modified habitat to succeed the old species.
type of succession is typified by smaller-scale disturbances that do not These newer species are superseded, in turn, by still newer species.
eliminate all life and nutrients from the environment, for example the
burning of heathland vegetation on fixed dunes. Zonation refers to the variation of species or communities over a
particular area. For example, variations in salt spray, inundation and the
Primary and secondary succession both create a continually changing height and salinity of the water table will result in different species
mix of species within communities as disturbances of different occupying (or tolerating) different areas of the dune. In Fig. 3 (on page
intensities, sizes, and frequencies alter the landscape. The sequential 2) you can see the dune slacks.
progression of species during succession, however, is not random. At
every stage, certain species have evolved life histories to exploit the As succession is a slow process (and change in soil and vegetation often
particular conditions of the community (niches). This situation imposes takes many tens or hundreds of years at any one location), zonation is
a partially predictable sequence of change in the species composition of often used in sand dune studies to show how successions can work. This
communities during succession. Fig. 2 (on page 2) helps identify the assumption is simply based on the fact that the dunes nearest the sea or
process of succession in a sand dune system or psammosere. Initially coast are young and they become progressively older as the distance
increases inland.
Fig. 2 Schematic diagram showing the development of a dune system.Sand dune structure
While the nature of dune systems varies considerably both spatially and
over time, the following components may be present (refer to Figs 2
Sediment moved onto the beach
and reworked by wind, wave and and 3).
tide energy
1. The Strand Line (youngest dunes). Extreme high tides or storm
tides may leave a zone of several metres landward of the normal
Tidal litter Sandy beach highwater mark. Sand is an inhospitable substrate for plant growth –
it is dry, salty, unstable and lacks nutrients. Only highly specialised
plants – pioneers – can colonise such environments, e.g. Sea
Strand line Bare dry sand Rocket Cakile maritima and Prickly Saltwort Salsola kali. These
annual plants may form miniature dunes as sand gradually
accumulates around the plant body (which is able to grow upwards
Embryo dunes through the accumulating sand).
2. Embryo Dunes. Sand accumulation which persists above the high
Mobile dunes tide line may be suitable for colonisation by the first perennial
plants in dune succession which are specialised grasses, e.g. Sand
Blow out Couch Elytrigia juncea and Lyme Grass Lymus arenarius. Both of
Semi-fixed these grasses are able to grow upwards through accumulating wind-
dunes blown sand, and as a result low, hummocky dunes are formed. The
Dune slack substrate is still extremely inhospitable for plant growth.
Fixed dunes succession
3. Fore-Dunes (or mobile dunes). Upward growth of the embryo
dunes allows the dune surface to be raised so that it is out of reach
of all but the highest storm tides. Incursion of rainwater results in a
Dune heath
slightly less salty substrate so that Marram Grass Ammophila
arenaria is able to colonise and becomes the dominant species. This
remarkable plant is able to grow upwards through accumulating
Scrub sand at rates of up to 1m/yr. The dead leaves of Marram add organic
matter to the soil, increasing its water-holding capacity and
releasing plant nutrients. On the more sheltered landward site of the
Woodland frontal dunes, plants such as Sea Holly Eryngium maritimum and
Sea Spurge Euphorbia papalias become established. The gradual
amelioration of conditions results in increasing species diversity
and less bare ground.
4. White or Yellow Dunes (semi-fixed dunes). If conditions remain stable, mosses will cover bare sand patches between the Marram and plants,
such as Restharrow Ononis repens and Sand Sedge Carex arenaria, will become common. Species diversity should continue to increase.
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