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INTRODUCTION TO SAND DUNES
Origins and definition
Sand dunes
develop when there is a supply of sand grains which are mobilised by the wind.
Typically in coastal location they depend on there being a sufficiently wide
beach which dries out at low tide allowing the sand grains to be picked up and
blown inland. The dune grows as the mobile sand grains come to rest in areas
where wind velocities are reduced in the lee of obstacles on the beach, open
dune ridges, or more typically in temperate regions, around specialist plants.
The most important of these is Ammophila arenaria
(marram grass) which is the most obvious species in the early stages of succession
on many dunes. Over a period of time other plants invade and full successional
development may be attained when the dune grassland/heathland turns to scrub
and ultimately woodland, though this latter stage is very rare in Great Britain.
The extent to which calcareous dune grassland develops depends on the inherent
calcium content of the sand grains which make up the system. Where the calcium
content is low dune heath is the main vegetation type. On the oldest systems
leaching
of the calcium from the surface layers of the dune in lime rich soils will also
result in the development of dune heath. Dune slacks are also an important component
occurring at or near the dune water table, which may be strongly domed.
Traditional management
Sand dunes have been managed
as far back as Neolithic times. Historically the most common use of dunes has
been the grazing
of domestic stock. On calcareous systems this has helped to create a rich native
flora and fauna similar to that of calcareous grasslands. As the dune develops,
other factors come into play, and under the influence of rainfall the vegetation
may change from calcareous dune grassland to acid dune heath as the nutrients
are leached
from the surface soil.
The Guide includes a section on the management and conservation of machair
(a special type of sand dune). The traditional land use is linked very closely
to its high nature conservation value.
Reasons to restore
Sand dunes have suffered considerably from habitat
loss throughout Europe. Over-grazing which when combined with other
human uses including use as rabbit warrens, cutting of thatch (marram) for housing
or can create highly unstable structures. In its turn this led to eroding dunes
overwhelming farms, farmhouses, villages and churches, which resulted in large-scale
stabilisation programmes, notably by afforestation.
The combined effect has been to greatly reduce the area of open dune landscapes.
Traditional management practices such as grazing
and the demise of the rabbit populations have led to many remaining open dunes
becoming overgrown
adding to the loss of dune grassland and heath. The traditional management has
been replaced by a great variety of other activities including recreation and
recreational development and the siting of airfields, housing and industry.
All of these uses have led to an overall decrease in natural and semi-natural
dune habitats.
States and values
Three states have been identified for sand dunes based on the degree of stability
of the dune.
- State
1 - Erosional dunes are those where the proportion of bare sand
is high and where sand movement is such that little vegetation can develop
and land and infrastructure is threatened by blowing sand;
- State
2 - This state includes a range of dune types with dune grassland
and heath in open or semi-stable dunes. These may or may not have significant
areas of mobile sand within them. However by virtue of the mosaic of open
communities they support a wide range of plants and animals. Their value if
often sustained by grazing management and/or periods of instability;
- State
3 - At the other end of the spectrum lie those dunes which are
considered to be over-stable.
This third state can take two forms:
- Dunes where grazing and other pressures, which help sustain the open dune
vegetation have been reduced or removed such that they become overgrown;
- Areas where deliberate planting (of scrub and trees, afforestation)
has resulted in the development of a closed canopy and the loss of most of
the open dune vegetation.
Guidance (routes to restoration)
Restoring
dunes can involve processes which help to increase stability on unstable
sand dunes (State
1 to State 2). This involves a variety of well proven methods
of restoration and include many 'soft' engineering approaches. Other
methods employed are not always as appropriate (as for example introducing 'hard'
engineering structures). It is also true that an over zealous approach
to stabilisation can (and has led) to loss of interest especially for nature
conservation. Reversing the process is possible but can be fraught with difficulties,
especially when the removal of mature forests (deforestation)
is contemplated. The reintroduction of grazing
animals is a well tried technique for restoring vegetation.
Several important elements need to be assessed before deciding on a change
in management which increases the dynamic
of the dune. Foremost amongst these is an understanding of the geomorphological
context of the dune system. Wind direction (prevailing or dominant), climate
(exposure and geographical position), sediment budget and sea level change are
also key factors in determining the form and dune dynamic.
Equally significant is the need to have an historical perspective on the influence
of grazing
animals and human actions. The latter include information on past
control measures for sea
defence. Armed with this information predictions can be made of the
likely consequences of a particular course of action and its cost. Whilst the
information need must be met locally (and be site specific) there are a number
of more general text books which provide important background reading (see below).
Key publications:
References
Carter, R.W.G., Curtis, T.G.F. & Sheehy-Skeffington, M.J.
1992.
Coastal Dunes. Geomorphology, Ecology and Management: Proceedings of the Third European Dune Congress, Galway, Ireland, 17-21 June 1992. A.A. Balkema, Rotterdam, 533 pp.
Doody, J.P.
1985.
Sand Dunes and their Management.
Focus on nature conservation, 13
NCC, Peterborough.
Packham, J.R. & Willis, A.J.
1997.
Ecology of Dunes, Salt Marsh and Shingle. Chapman & Hall, London, 335 pp.
Pye, K. & Psoar, H.
1990.
Aeolian Sand and Sand Dunes. Unwin Hyman Ltd., London, 396 pp.
Ranwell, D.S. & Boar, R.
1986.
Coast Dune Management Guide. Institute of Terrestrial Ecology, NERC, 105 pp.
Ranwell, D.S.
1972a.
Ecology of Salt Marshes and Sand Dunes. Chapman and Hall, London, 258 pp.
van der Meulen, F., Jungerius, P.D. & Visser, J.H.
1989.
Perspectives in Coastal Dune Management. SPB Academic Publishing bv, The Hague, 333 pp.
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