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I NT RO D U CT IO N
“A sand dune can simply be classified as a hill or
ridge piled up with sand” (Pye et Tsoar 1987). Sand
dunes are formed when a wind speed of 10 mph
or stronger picks up small grain particles and
deposit them when resistance is met or obstructed
by an obstacle.(Cros et Serra 1993). Sand dunes
are a mobile habitat; over time as vegetation
colonizes an area of dune land it becomes more
static. Many topographic obstacles such as
boulders, escarpments and hills have the ability to
induce zones of airflow acceleration, deceleration
and enhanced turbulence.(Gaylord and Dawson,
1987)
The formation of a sand dune is mostly related to the vegetation on the plains
area and topographic
Obstacles. (Cros et Serra). The process by which vegetation colonizes a sand dune is called succession. Successions can further be classified into four types – psammoseres, lithoseres, hydroseres and haloseres. A sere refers to a specific type of vegetation succession. A Psammosere is a community that started its life on exposed coastal sand. (Agate, E 2005). A Lithosere is a community that began its life on an exposed rocky surface. (Codrington, S,B. 2005). A hydrosere is a community that began its life in water. (Offwell Woodland & Wildlife Trust 1998). A halosere is a community that develops within a saline environment, for example a salt marsh. (Collard, R 1998) Succession can be further divided into primary or prisere and secondary or subsere, differentiating to when or where on a dune it occurs. Primary succession occurs first, this colonization occurs
A study investigating changes in
vegetation on a manmade
Psammosere (Porthtowan)
MAX ADCOCK
Falmouth Marine School, Killigrew St, Falmouth, Cornwall, TR11 3QS.
Abstract
Marram grass, Ammophilaarenaria plays a very important role in stabilising dune systems. They have
extensive root systems that help bind the sandy sediment together, making conditions more favourable for
less ‘hardy’ plants to colonize. Porthtowan sand dune is located on the North coast of Cornwall, it is an
artificial dune which was planted in the 1970’s. The dune is heavily exposed to human activity through
tourism and has restricted growth from residential and commercial buildings. Fixed point photography was
performed at 4 different locations using a high powered digital camera. Beach profile transects were
performed at three different locations along the dunes lateral structure. Photographs taken over a period of
six months were used to correlate vegetation cover into a phase 1 habitat survey.
Keywords: Psammosere, Vegetation cover, Phase 1 Habitat survey
where no soil or vegetation that has previously existed.(Bishop, V 1997).The term given to a species that is the initializing species is a pioneer species; these are usually extremely adapted organisms and are very resistant to harsh environmental and human activity. (Harper et al 1961) Succession within an ecosystem is the number of changes that occur to the ecosystems community over a period of time. Primary succession takes place on a surface where no soil or vegetation has formerly existed. (Skinner, Redfern et Farmer 2003). Marram Grass (Ammophilaarenaria) is usually one of the first species to settle on a dune system. It is highly adapted and its leaves are rolled along the vertical length of the plant. This adaptation prevents moisture loss through the underside of the leaf. Also, its long roots dig in deep to find water found far below the surface. The roots also act as a binding system, giving it grip and securing small sediment grains together.(Rae, A 2008). (Horwood 1919) states that marram grass requires algae and lichen to start the nutrient cycle off, suggesting that marram grass is not a ‘True Pioneer species’. However, marram grass is not adapted for life in stable dunes, and is easily out competed with lichens, grasses, mosses and shrubs. (Prosser, R 1997). As plants increase in number and size, sand becomes stabilized. Over time, ecological colonization leads to changes in the substrate chemical makeup allowing new species to invade.(Brooks, A 1979). As plants die off over time, they change the sandy sediment into more coarse acidic earthy sediment, making conditions more 'favourable' for more field ‘pasture’ species to colonize.(Agate, E 2005) Once succession has initiated, over time it will
develop into continuously less biologically adapted
plants. However, drifting sand can cause either
advances or retreats in the succession of dune
plants. (Kumler 1969)Dunes are also susceptible to
damage from extreme weather events and human
interaction that can also contribute to hindering
succession or accelerating it.
Sand dunes are a delicate mobile ecosystem; they
are unlike hard stable rocky shorelines. Dunes
contain hinterland shrubbery, predominantly a
coastal species that is not found in any other
location in Britain. (Brookes, A 1969). Small
changes introduced by man or the natural
environment often contribute to deterioration or
acceleration of the coastal environment. Victoria
Bishop, (1997), states that the most seaward dune
ridges are the most unstable. They are influenced
and modified by coastal storms, high tides, or
breaks in marram cover. Robert Prosser (1997),
suggests that once an area of dune is exposed to
wind, depressions called ‘blow-outs’ can occur.
Once a blow-out has occurred, wind is quick to
sweep into the fault of the dune ridge and
drastically alter its shape.
This project will focus on Porthtowan, which is a
small village located on the north coast of
Cornwall, it has a unique dune ecosystem in
comparison to its natural neighbouring dune;
Gwithian. Porthtowan is an artificial dune which
was seeded in the 1970’s. It is a small dune
ecosystem, enclosed by two cliffs running
horizontal either side of the dune border. Human
activity and the seasonal tourist activity suggest
that the dune will never meet its climatic
community. Its growth is restricted by residential
and commercial buildings. Over the past 30 years
the once healthy dune system is now retreating, it
can be classified as a declining dune ecosystem.
Figure 1 – Location of study area on google map -
Porthtowan.
Figure 2 – Location of Porthtowan.
Human trampling and weather events have led to
a blowout developing, running right through the
centre of the already unstable fore-dune. Using
Carters blow-out model, you can apply the case
study of Porthtowan and evaluate that the dune is
in stage B of the blowout cycle, where the wind is
eating down until it meets resistance. In addition
to the blowout, trampling and weather conditions
have led to a decrease in the abundance of
marram grass.
Usually the typical management strategy for
restoring a dune, is to attempt to restore the dune
to its natural processes (Rooney 2010), however
this is unachievable at Porthtowan. The dune is too
restricted by human activity; it is unlikely that it
would be possible to restore the dune system by
leaving it to its natural processes, because it is an
artificial dune with far too many human inputs.
There has been some previous management put in
place at Porthtowan, (cornwall.gov.uk). The dunes
were once fenced off, sediment trapping nets
were placed to prevent sand loss, and a designated
path was established, to keep trampling to a
minimum. Unfortunately the path was destroyed
by a storm. This led to the remaining debris of the
path being bulldozed away, which could have
contributed to the large blowout that is located
near where the path once was placed.
This project will focus on vegetation change over a
6 month period, whilst my study peer Dafyd
Propert-Lewis will focus on change in topography
over a 6 month period.
M A T E R I A L S A N D M E T H O D S
The joint Nature Conservation Committee released
a guide on how best to monitor sand dune health
over a period of time in August 2004. The common
standards monitoring guidance recommends that
firstly once a site is chosen. All historical
information should be gathered and evaluated
where possible. Aerial photographs from a fixed
location over a period of time can be compared,
and plotted on GIS Software. (Dargie, 2000). The
JNCC recommends that all field studies should be
compared to the ‘Sand Dune Survey of Great
Britain’ (Radley, 1994) which contains many
records of previously surveyed dunes.
The JNCC’s recommended biodiversity assessment
is also linked in with aerial photos to give a
baseline to any scientific field study, plotting
vegetation cover onto simple GIS software which
can then be compared to survey databases such as
(Dargie, 2000).
There are varying degrees of accuracy for
measuring and mapping a dune system, ranging
from simple transects using ranging poles and a
clinometer. (Department of Education 2010) On
the other end of the scale there is extremely
accurate three dimensional laser mapping
technology. (Nagihara, S., Mulligan, K. R. and
Xiong, W. (2004)
There were two main methods used by myself and
my peer to be able to monitor successfully over
the 6 month period. Sampling was decided to fit in
with the end of the tourist season at Porthtowan
to give any indication whether the intense human
interaction with the dune in summer is having any
direct linkages with sediment movement or
vegetation development.
Fixed point photography for visual representation
of vegetation change was taken from three
different locations around the sand dune and one
was taken from the cliffs overlooking for an aerial
view. Photographic surveys were carried out in
December, February and April.
Figure 3 – Google Earth map demonstrating F.P.P
locations.
The camera used was a high powered digital
camera, photographs were taken as close to mid-
day as possible to ensure the best lighting
conditions. Photographs were then used to plot
vegetation cover directly onto a gridded map, to
evaluate change over time.
Three beach transects were conducted
intersecting with the dune at three different cross
sections of the dune, to monitor sediment
movement in December, February and April.
Figure 4 – Google Earth map demonstrating three
transect locations.
Data was gathered by use of ranging poles and a
clinometer from mean low water to establish
change in height of sediment.
Data can then be correlated into graphs in order
compare change in height over the 6 month
monitoring period.
The fixed point photography showed very little
change in vegetation over the 6 months. The
communities on the dune stayed relatively stable,
however the sand movement was fairly dramatic.
The project was adapted halfway through to
examine historical aerial photography to gather
more information to suggest what activity is
occurring with the vegetation. The fixed point
photography did not show any change on the
marram grass communities on the frontal dunes,
but demonstrated an increase in the blowout, and
between February and April there is an increase in
grass communities behind the frontal dune.
R E S U L T S
Figure 5 – December Fixed Point Photograph
overlooking cliffs.
Figure 6 - February Fixed Point Photograph
overlooking cliffs.
Figure 7 – April Fixed Point Photograph
overlooking cliffs.
Figure 8 – Google Earth 2001 Historical aerial view of Porthtowan Dunes. Green dot resembles vegetation present. 171
Figure 9– Google Earth 2005 Historical aerial view of Porthtowan Dunes. Green dot resembles vegetation present. 136
U s i n g
Using Google Earths historical data, the
investigation was able to examine three aerial
photographs from the years 2001, 2005, and 2009.
A grid was placed over the map, and each vector
within the grid relates to an area of 8 x 8 m².
Using a marker on each grid to show an indication
of vegetation, it allows vegetation cover of the
entire dune system to be estimated and compared
to the different years.
In 2001 there were 171 squares of vegetation
cover. 8 x 171 = 1368 m².
In 2005 there were 136 squares of vegetation
cover. 8 x 136 = 1088 m².
In 2009 there were 127 squares of vegetation
cover. 8 s 127 = 1016 m².
The general trend of vegetation coverage from the
year 2001 to 2005 was a steep drop. From 2005 to
2009 the decline slows dramatically.
#
Figure 11 – Graph demonstrating the different
vegetation coverage over 8 year period.
The frontal dune was dominated by mainly
marram grass; the area behind the dune has very
stable communities of grass, in a six month period
the grass actually increases in coverage, which is
surprising considering the sand input from the
widening blowout.
0
200
400
600
800
1000
1200
1400
1600
2001 2005 2009
Me
ters
Sq
uar
ed
Year
Area
Poly. (Area)
Figure 10 – Google Earth 2001 Historical aerial view of Porthtowan Dunes. Green dot resembles vegetation present. 127
D I S C U S S I O N
Sediment Characteristics
Over the 6 month monitoring period, there were
noticeable changes in the dune structure. Daffyd’s
project highlights in detail that the frontal dune is
not retreating. His investigation demonstrates the
dune is losing sediment. The dune overall is lower
down and the blowout is increasing in size. Where
the sediment is being transferred to opens up
room for a further investigation. The council runs a
sediment relocation scheme to keep access roads
clear of potentially hazardous sediment build up.
The start of the project looked at the damage done
to the dunes over the busy summer period, the
trampling cause’s sand to become dislodged and
allows it to be picked up by wind and relocated
elsewhere in the dune system. April’s photograph
demonstrates the human use on the frontal dune.
The blowout is made up purely of soft sand, and is
exposed to the sun throughout the whole day,
making it an ideal location for beach users to
sunbathe.
April’s fixed photograph clearly demonstrates that
the sediment is moving away from the frontal
dune to the colonized area of vegetation behind
the dune. There is increased % cover of sand in
comparison to grassland.
Long term Vegetation Monitoring
Visually there is a clear indication of vegetation
decline over the period of nine years. The frontal
dune shifts back by a considerable distance from
2001. The marram grass communities on the front
thin out and become very patchy. There are many
factors that may have contributed to the decline in
marram, environmental and human influence. The
lack of stable marram could explain to some extent
the lowering of the dune sediment. With no
vegetation to keep the sediment bound together,
sand is exposed to high wind speeds that have
enough energy to dramatically change the dune
topography.
The thinning vegetation on the frontal dune could
also explain the increasing amount of sand input
onto the access roads, and onto the grassland
habitat behind the dune system.
Short term Vegetation Monitoring
There was very little change in vegetation over the
6 month period of monitoring. This could have
been due to the lack of tourists over winter;
however there was still a considerable amount of
trampling. Although there was little change in
vegetation communities it is worth noting that
there was a considerable change in dune
topography.
The dune became much flatter and the blowout
had retreated further back, encouraging a build up
of sand behind the frontal dune, making it a much
more gradual decline in height leading onto the
built up grasslands.
The grasslands behind the dune in April show that
a large amount of sand had been deposited
directly onto an area of built up vegetation.
In future investigations, it would be practical to
gain access to direct aerial photographs, to directly
compare data accurately through habitat surveys,
in comparison to estimating vegetation cover and
plotting onto a rough aerial map.
Additionally it would be recommended to monitor
individual communities on the frontal dune, to see
if the microhabitats are as dynamic as the whole
artificial dune.
A K N O W L E D G E M E N T S
We would like to thank L.Hockley for her
assistance in planning this study, aiding us in
developing rigorous methodology and her
invaluable experience in the field. We would also
like to thank the Falmouth Marine School for
lending us equipment over the 6 month period to
gather our data.
.
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F i g u r e s
Figure 1 – Google map UK
Figure 2- Google map Porthtowan
Figure 3- Google Earth map
Figure 4- Google Earth map
Figure 5- FPP Dec
Figure 6- FPP Feb
Figure 7- FPP April
Figure 8- Habitat survey Dec
Figure 9- Habitat survey Feb
Figure 10- Habitat survey April
Figure 11- Comparison graph