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
INTRODUCTION “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
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.
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.
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. MATERIALS AND METHODS 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.
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 midday 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.
RESULTS
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 5 – December Fixed Point Photograph overlooking cliffs.
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.
Figure 6 - February Fixed Point Photograph overlooking cliffs.
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.
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
#
Figure 10 – Google Earth 2001 Historical aerial view of Porthtowan Dunes. Green dot resembles vegetation present. 127 Using
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.
1600 1400 Meters Squared
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.
1200 1000 800
Area
600
Poly. (Area)
400 200 0 2001
2005
2009
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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.
DISCUSSION
Sediment Characteristics
Short term Vegetation Monitoring
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.
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 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.
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.
AKNOWLEDGEMENTS 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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