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Succession on Portstewart dunes

Tree Autecology and Biology

Temperate Forest Ecology

Tropical Forest Ecology

Forestry

Sand Dune Ecology

East Usambaras

Pitcairn Islands

Binggeli, P., Debost, M., Eakin, M., Gilbert, R., Hamilton, A.C., Nevin, C. & Wilson, C.

Appendix 5 in Binggeli (1992)

Introduction

Portstewart dunes are situated on the north coast of Ireland, between the River Bann and the sea where the beach is 2.5 km long. They were formed by an aeolian sand accumulation on a Holocene gravel storm beach resulting in unimpeded drainage. Three phases of dune formation are represented in the eastern part of the dunes, the earliest dating from about 5300 BP and two post-dating the Bronze Age. The western end near the Bann estuary consist of shallow aeolian sands which extend to the higher ground above Portstewart. By 4000 BP it would appear that the dunes had stabilised (Carter & Wilson 1991). The present day dune morphology is dominated by high and anomalously orientated transverse (North-South) ridges.

The site has a long history of human occupation but until the early part of the 19th century the impact of man is not known. However, the exotic rabbit (Orictolagus cuniculus L.) is likely to have been introduced during the 12th century and at the latest by the 17th century the Portstewart dunes became a rabbit warren. The OS Map of 1830 shows that the Portstewart dunes were managed as a rabbit warren.

By 1900 the eastern end of the rabbit warren had been reclaimed for housing and agriculture. Some of this land and a large area of natural dune was used for recreation in the 1920s when the Portstewart Golf Course was created. The northern and western parts of the dunes (ca. 79ha) was acquired by the National Trust in 1981.

Methods

Temporal changes in areas of bare sand and sea buckthorn (Hippophae rhamnoides) were estimated from vertical areal photographs taken in 1949, 1963, 1975 and 1989, and their respective cover was estimated using a leaf area machine. Because of the large amount of shadows on the 1949 aerial photograph a certain degree of error is likely in the cover estimates for that year. All sea buckthorn clumps recognized on the aerial photographs were checked for their existence in the field.

Every tree and shrub at the exception of elder in sea buckthorn thickets and sea buckthorn were located and mapped. The age of some woody plants was estimated by ring counts.

Two transects through sea buckthorn were set up to investigate woody plant succession. The transects were laid out in two sea buckthorn patches. These patches were chosen as representative of the apparent successional changes and to include presently expanding stands of sea buckthorn. Transect A was located in a dune slack and B on a gently sloping dune. In December 1989, each transect was surveyed from the edge to the centre of the clump. The position and height of all woody plants within a 1m band for sea buckthorn and 2m for other species, extending along the line of the transect were recorded. A section was taken from the base of each stem and their age subsequently estimated by ring count.

Results

From aerial photographs it was estimated that in 1949 areas of bare sand covered about 31% of the Portstewart dunes and decreased to 7.5% in 1989 while sea buckthorn cover increased from 1.6% to 13.1% during that time (Table A5.1).

Table A5.1. Changes in bare sand and sea buckthorn cover (in % of total area) from 1949 to 1989 at Portstewart.

Fig. A5.1a,b shows that in both transects elder (Sambucus nigra) regenerated below sea buckthorn with an average timelag of about 15 years. In Transect B, three Solanum dulcamara also became established below sea buckthorn. In the two transects, the maximum height recorded for sea buckthorn is c. 3.5-4m. Elder grows beneath the sea buckthorn canopy, but eventually usurps it to reach a height of 5m. Elder reaches canopy height c.15 years after establishment and subsequent increment growth is small due to its intolerance to above-canopy exposure (wind and salt damage).

Discussion

Table A5.1 clearly shows a recovery of vegetation on Portstewart dunes since the 1960's with bare sand cover decreasing from 31% in 1963 to 7% in 1989. All published evidence (e.g. Knowles 1887, May & Batty 1947) suggest that the dunes have had a very large amount of sand mobility since the mid-19th century with a possible worsening during the first half of 20th century.

There must have been shorter periods of very severe disturbance or slight vegetation recovery as populations of rabbits have fluctuated greatly. Such variations in rabbit populations and particularly population crashes occur during periods of low food supply and extreme weather conditions (drought and long cold spells). Other factors will have also affected the degree of instability and include changes in wind climate and possibly blow-out development following events vegetation dieback and soil exhaustion as suggested by Jungerius et al. (1981).

Our evidence of bare sand changes from 1949 to 1989 and the historical record do not support Oldfield et al. (1973) and Carter's (1975) view that in the Portstewart dune 'trampling' pressures have caused the die-back of vegetation followed by selective wind erosion. They conclude that recreation pressure and related paths and tracks increase sand mobility. It is more likely that recreation pressure has delayed or halted vegetation recovery on the most used part of the dunes, following a long period of dune instability caused by large rabbit populations. On stabilized dunes with well-established vegetation, the driving force behind ecosystem stability and environmental disturbance is grazing; this is particularly true of unstable ecosystems. Because of the wide ranging cycle in rabbit numbers, dune vegetation is likely to suffer pronounced short-term changes.

Other changes have occurred on the dunes and may be linked with the decrease in sand mobility and related increase in vegetation cover and density. Since the 1970s the number of Cepea has remarkably decreased (A. Macfadyen pers. comm., 1990) and the species diversity and large densities of land shells recorded last century by Welch (1898) are not observable today.

The advent of the dune colonization by woody plants at Portstewart coincides with the advent of myxomatosis in the 1950s, which decimated rabbit population numbers, and a pattern similar to many other sites in the British Isles (e.g. Whatmough, 1979; Hodgkin, 1983).

Most of the decrease in bare sand has occurred since the invaded by sea buckthorn, a 1960s and a large proportion has been process still important in several areas todayon at least two occasions. . It is likely that it was planted Some of the eastern clumps around the golf course were planted later than those to the west, probably in the clones are male or strongly 1930's, while some planting must have taken place later. Because all male there has been very little sexual regeneration on the dunes. This is in stark contrast with what has happened elsewhere (e.g. Whatmough, 1978; Baker & de K. Wislocka,1990)

The observed difference in the timing of seedling establishment of woody species under the sea buckthorn appears to be related to the disappearance, through shading, of dense dune vegetation consisting mostly of grasses. In Transects A and B, grasses grow beneath the colonising sea buckthorn to a distance of 8m and 16m, respectively (Fig. 1 a,b). Following its establishment below sea buckthorn, elder grows through its canopy and eventually shades it out. One of the original sea buckthorn patches planted c.1900 has now developed into a woodland dominated by elder, and sea buckthorn survives only where elder failed to become established (Fig. 1, Transect C). In this woodland elder became established prior to myxomatosis, and its regeneration may have been enhanced by the presence of rabbits. Elder is notably avoided by rabbits, and is often found growing in the bare soil between rabbit burrows (Tansley, 1953).

Figure 1. Age distribution of woody plants along transects through two sea buckthorn thickets on Portstewart dunes.

At present, it is not clear what will happen to the elder woodland but when one considers the lack of tree regeneration, it is likely that it will revert to low vegetation, just like the senescent sea buckthorn patches at Murlough Bay (Co. Down). This low vegetation will of course be very different from the original dune vegetation because of the changes in soil, flora and fauna that have occurred during succession and aging of the stands (Binggeli et al. 1992).

The successional pathway observed at Portstewart is different from the one recorded at Murlough Bay (Co. Down) or elsewhere in western Europe. These differences can be explained in terms of seed availability and dispersal pattern of individual species. At Portstewart, the nearest seed source is to the south of the River Bann, and this considerable distance means that few wind-dispersed seeds are likely to reach the dunes. Extensive use of the sea buckthorn habitat at Portstewart by birds means that large quantities of elder fruits are dispersed. The pattern of seed dispersal and the suitable habitat provided by sea buckthorn for seed germination and seedling establishment explain the initial stages of elder invasion. At Murlough, the potential seed source and supply is completely different, as numerous trees (including a large patch of sycamore) and shrub species were planted around Murlough House (see Section 5.2). The nearby sea buckthorn patches appear to have fewer visiting birds but receive large quantities of wind-dispersed seeds. This situation will, therefore, favour the regeneration of sycamore which has similar requirements for seed germination and seedling establishment as elder, both species being intolerant of grass competition.

The distribution of some of the other species is worth discussing. The occurrence of a few Alnus glutinosa near the seaside part of dunes may appear surprising as this species is usually associated with wet habitats. However, because of its nitrogen fixing properties it has been extensively used to revegetate sand dunes in Germany (Kohnke, 1937). Cotoneaster was recorded by Praeger in 1935 (Praeger & Megaw, 1938), and has clearly failed to increase its hold since.

Our study clearly shows that woodland vegetation can develop throughout the dune system and similar development could occur on all Irish dunes. Historically, woodland type vegetation has been recorded. Hamilton & Carter (1983) found evidence of a fossil oakwood (dated 5315BP) at the mouth of the Articlave River (southern shore of the Bann opposite the Portstewart Dunes) and Sampson (1832) described the Magilligan dunes as once being a Juniper scrub.

References

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Sampson, G.V. (1814) Memoir, explanatory of the chart and survey of the County of London-Derry, Ireland. Nicol, London.
Tansley, A.G. (1953) The British Islands and their vegetation. Cambridge University Press, London.
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