Sycamore invasion of sand dunes
|Invasive Woody Plants
Sand Dune Ecology
|(this is section 5.3 in
In the British Isles sand dune systems have been extensively used by humans for centuries. After the introduction of rabbits, most of the dune systems became rabbit warrens, while others were grazed by sheep or cattle, and this pattern of land use remained stable for a long time. In Ireland during the early part of the 19th century some sand dune systems were extensively used for cultivation because of land shortage, and in many instances severe movement of sand was recorded in some barely vegetated dunes (e.g. Kinahan & McHenry 1882, Lamb 1991). Following the famine and its related decrease in number of people many areas were abandoned (Carter 1991).
This century a change in land use has been observed including the use of sand dunes for forestry, golf courses, reclamation schemes and less intensive agriculture. More importantly the advent of myxomatosis in the 1950s led to the near extinction of rabbit populations. This population crash favoured the regeneration of woody vegetation on many sand dune systems throughout the British Isles (e.g. Hope-Jones 1965). The species spreading on to the dunes reflected the type of seed source available, either from natural woody vegetation, forestry plantations, or amenity planting. Sea buckthorn (Hippophae rhamnoides), a species native to eastern England, is often the most noticeable invasive species. However, its rate of spread and the timing of its subsequent replacement by later successional species, such as sycamore (Acer pseudoplatanus), has never been accurately reported.
This section examines the pattern of invasion and succession by sycamore and co-occurring woody plants on two coastal sand dune systems in Northern Ireland. At one site an unusual secondary succession is described, which, in some parts of the dunes, does not include any native woody species.
The Murlough dune system (IGR J4135) extends along parts of the eastern coastline of Co. Down, Northern Ireland (Fig. 1). The dunes are part of a National Nature Reserve and its vegetation has been described by McManus (1988), and Kelly (1977) recorded sycamore in many of his vegetation quadrats particularly in sea buckthorn stands. During the second part of the 19th century Murlough House was built to the north-east of the dunes near the sea (Fig. 1). Numerous exotic species were planted around the house. Many woody species have now spread into the surrounding semi-natural vegetation (Whatmough 1978).
Portstewart dunes (IGR C8036) are situated on the north coast of Ireland, between the sea and the River Bann, and were formed as an aeolian sand accumulation on a Holocene gravel storm beach. Present day dune morphology is dominated by high and anomalously orientated transverse (North-South) ridges (Fig. 5).
Sea buckthorn was introduced to the Murlough dune system for sand stabilization in the late 19th century and about 1900 at Portstewart. During the first few decades after planting sea buckthorn spread slowly, mostly by means of vegetative propagation. Following the introduction of myxomatosis in the 1950s, which caused a dramatic reduction in rabbit numbers, sea buckthorn spread rapidly over large areas of the dune systems, both by seed and vegetatively. Particularly at Murlough, several new clones became established following seed dispersal by birds (Whatmough 1979).
At Portstewart the sea buckthorn cover has increased from 1.6% to 13%, while bare sand decreased from 31% to 7% of the dune area between 1949 and 1989 (Appendix 5). The impacts of sea buckthorn on dune ecosystems have been documented at Portstewart by Binggeli et al. (1992).
Using vertical aerial photographs of Murlough dunes for 1942, 1963, 1975 and 1988 the changes in the shape and size of the spit, in cover of bare sand, marram grass (Ammophila arenaria), sea buckthorn and sycamore was estimated for the area of dune (including the spit) which prograded since the 19th century. The spit area is characterized by relatively high pH (pH ± 6), whereas the rest of the dunes are acidic with pH ± 4 (measured following dilution by 1:2.5 with distilled water). The estimates of sycamore cover for 1942 and 1963 are likely to be underestimates because of the difficulties in differentiating sycamore from sea buckthorn on the aerial photographs.
To determine the pattern of succession of woody plants, transects were laid out in two sea buckthorn patches (Fig. 1). These patches were chosen as representative of the apparent successional changes and, at the spit, to include a presently expanding stand of sea buckthorn. Transect A was situated on the expanding spit and was surveyed in December 1989 from the edge to the centre of the clump. Transect B was on the top of a small dune ridge and traversed right across a mature stand towards a large sycamore. The position and height of all woody plants within a 1m band for sea buckthorn and 2m band 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. The yearly variation in flowering of the large sycamore of Transect B was estimated using the method described in sycamore flowering periodicity.
At Portstewart the dune system was systematically surveyed and all sycamores and other woody species were recorded and located. Individual sea buckthorn and elder occurring outside large sea buckthorn/elder stands were only recorded.
Changes in size and shape of the Murlough spit
The area to the east of Murlough House (Fig. 1 and in detail Fig. 2) has gone through cycles of sand accretion and erosion (Nairn 1977). In 1803 a spit existed but had eroded away by 1834. During the next two decades the coastline prograded and eventually a new spit appeared (as recorded on the 1900 Ordnance Survey 6" map). Since the start of the century the spit has been extending northwards while its southern part and continuing sandy coastline have been eroded. Changes in spit size and shape since 1942 are illustrated in Fig. 2. Between 1942 and 1963 about 4.3% of the spit area eroded but there was a 4.6% increase of accretion to the north. Between 1963 and 1988 further sand accretion occurred to the north (increase of 7.5%) with no erosion.
Changes in bare sand and vegetation cover on Murlough spit
There have been many changes in vegetation and in areas of bare sand on the Murlough dunes this century (see Cronk 1981, McManus 1988, and Whatmough 1978, 1979 for details). The changes in cover vegetation and bare sand between 1942 and 1975, in the area which has accreted since 1834, are described. Fig. 3 shows the respective covers of sea buckthorn, sycamore, typical sand dune vegetation (dominated by Ammophila arenaria) and bare sand in 1942, 1963 and 1975, and the estimated percentage covers are given in Table 1.
Table 1. Percentage cover of bare sand and three vegetation types on the Murlough spit and adjacent area 1942, 1963 and 1975.
On the spit the vegetation changed dramatically from 1942 to 1963 with the replacement of typical dune vegetation by sea buckthorn. There has been an important increase in sycamore cover since 1963 (but note that the figures for 1942 and 1963 are likely to be underestimates). The increase in bare sand observed between 1963 and 1975 is due to sand accretion to the north of the spit (see Fig. 2).
In the area adjacent to the spit sea buckthorn covered 78.3% of the area in 1942 and according to Whatmough (1979) sea buckthorn was originally planted in the 19th century in this area. Kelly (1977) aged a small number of stems and found that the oldest was at least 70 years. In 1942 bare sand covered about 15% of the area and was to a great extent restricted around American army barracks and related to human disturbance (Fig. 3). During the subsequent decades the main changes have been the disappearance of bare sand and the strong increase in cover by sycamore (increasing up to 38% in 1975). The spread of sycamore has caused a decrease in sea buckthorn since 1963. The vegetation map for 1975 shows that only a few areas are totally dominated by sycamore, but its distribution and the small canopy size of most of the trees indicate that in the future much larger patches will be dominated by the species. Furthermore, saplings and small individuals cannot be detected from aerial photographs and the sycamore distribution shown in Fig. 3 does not show the full distribution of the species.
Timing of woody plant succession at Murlough
In Transect A (Fig. 4) sycamore and Clematis vitalba, a woody climber, became established below sea buckthorn; the age of clematis is in some cases only one year younger than the nearby dominant sea buckthorn, while all sycamores were less than seven years old, even under mature sea buckthorn. In Transect B (Fig. 4), all saplings, with the exception of one elder, were sycamores with ages ranging between two and 14 years, with a large cohort occurring in 1985. This cohort follows an exceptional flowering year in 1984 in Ireland (see Sycamore Flowering Periodicity). The yearly estimates of flowering of the large sycamore situated at the end of the transect are given in Table 2 and show that 1984 was also an exceptionally good flowering year for that tree.
Figure 4. Age distribution of woody plants along transects through two sea buckthorn thickets on Murlough dunes. Left: Transect A, spreading sea buckthorn stand on the Murlough spit. Right: Transect B, old sea buckthorn stand on the mature dunes. (Red triangle is Sea Buckthorn, green circle is sycamore and black square clematis).
Table 2. Yearly variation in flowering of the large sycamore situated at the end of Transect B (flowering intensity is expressed as the percentage of buds flowering).
The maximum height recorded for sea buckthorn is ca. 3.5-4m.
Sycamore grows beneath the sea buckthorn canopy, but
eventually usurps it to reach, in sheltered places, a
height of around 10m. Sycamore growth is not so affected
by wind and salt damage and protrudes well above sea
Distribution and frequency of woody plants at Portstewart
Sycamore was the second most abundant species recorded outside sea buckthorn at Portstewart (Table 3). Crataegus monogyna was the commonest species and among the eight other recorded species, only Alnus glutinosa and ash were well represented.
Table 4.2.3. Number of individuals of different tree species on the Portstewart dunes (elder, found in sea buckthorn thickets, is not included.)
The sycamore distribution is shown on Fig. 5. Sycamore trees, like trees of all other species, are clumped mostly towards the seaward side of the dunes and appear to have become established in areas which have been vegetated since 1949. Few sycamores became established within sea buckthorn or sea buckthorn / elder stands.
At Murlough, the spread of sea buckthorn has now nearly completely stopped and this appears to be related to the dense vegetation presently dominating most of the dunes. The two transects show that sea buckthorn is replaced by sycamore or a mixture of sycamore and clematis in areas with higher pH. Sycamore eventually eliminates sea buckthorn through shading. This process has its ultimate expression to the west of Murlough House where a tall sycamore woodland has replaced a large area originally dominated by sea buckthorn. However, another successional pathway also exists at Murlough although it is not so common and consists of the regeneration of elder below sea buckthorn. On the other hand, such a pathway is the only type of succession observed on the Portstewart dunes on the north coast of Ireland (Appendix 5).
The different successional pathways can be explained in terms of seed availability and dispersal pattern of individual species (see Appendix 5 for details). Portstewart dunes have few woody plants and most do not produce seeds. 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. Also, most of the woody species are non native, leading to a secondary succession dominated by exotics, which is quite unique. 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, with both species being intolerant of grass competition.
After the establishment of sycamore as a dominant species in dune thickets the remaining sea buckthorn dies back leaving gaps in the canopy. On the spit, clematis appears to eliminate sea buckthorn by shading, and the resulting gaps consist of a carpet of clematis. At present, it is not clear what will happen to the patches of clematis and sycamore woodland but it is likely that the areas will revert to low vegetation as no regeneration is presently observed. This low vegetation will of course be very different from the original dune vegetation because of the increase in organic matter and nitrogen fixation in the immature soils by sea buckthorn.
Areas of dunes free of sea buckthorn are subject to a different successional pathway. At Murlough, in vegetation free of Pteridium aquilinum (bracken), Pinus sylvestris and Cotoneaster simonsii regenerate freely (Whatmough 1978). In bracken dominated areas little regeneration is observed. At both Murlough and Portstewart sycamore establishment is possible, although at lower densities, in typical sand dune vegetation found outside sea buckthorn thicket. At Murlough, Whatmough (1978) recorded sycamore in 61% of 10m x 10m squares sampled in the reserve and it was the commonest species. Still, it is apparent that sea buckthorn provides a better substrate for seed germination and seedling growth than open dune vegetation, therefore facilitating its establishment. The conditions believed to be favourable to sycamore establishment include: high humus content, relatively high humidity, lack of ground vegetation (mainly grass spp.) competition, less exposure to wind and salt desiccation and high nitrogen content (van den Burg & Peeters (1977) have shown that sycamore saplings height growth responded strongly to the application of nitrogen to sandy calcareous soils). Because shading is relatively high, seedling growth rates are low, but similar to those recorded in beech dominated forests in native habitats (see Section 4). Once sycamore has reached canopy height, growth rates increase dramatically in sheltered areas.
The impact of sycamore on dune systems has not been investigated in detail, but a few points can be made. It obviously alters dune systems from open communities to typical woodland habitats. When compared to sea buckthorn, sycamore will cast higher shade than most of Murlough sea buckthorn stands (but note that Porstewart sea buckthorn stands are much denser) and canopy height will reach at least 10m in most areas instead of about 3-4m. On the spit it was noted that the ground vegetation under sycamore varies greatly, some patches being dominated by bluebells (Hyacinthoides non-scriptus), others by mosses, bare soil and ferns or ivy (Hedera helix). Because of a leaf C/N ratio of around 10 sycamore appears to increase soil pH (see Sections 4.3 & 188.8.131.52), it is probable that following a decrease in soil pH caused by sea buckthorn sycamore will reverse the trend. This process would obviously be more significant in areas where the soil pH has decreased down to around 4. However van den Burg & Peeters (1977) found that on sandy calcareous soils foliar nitrogen was low, which would probably increase the C/N ratio resulting in no change in soil pH. This decrease in leaf nitrogen may not occur in sycamore growing in sea buckthorn stands and needs to be investigated further.
The impact of sycamore on bird diversity has been indirectly investigated by Nairn & Whatmough (1978), who compared two dune areas, one consisting of typical dune vegetation with a large patch of young sea buckthorn and the other was a large area of mature sea buckthorn with protruding immature sycamore. They found that the latter area contained more than three times as many breeding pairs of birds per km2, but the total number of species was similar with differences in the type of species recorded. The area of sea buckthorn and sycamore has now become a sycamore wood, and casual observations clearly indicate that both the number of species and the number of breeding pairs has decreased, although detailed investigations have yet to be carried out. It appears that a mixture of sea buckthorn and sycamore, with its associated heterogeneity in canopy height, food source, tree architecture and nesting sites is most favourable for avian diversity and high breeding populations. When sycamore reaches maturity and produces a continuous canopy it appears to be deleterious to both the number of breeding birds and bird species richness. However, whenever gaps start forming in the sycamore canopy, as a result of tree death or windthrow, habitat heterogeneity will once again increase and will probably once again increase bird diversity.
|Copyright © 1999 Pierre Binggeli. All rights reserved.|