Journal of Coastal Conservation 7: 171-182, 2001
© EUCC; Opulus Press Uppsala. Printed in Sweden
            - The role of European marram grass in dune stabilisation and succession -            171


      The role of European marram grass in dune stabilization
        and succession near Cape Agulhas, South Africa

                     Lubke, R.A.1* & Hertling, U.M.1,2

             1Department of Botany, Rhodes University, 6140 Grahamstown, South Africa;
          2Present address: Urban & Fischer Verlag, P. O. Box 100537, D-07705 Jena, Germany;
         E-mail u.hertling@urbanfischer.de; *Corresponding author; E-mail borl@rhobot.ru.ac.za




Abstract. The coastline near the southern tip of Africa is     Introduction
characterized by large mobile dunes that are driven along wide
beaches by strong winds throughout the year. This results in      Near the southern tip of Africa, Cape Agulhas, the
the blockage of the river mouths causing severe flooding of the   early European settlers encountered vast, mobile dune
low-lying farmland of the Agulhas Plain during the rainy
                                  fields, coastal fynbos, extensive flood plains fringed
winter season. Large parts of the driftsands were stabilized
with the European dune pioneer species Marram grass         with salt marsh and flat fertile plains where they could
(Ammophila arenaria), which has proved highly invasive       graze their livestock and grow crops. Along this coast-
along the North American west coast. In order to establish the   line, strong winds drive the mobile dunes along wide
potential invasiveness of A. arenaria in South African coastal   beaches resulting in the blockage of the river mouths.
dune systems and its role in the succession of a large       This leads to extensive flooding of the low-lying farm-
stabilization area, studies were carried out on De Mond Nature   land during the rainy winter season. In the 1870s Marram
Reserve. Using aerial photos, maps and planting records, the    grass (Ammophila arenaria) was introduced to South
vegetation of sites of various ages were sampled. By means of    Africa for the purpose of artificial dune stabilization
this chronosequence of stands, there is clear evidence that
                                  (Heywood 1894). Since the 1930s large parts of the
succession takes place at De Mond. Four communities are
distinguished, varying from recent plantings of A. arenaria to   coastline of the Agulhas Plain have been stabilized with
mature dune thicket or dune fynbos (heath) vegetation. These    A. arenaria to fix driftsands and prevent the further
relate to four different stages of succession, A. arenaria occur-  blockage of the river mouths.
ring in reduced abundance in the older communities. After 50      A. arenaria has proved to be a highly invasive spe-
years, former A. arenaria areas are usually covered by dense    cies along the North American west coast (Wiedemann
dune scrub and in some places even in asteraceous dune       & Pickart 1996). Research on the potential invasiveness
fynbos. Succession is most rapid in sheltered, moist dune      of A. arenaria in South African coastal dune systems
slacks, but A. arenaria remains vigorous in conditions favour-   was initiated in 1995 (Lubke & Hertling 1995; Hertling
able for its growth, i.e. on exposed, steep dune slopes with
                                  1997; Hertling & Lubke 1999a). To examine long-term
strong sand movement. A. arenaria does not appear to spread
unaidedly at De Mond and has been successfully used for       changes in large stabilization areas involving A. arenaria
temporary dune stabilization.                    and determine the succession in such initially mono-
                                  specific A. arenaria stands, studies were carried out in a
                                  stabilization area in the vicinity of the mouth of the
Keywords: Biological invasion; Chronosequence; De Mond       Heuningnes River. The mouth is part of the De Mond
Nature Reserve; Driftsand; Dune stabilization; Fynbos: Suc-     Nature Reserve, situated on the coast of the Agulhas
cession.                              Plain near Bredasdorp, and surrounded by extensive
                                  dune fields to the northwest and southeast (Fig. 1). The
                                  vegetation at De Mond has been described to some
Nomenclature: Arnold & de Wet (1993).                extent by Walsh (1968) and Bickerton (1984), but no
                                  account as yet has been given on succession in the De
                                  Mond dunes since they were stabilized. This study aims
                                  at establishing whether the extensive A. arenaria stands
                                  at De Mond have been replaced by indigenous plants or
                                  whether A. arenaria has spread and invaded the fynbos
                                  hinterland of the reserve.
172                     Lubke, R.A. & Hertling, U.M.




                                           Fig. 1. Situation of the Heuningnes
                                           River mouth in the De Mond Nature
                                           Reserve, and distribution of other im-
                                           portant Ramsar sites in South Africa
                                           (Cowan & Marneweck 1996).



Study area                          along the De Mond shore is only slightly higher to the
                               northeast than to the southwest. (Bickerton 1984).
Physiography, climate and vegetation               A recently published field guide describes the veg-
                               etation and plant species of the region (southern
  The mouth of the Heuningnes River is situated at 34∞   Overberg), and interestingly, no mention of A. arenaria
43' S; 20∞ 07' E in the Bredasdorp district of the Western  is made (Mustart et al. 1997). The vegetation of the
Cape province of South Africa (Fig. 1). The estuary is    reserve consists of coastal strand vegetation along the
the southernmost of the African continent, not more      shore line, dune scrub and dune asteraceous fynbos
than 15 km north of Cape Agulhas, and was designated     with thicket patches (Mustart et al. 1997). Fynbos is the
in 1986 at the Ramsar Convention as a Wetland of       characteristic heath vegetation of the Cape floral region
International Importance (Cowan & Marneweck 1996).      which is identified by sclerophyllous or ericoid shrubs
The lower reaches of the estuary fall within the De      and plant species of the Proteaceae, Ericaceae, Re-
Mond Nature Reserve (1768 ha). From 1939 onwards       stionaceae (Cape reeds) and many endemic species and
the Minister of Agriculture and Forestry started buying    genera. The coastal fynbos types are particularly di-
land around the Heuningnes estuary with the intention     verse in this region and protected in a number of re-
of stabilizing the driftsands (Bickerton 1984). The area   serves. The vegetation inland of the reserve is mostly
was managed by the Department of Forestry until Cape     coastal scrub or coastal fynbos or a mix of both if it is not
Nature Conservation took over coastal state forestry     under cultivation for wheat lands or grazing pastures.
lands in the mid-1980s.                      The coastal strand vegetation (Mustart et al. 1997)
  The soil is sandy with limestone outcrops (Day 1981).   consists of hummock dunes and linear dune ridges colo-
De Mond Nature Reserve falls within South Africa’s      nized by indigenous grass pioneers, Ehrharta villosa and
winter rainfall area, with wet winters and mostly hot and   Elymus distichus. Hummock forming asteraceous herbs
dry summers. The mean annual precipitation for the      include Arctotheca populifolia and Didelta carnosa.
Heuningnes drainage system is around 400 mm, maxi-      Other herbs in this fore-dune zone are Chironia baccifera,
mum daily temperature means are 28 ∞C for January and     Dasispermum suffruticosum, Senecio elegans, Heben-
17 ∞C for July, winds are mainly from the west in winter   stretia cordata and Thesium fragile. The succulent
and from the southwest and southeast in summer. It is     Tetragonia decumbens forms large hummock dunes and
especially the high-velocity southeasterly winds during    the succulent creeper Carpobrotus acinaciformis is com-
summer that cause shifting of the then dry and hot dune    mon. In the scrub zone Chrysanthemoides monilifera,
sands. Long-term observations of deep sea waves for the    Myrica cordifolia, Rhus crenata, Passerina rigida and
coast off Struisbaai near De Mond Nature Reserve have     Metalasia muricata are common.
shown that the predominant direction of deep sea waves      The dune asteraceous fynbos (Mustart et al. 1997) is
is from the southwest. Nevertheless, sediment transport    dominant on inland sites with patches of dune thicket in
           - The role of European marram grass in dune stabilisation and succession -           173




Fig. 2. Estuaries of the
Heuningnes River and Salt
River on the Agulhas plain, and
areas that are exposed to their
flooding. After the ‘Divisional
map of Bredasdorp 1901’ (Cape
Nature Conservation, De Hoop).




nutrient-rich sites. The asteraceous shrubs have ericoid    mouth would cause flooding of about 90 square miles of
leaves and include Metalasia muricata, Helichrysum       inland farms” (Walsh 1968). Fig. 2 illustrates the exten-
spp. and Stoebe plumosa. Other ericoids include        sion of farmland of the Agulhas plain that was so low-
Agathosma collina, Muraltia satureoides and Passerina     lying as to be prone to inundation. Severe floodings
ericoides; Pelargonium spp. and Salvia africana-lutea     occurred in 1871, 1880, 1902, 1903, 1906 and 1920
are other common dune fynbos shrubs. The restioids       (Bickerton 1984). In 1937 the Minister of Agriculture
(Cape reeds, which replace grasses in this fynbos or      and Forestry was approached by farmers of the district
heath-like vegetation) include Ischyrolepis eleocharis     who requested the reclamation of the driftsands. Be-
and Chondropetalum microcarpum. Many geophytes,        cause of the known invasibility of alien Acacia spp. they
common Cape bulbous species, are also found in dune      requested that these plants not be used in the dune
fynbos.                            stabilization (Hertling 1997). From the late 1930s the
  Thicket patches are characterized by shrubs of Rhus    stabilization of driftsands at De Mond with A. arenaria
glauca, R. crenata, Euclea racemosa, Olea exasparata,     has been continued until present times. Between 1942
Cassine maritima, Maytenus procumbens and vines        and 1958 alone a total of 283 ha were stabilized (Walsh
and climbers such as Cynanchum obtusifolium, Sola-       1968), today (1996) the stabilized area extends over
num quadrangulare and Asparagus asparagoides. In        some 900 ha.
some patches small trees of Sideroxylon inerme, Ptero-
celastrus tricuspidatus and Tarchonanthus camphoratus     Stabilization techniques at De Mond Nature Reserve
may be found.
                                  Since the first efforts in the 1930s until the present
History of the area                      day, the same stabilization technique has been applied at
                                De Mond with only slight variations. In view of other
  In the past, driftsands used to block the mouth of the   stabilization practices of the time, particularly the plant-
Heuningnes River in summer, when sand movement is       ing of invasive Australian acacias in many other areas,
highest and the flow of the river low. This would cause    this method was very advanced for the 1930s: between
extensive floodings of the low-lying farmland behind      the planted clumps of A. arenaria (Fig. 3), seed of
the reserve during the rainy winter season. Farmers      indigenous dune plants was sown, mostly of shrub species
were severely afflicted by the frequent blocking of the    like Metalasia muricata and Chrysanthemoides moni-
Heuningnes River mouth. At the beginning of the twen-     lifera.
tieth century “a rise of 20 feet in water level at the river    Therefore a seed bank of indigenous species was
174                      Lubke, R.A. & Hertling, U.M.

                                mostly on the more exposed areas of higher elevation. It
                                is noteworthy that the indigenous dune grass species Elymus
                                distichus and Ehrharta villosa occurred naturally at several
                                spots, which were consequently spared A. arenaria
                                plantings. Dune scrub species like waxberry (Myrica
                                cordifolia) were sown mostly in the dune valleys.
                                  Aerial photographs from 1938 and 1981 (Fig. 5)
                                show clearly the change from completely mobile drift-
                                sands (Fig. 5A) to densely vegetated dunes on either
                                side of the now open river mouth (Fig. 5B). The positions
                                of the plantings of dune stabilizing species in Fig. 4 can
                                be related to the aerial photos (Fig. 5). The opening of
                                the river mouth is, according to Bickerton (1984), pri-
                                marily due to an artificial littoral dune being established
Fig. 3. Government Forester Hendrik O. Swart among fresh A.  between the estuary and the sea (Fig. 4). It was built
arenaria plantings at De Mond in the mid-1980s (courtesy of
                                parallel to the coast by erecting ‘droppers’ and palings
Cape Nature Conservation, De Hoop).
                                along the beach to support brushwood which trapped
                                sand that was then artificially vegetated. The asteraceous
created in the stabilization area which would encourage    indigenous pioneers of the foredunes, Didelta carnosa
the succession of the mostly monospecific A. arenaria     and Arctotheca populifolia as well as the grasses Elymus
stands by indigenous dune plants. Once the driftsands     distichus and Ehrharta villosa (more common in rear
were fixed to a certain extent by A. arenaria, seedlings    dunes at this site) are unable to stabilize the sands at the
of indigenous dune plants would establish successfully     mouth of the river due to excessive sand movement. The
and ‘take over’ the area from A. arenaria.           large scale stabilization of driftsands in the vicinity of
  Fig. 4 illustrates the stabilization efforts at De Mond  the river mouth is preventing any further blockage of the
in the 1930s and 1940s The foredunes were stabilized      mouth, but it is the artificial littoral dune to the front of
with A. arenaria in three continuous rows between 1940     the mouth which keeps it open in the first place. It must
and 1944, while the stabilization of the larger and more    therefore be constantly maintained using brushwood
inland part of the dune field is patchier and focuses     and A. arenaria plantings.




                                               Fig. 4. Stabilization efforts at
                                               De Mond Nature Reserve (then
                                               De Mond State Forest) in the
                                               1930s and 1940s. After the
                                               ‘Stock map of De Mond Forest
                                               Reserve 1940’ by J. De Genis
                                               (Cape Nature Conservation, De
                                               Hoop).
            - The role of European marram grass in dune stabilisation and succession -  175

                A




                B




Fig. 5. Aerial photographs of
the De Mond Estuary and Na-
ture Reserve from 1938 (A: bare
driftsands, river mouth blocked)
and 1981 (B: a large proportion
of the mobile sands has been
stabilised, the river mouth is
open). The letters and numbers
marked on Fig. 5B are the sam-
ple sites and transects for this
study.
176                      Lubke, R.A. & Hertling, U.M.

Methods                             ity, organic matter content of soil, time of stabilization of
                                the stands – ordinal data ranked from 1 to 9:
Sampling                            1 = 1931; 2 = 1939; 3 = before 1961; 4 = 1961-1973; 5 = 1974-1979;
                                6 = 1980/1981; 7 = 1982-1989; 8 = early 1990s; 9 = never stabilized.
  Sampling was carried out in February and June
                                As to the topographical situation of the stands the ordi-
1996, some 65 years since the original planting of A.
                                nal data were ranked from 1 to 6:
arenaria in the region. Vegetation and soil were sampled
                                1 = within 150 m from the high water mark, 2 = dry dune slack, 3 =
in 42 stands of 10 m ¥ 10 m throughout the reserve.       backdune with level surface, 4 = top of backdune, 5 = slope of
Within each of the 100-m2 stands, 20 1-m2 quadrats       backdune, 6 = moist dune slack.
were placed according to random numbers (Avis 1995;       A dune slack is here understood as a hollow between
Avis & Lubke 1996). For each plant species the percent-     dune ridges which is often influenced by salt in the early
age aerial cover was estimated and the number of indi-     stages of formation, generally influenced by moderate
viduals counted (number of culms or shoots) in order to     accretion and subject to the opposing influences of
establish cover, density and frequency of each species.     submergence or drought seasonally or at different stages
The data were then used to determine species richness      of its development (Ranwell 1972).
and species diversity for each stand as well as impor-
tance values of species for each stand (Brower et al.
1990). Soil samples were tested for pH, organic matter     Results
content (% of dry weight) and conductivity (mS cm–1)
according to standard methods (Brower et al. 1990).
  We laid out 23 stands (A, B, C, D, ..., W) in a       Evidence of succession
stratified random fashion throughout the reserve to en-       Evidence of succession was reflected by Detrended
sure sampling of the various habitats, on the foredunes as   Correspondence Analysis (DCA) of all 42 stands. The
well as in dense dune scrub, on steep slopes and dune      stands were split into six distinct communities, as iden-
summits as well as at the edge of the salt marsh (Fig. 5B).   tified by TWINSPAN, namely A. arenaria foredunes or
Another 19 stands were laid out along four transects (A1    recent plantings, stable dunes with some foredune ele-
to A4, B1 to B6, C1 to C5, D1 to D4) perpendicular to the    ments, dune scrub, dune scrub/dune fynbos, saltmarsh
coastline, about 1330 to 2000 m to the northeast of the     and indigenous foredunes. While the first four commu-
mouth of the Heuningnes River, extending from 52 m to      nities were lined along a gradient representing sequen-
850 m from the high-water mark. According to old plant-     tial successional stages, the communities ‘saltmarsh’
ing records and aerial photos, the stabilization times of all  and ‘indigenous foredunes’ cannot be placed within
stands are known. They are situated in patches that were    this line. A second DCA was therefore carried out with-
stabilized in 1931, 1939, between 1942 and 1961, be-      out these stands (Fig. 6). The four remaining communi-
tween 1962 and 1973, between 1974 and 1981, between       ties can be distinguished more clearly, and it is possible
1982 and 1989, and in the 1990s. In addition, a 150-m      to attribute their times of stabilization to them. Stands of
transect was laid out at the same site as transect A. The    the early stage of foredunes dominated by A. arenaria
vegetation along this transect was sampled continuously     were all stabilized in the 1980s and 1990s (3 - 10 yr old).
in 1-m2 stands for density, cover and frequency of each     Stands of the more stable dunes with only few foredune
species.                            elements were mostly stabilized in the 1980s (6 - 20 yr
                                old). Dune scrub stands were stabilized in the 1980s or
Data analysis                          earlier, in the 1960s and 1970s or even in 1939 (mostly
                                13 - 35 yr). The last community contains stands of an
  All 42 stands were combined for interpretation and     advanced dune vegetation, leading either to a species-
subjected to comparisons of species richness, Simpson’s     rich scrub vegetation, or to dune fynbos. These were
diversity index, importance values of each species and     stabilized in 1931, 1939 or in the 1950s and 1960s
edaphic factors. The dependence of these values on the     (mostly 22 - 60 yr).
age of the stands was examined in regression analyses.
To detect patterns of succession the stands were sub-
jected to the classification and ordination techniques     Vigour of A. arenaria related to age of the stabilized
TWINSPAN (Hill 1979) and CANOCO (ter Braak 1988).        area and dune form
The data used in both programmes were importance          More information about the pace of the replacement
values of species for each stand. In an indirect gradient    of A. arenaria at De Mond is given in Fig. 7. The vigour
analysis the ordination of stands was then interpreted     of A. arenaria in all sampled stands was grouped into
with the environmental variables soil pH, soil conductiv-    six categories and plotted along an axis of the stands in
            - The role of European marram grass in dune stabilisation and succession -          177




Fig. 6. DCA of all De Mond
stands (except saltmarsh and
indigenous foredunes) results in
the clear differentiation of suc-
cessional stages at De Mond
Nature Reserve. The important
characteristic species are listed
for the related communities. The
ordination axes were also related
indirectly to five environmental
variables. The four stages relate
well to the stabilisation times of
respective stands. See Table 1
for corresponding regression co-
efficients.



chronological order. It is obvious that A. arenaria is     tively (Fig. 8B). Stand N was stabilized before 1961, yet
more vigorous in sites of a younger stabilization time.     A. arenaria is the dominant plant, if mostly medium-
Most stands that were stabilized between the 1930s and     aged. The stand is situated on top of a transverse dune in
1970s carry few, thin culms or some clumps of A.        relative closeness to the beach and thus exposed to
arenaria (categories 1 and 2), while stands from the      strong winds as well as frequent sand burial. Stand Q, on
1980s and 1990s contain vigorous, strong and often       the other hand, was only stabilized in the 1980s, yet no
dominant A. arenaria. The unexpected high or low        A. arenaria is left today. In this case, the stand is
vigour of A. arenaria in some stands, e.g. stands N and     situated in a sheltered and moist dune slack, in which
Q, can be explained with very exposed dune summit        indigenous plant species less tolerant of sand burial can
(Fig. 8A) or very sheltered dune slack locations respec-    easily be established.




Fig. 7. Vigour of Ammophila
arenaria in all De Mond stands (in
chronological order, excluding
stand V) according to six catego-
ries from 0 (no A. arenaria) to 5
(very vigorous).
178                      Lubke, R.A. & Hertling, U.M.


(A)                                 (B)




Fig. 8. A. Medium-aged but dominant A. arenaria (Vigour category 4) on a steep dune slope, in the background enriched dune scrub/
fynbos. B. Dead culms of A. arenaria (Vigour category 2) covering the ground of a sheltered and moist dune slack.



Vegetation of the different successional stages           the ordination axis 1, along which a succession was
                                  detected: the more recent stands were stabilized (vari-
  A typical toposequence of the vegetation at De Mond       able ‘time of stabilization’), the further to the right are
is reflected in Fig. 9 along a 150-m transect (transect A,     these stands found in the plot; in contrast, stands to the
Fig. 5B) perpendicular to the coastline. The first species     left of axis 2, vegetated by dune scrub and dune fynbos,
on the beach is Arctotheca populifolia. After a sandy,       are characterized by a high organic matter content and
unvegetated zone on the back-beach large amounts of A.       more sheltered and moist locations. However, the or-
arenaria occur with very few Elymus distichus on the        ganic matter is not equally well related to these stands:
fore dunes. A. arenaria is a prominent plant throughout       the dune fynbos stands appear to have a lower organic
the transect, occurring up to a distance of 210 m from       matter content than stands from the advanced dune scrub
the high-water mark. An important plant in the middle        stage. This is confirmed by the highly significant (P <
part of the transect is Psoralea repens. Chironia baccifera     0.01) regression of the organic matter content with axis 2
is not quite as prominent, but extends further to the back     (Table 1), which therefore accounts mostly for the diver-
than Psoralea repens. Similarly common throughout          gence of vegetation in later successional stages. Of the
the greater part of the transect are Ficinia lateralis and     five environmental variables examined, the conductivity
Helichrysum patulum. Of the shrub species, Metalasia        and pH values appear to have the least influence on the
muricata and Chrysanthemoides monilifera appear only        stand ordination. This is confirmed by their low regres-
sporadically, while Myrica cordifolia is very common,        sion coefficients (Table 1). The strongest influence on the
occurring vigorously especially in the back parts of the      ordination of De Mond stands is shown by their
transect. The restio Ischyrolepis eleocharis was recorded      stabilization time and topographical situation, both vari-
from 214 m. It is one of the first fynbos plants to appear     ables bear highly significant (P < 0.01) regression coeffi-
in the advanced dune scrub vegetation at De Mond.          cients. This confirms the decline in vigour and replace-
Fynbos elements found along other transects are Thamno-       ment of A. arenaria at De Mond with indigenous dune
chortus insignis, Euclea racemosa, Ficinia ramosissima       plant species as dependent on time and habitat.
and Phylica ericoides. These species are the characteris-
tic or diagnostic species of the various communities
identified by TWINSPAN and are listed in Fig. 6.          Table 1. Regression coefficients (multiplied by 100) of five
                                  environmental variables with the first and third axis resulting
                                  from indirect gradient analysis (DCA) of all De Mond stands
Edaphic and other environmental factors
                                  (df = 30). See Fig. 10 for the plot. Significance at a = 0.01 (*)
                                  as indicated through t-values supplied by the same analysis.
  To examine the influence of edaphic soil factors
                                  No values were significant at a = 0.05 or a = 0.1.
(pH, conductivity and organic matter content), of the
time of stabilization and of the topographical situation                        Axis 1      Axis 2

per stand on the differentiation of De Mond stands in        Soil pH             + 136      + 240
                                  Soil conductivity        + 226      + 139
CANOCO, the ordination axes of the original plot were        Organic matter content of soil  - 240     + 546 *
subsequently related to these five variables (Fig. 6). The     Stabilization time        + 907 *     + 161
environmental variables correspond particularly well to       Topographical situation     - 627 *     + 118
           - The role of European marram grass in dune stabilisation and succession -               179




Fig. 9. Profile of a 150-m dune transect (A in Fig. 5B) at De Mond Nature Reserve with values of species cover for every m.



Discussion                             Ammophila arenaria communities on foredunes or as
                                  recent plantings further inland, (2) mixed Ammophila
Succession at De Mond                       arenaria communities on stabilized dunes, (3) dune
                                  scrub, (4) dune scrub with fynbos elements, (5) saltmarsh
  The results of the vegetation analysis at De Mond       and (6) indigenous foredunes. The first four stages show
Nature Reserve prove that succession is taking place in      an increase of species richness and species diversity and
this large-scale stabilization site. Monospecific A.        can be lined up along a successional gradient related to
arenaria stands can be transformed into dense dune         a chronosequence (Fig. 6). Most areas at De Mond that
scrub/dune fynbos within 50 - 60 yr. Six communities        were stabilized in the 1930s and 1940s are today veg-
were identified at De Mond: (1) young, vigorous          etated by a dense, species-rich dune scrub with many
180                      Lubke, R.A. & Hertling, U.M.

elements of area-specific fynbos. Stands that were stabi-   was observed to grow vigorously at a distance of up to 2
lized from the 1950s to 1970s are mostly vegetated by     km inland in areas of active driftsands and around blow-
dense dune scrub. More recently stabilized stands are     outs caused by human or animal trampling (Hertling
often still dominated by A. arenaria.             1997; Knevel 2001). The niche advantage that moist,
  The succession of species at De Mond in the above     sheltered dune slack locations offer colonizing species
described chronosequence is reflected in a related topo-    in A. arenaria stabilization sites is reflected in a Dutch
sequence. Transects from the shore into the backdune      study: van Dorp et al. (1985) report that Alnus glutinosa
area confirm the transformation of vegetation from A.     woodland can succeed A. arenaria plantings if it devel-
arenaria foredunes to dense dune scrub along a spatial     ops in adjacent lower-lying slacks from which it would
gradient (Fig. 9). Psoralea repens and Chironia baccifera   extend into the A. arenaria zone. Sowing of woody
are common species among stands of A. arenaria, while     indigenous species, such as the waxberry Myrica
Ficinia lateralis and Helichrysum patulum become more     cordifolia, in the De Mond stabilization area was carried
frequent as the cover of Myrica cordifolia increases.     out preferentially in moist dune valleys, in which simi-
The restio Ischyrolepis eleocharis is the first fynbos     lar initial spots of successor species were developed.
species to appear in the stabilization sites at De Mond.    The colonization of A. arenaria dunes at De Mond has
The succession of the species in the identified commu-     possibly proceeded out of these dune slack locations.
nities does not relate as clearly to a toposequence as do     Along transects perpendicular to the sea, A. arenaria
other successional gradients in South African dune fields,   occurs generally further back than is observed in other
for example at Mtunzini in KwaZulu-Natal (see below,      situations on the Cape Coast (Hertling 1997; Hertling &
Avis 1992).                          Lubke 1999b). Due to the recent origin of the vegetated
                                De Mond dunes, the greater part of a 235-m transect
Location of stands on dunes and rate of succession       sampled from the high water mark is covered with A.
                                arenaria (Fig. 9). This is similar to the situation in
  Even stands that were stabilized only in the 1980s     Europe where A. arenaria covers vast areas of the
can carry a rich and dense dune scrub vegetation. This     yellow dune, for example, at Braunton Burrows (Willis
would be due to facilitating habitat features such as a    et al. 1959) and on the North-Sea coastline at Meijendel
sheltered dune slack location with a higher organic      in The Netherlands (Lubke pers. obs.). More often on
matter accumulation and greater moisture. On the other     the Cape coast where A. arenaria has been planted, it
hand, sites of an early stabilization date can bear persist-  forms a 50 - 100 m maximum belt along the foredunes
ently vigorous A. arenaria populations if they are situ-    (Hertling & Lubke 1999b).
ated on exposed dune slopes and therefore characterized      Studies on the vegetation of European coastal
by a greater sand movement. A. arenaria profits at such    foredunes show that soil-borne diseases, especially nema-
sites from its superior sand burial tolerance. In sheltered  tode parasites, are closely linked to the succession of
dune slack locations, A. arenaria does not have this      species on the dunes. The root zones of sequential
niche advantage and is outcompeted by other species      foredune plant species contain nematode species that
(Figs. 7 and 8). The interpretation of the ordination of all  are specific for their host and pre-successional plant
De Mond stands with environmental factors confirms       species, but affect the next species in the succession to a
that, beside the time of stabilization, the topographical   much lesser extent (van der Putten et al. 1993). The
position of the stands has the strongest influence on their  potential importance of these pathogens in the succes-
vegetation (Fig. 6).                      sional process has not been overlooked in this study
                                and both field trials and growth studies have been car-
A comparison with succession of A. arenaria stands in     ried out in the South African situation (Hertling 1997;
Europe                             Knevel 2001). A. arenaria was introduced as seed to
                                South Africa (Heywood 1984) presumably without the
  An analysis of the colonization of a British stabili-   nematode parasites, and those now found on the roots of
zation site at Braunton Burrows, North Devon, involv-     A. arenaria have possibly come from other grasses such
ing A. arenaria confirms the De Mond results (Hewett      as Ehrharta villosa which shared ca. 36% of the ap-
1970): within 14 years of planting, the frequency of A.    proximately 12 species of nematodes found in five dune
arenaria had decreased from more than 70% to less than     pioneer species. The impact of these pathogens, which
20%, while that of other grass species like Festuca      exclude endoparasites, have been analysed from a growth
rubra and Phleum arenarium increased from 0% to        experiment (Knevel 2001).
100%. A. arenaria remained vigorous only on the sea-
ward plantations and in small areas of mobile sand. On
Sylt Island off the German North Sea coast, the grass
           - The role of European marram grass in dune stabilisation and succession -            181

A comparison with succession of vegetation in other      dates confirms the transformation of vegetation from A.
South African dune fields                   arenaria foredunes to dense dune scrub along both a
                                spatial and temporal gradient. A Detrended Correspond-
  The pattern of succession along a chronological      ence Analysis (DCA) of all stands at De Mond shows
gradient found for the A. arenaria plantings at De Mond    that their topographical situation has more influence on
is comparable to that recorded on the Mtunzini dunes in    their ordination along an environmental gradient than
KwaZulu-Natal, South Africa (Avis 1992): both a-        edaphic factors. This confirms that the replacement of A.
diversity (species diversity) and b-diversity (commu-     arenaria is dependent on the time and the habitat of the
nity diversity) increase with the age of the dunes. In the   site. The longer a site has been stabilized and the more
Mtunzini study an ordination of the sampled stands       sheltered its location, the sooner indigenous dune plant
resulted in a pattern of a unilinear sequence of young     species colonize the A. arenaria plantings and the faster
and medium-aged communities (pioneer, enriched pio-      the grass degenerates.
neer, open dune scrub, closed dune scrub) which then        The De Mond Nature Reserve offers one of the most
diverged like a fork into a variety of older, more ad-     significant examples of succession in a stabilization
vanced communities (bush clumps, forest margin, forest).    area involving A. arenaria in South Africa. It appears as
The divergence of community types into at least two      though A. arenaria has been successfully used at De
different directions is even more distinct at De Mond,     Mond, providing temporary stability of dune sands un-
where the oldest dunes can carry either an enriched dune    til indigenous dune plants take over. On a smaller scale,
scrub vegetation or a very differently composed dune      the succession of A. arenaria by indigenous plant spe-
fynbos vegetation (Fig. 8). In both studies, the increase in  cies has been observed at several other sites along the
a-diversity of the communities is therefore correlated     coast (Hertling 1997; Hertling & Lubke 1999b), such as
with an increase in their b-diversity.             Kleinkrantz near Wilderness (southern Cape) or in the
  The mosaic of dune thicket (enriched dune scrub)      Alexandria dune field near Port Elizabeth (eastern Cape).
and dune fynbos has been described by Cowling (1984)        By maintaining a continuous management pro-
further to the east in the Humansdorp district and by     gramme using mainly A. arenaria the littoral foredune
Hoare (1994) in Goukamma Nature Reserve near          has kept the mouth of the Heunings River open through-
Knysna. The shrubs and trees are favoured with the       out the year. This has resulted in a dynamic estuarine
development of organically enriched soil patches in the    system with no further flooding to the interior Agulhas
dunes, whereas fynbos species are more commonly        Plain since the 1940s. In contrast the Salt River now
found in nutrient-poor soils (Cowling 1992).          remains closed and forms the De Hoopvlei and
                                Papiesfontein Marsh within the De Hoop Nature Re-
                                serve (Fig. 2), which is also an important Ramsar site.
Conclusions

  The case study at De Mond Nature Reserve proves      Acknowledgements. Thanks are due to the personnel of the
                                De Mond and De Hoop Nature Reserves (Cape Nature Con-
that succession of monospecific A. arenaria stabilization
                                servation). The authors were able to attend the 28th Interna-
areas by indigenous dune plant species can take place     tional Geographical Congress in The Hague, where this paper
in South Africa. Six communities were identified at De     was presented, with the aid of a Rhodes University Travel
Mond, which relate to six successional stages and con-     Bursary (Ursula M. Hertling) and the Chairman’s Fund Edu-
stitute a clear chronosequence. Areas at De Mond that     cational Trust: Anglo American, De Beers (Roy A. Lubke).
were stabilized in the 1930s and 1940s are today veg-     Funds for research in the field were made available from
etated by species-rich dune scrub or dune fynbos.       Rhodes University Council and the European Commission
Stands that were stabilized from the 1950s to 1970s are    through their RTD programme INCO-DC (research on eco-
mostly vegetated by dune scrub. More recently stabi-      systems).
lized stands are often still dominated by A. arenaria.
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                                                  Received 8 December 1999;
                                              Revision received 18 October 2001;
                                                  Accepted 18 October 2001.
                                            Coordinating Editor: F. van der Meulen.