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The Impact of Exotic Dune Grass Species on Foredune Development in
Australia and New Zealand: a case study of Ammophila arenaria and
Thinopyrum junceiforme
Mike Hilton a; Nick Harvey a; Andrew Hart a; Kris James a; Chris Arbuckle a
a
University of OtagoUniversity of AdelaideUniversity of OtagoUniversity of AdelaideOtago Regional Council,
Dunedin, New Zealand, Australia, New Zealand, Australia, New Zealand
Online Publication Date: 01 November 2006
To cite this Article Hilton, Mike, Harvey, Nick, Hart, Andrew, James, Kris and Arbuckle, Chris(2006)'The Impact of Exotic Dune Grass
Species on Foredune Development in Australia and New Zealand: a case study of Ammophila arenaria and Thinopyrum
junceiforme',Australian Geographer,37:3,313 — 334
To link to this Article: DOI: 10.1080/00049180600954765
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Australian Geographer, Vol. 37, No. 3,
pp. 313 Á334, November 2006
The Impact of Exotic Dune Grass Species on
Foredune Development in Australia and
New Zealand: a case study of Ammophila
arenaria and Thinopyrum junceiforme
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MIKE HILTON, NICK HARVEY, ANDREW HART, KRIS JAMES
& CHRIS ARBUCKLE, University of Otago, New Zealand; University of
Adelaide, Australia; University of Otago, New Zealand; University of Adelaide,
Australia; Otago Regional Council, Dunedin, New Zealand
ABSTRACT Marram grass (Ammophila arenaria) and sea-wheat grass ( Thino-
pyrum junceiforme) have been introduced to Australia and New Zealand. This study
examines the morphology of incipient foredunes and established foredunes associated with
these species at two sites, Mason Bay in southern New Zealand, and the Younghusband
Peninsula in South Australia. Both species invaded the existing foredunes very rapidly. In
both cases the antecedent topography comprised relatively sparsely vegetated, irregular
foredunes. Invasion resulted in continuous, regular, evenly vegetated foredunes. At Mason
Bay a massive foredune has formed since 1958, in conjunction with Ammophila.
Thinopyrum has formed an incipient foredune, with a ramp or terrace morphology, along
the Younghusband Peninsula, South Australia. In both cases gaps in the former foredune
have been closed and the indigenous foredune vegetation has been displaced. Both species
may decrease the frequency and severity of blowout development. They are likely to be
resilient to aeolian processes of sedimentation compared with dunes formed by indigenous
species. Ammophila survives burial, is tolerant of drought and is resistant to erosion
associated with storm surge and high waves. Thinopyrum is very tolerant of salinity.
These species may adversely affect the long-term development of coastal barriers by
inhibiting transgressive dune development.
KEY WORDS Marram grass; sea-wheat grass; invasive species impacts; coastal dune
development; Australia; New Zealand.
Introduction
The development and morphology of established foredunes has been well
documented in relation to climate, sediment supply and type, wave climate and
onshore winds (see Hesp 2002 for a review), but less attention has been paid to the
important role of sand-binding grasses, sedges and herbs. Dense, tall, erect grasses
ISSN 0004-9182 print/ISSN 1465-3311 online/06/030313-22 # 2006 Geographical Society of New South Wales Inc.
DOI: 10.1080/00049180600954765
314 M. Hilton et al.
are associated with high, steep-sided, asymmetric foredunes (Hesp 1983, 1989;
Van Dijk et al . 1999). In contrast, species with a lower, more spreading rhizomatous
growth form tend to produce lower, less hummocky dunes (Hesp 2002). The
alongshore morphology of established foredunes is likewise influenced by vegeta-
tion cover, which may be more or less uniform, depending on patterns of species
type, vigour or cover (Carter 1988; Hesp 1999). Over the last 50 years there is
evidence for the global redistribution of dune grass species (Hilton & Harvey 2005)
which may change the dominant foredune grass species and hence affect the dune
morphology. This paper explores such changes using examples from Australia and
New Zealand, and considers the long-term implications of these species for dune
system and barrier genesis, in particular the impact of these grasses on transgressive
dune systems.
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The beaches and dunes of the temperate coasts of Australia and New Zealand
contained only a handful of marine-dispersed species capable of colonising and
occupying the back-beach, between the strandline and the established foredune:
Spinifex sericeus , in both countries; Atriplex billardierei in Australia; and Atriplex
hollowayi (a crystalwort) in New Zealand. In contrast, the established foredune and
back dune floras of both countries contain a rich diversity of species. Of those
indigenous species capable of forming incipient foredunes and established
foredunes, just four are widespread. Spinifex sericeus (spinifex) and Austrofestuca
littoralis (sand tussock) occur in both countries. Desmoschoenus spiralis (pingao or
pikao), a sedge, is endemic to New Zealand. Spinifex longifolia is dominant in
Western Australia. In addition, Atriplex cinerea (grey salt bush), is locally important
in southeast Australia.
The dune flora of Australia and New Zealand changed rapidly over a 50-year
period, from the late 1800s, with the introduction of European, American and
South African dune plants (Heyligers 1985; Hilton & Harvey 2005). Sea spurge
(Euphorbia paralias ), European sea rocket (Cakile maritima ) and American sea
rocket (Cakile edentula ) were accidentally introduced in the ballast of ships. The
seeds of these species apparently survive in sea water for long periods. Sea wheat-
grass (Thinopyrum junceiforme ) and marram grass (Ammophila arenaria ) were
deliberately introduced to New Zealand and Australia to stabilise mobile dunes.
Two South African species, bitou bush (Chrysanthemoides monilifera ) and pyp grass
(Ehrharta villosa ) were also introduced for this purpose. Arctotheca populifolia
(beach daisy), another South African plant, was probably brought to Australia as an
ornamental. These introductions contributed four new species to the strandline Á
back-beach community in southeast Australia (the two sea rockets, beach daisy and
sea spurge); two new species capable of forming incipient and established foredunes
(Ammophila and Thinopyrum ) and two species more commonly associated with
backdune environments or established foredunes (bitou bush and pyp grass). Sea
spurge occurs in all environments, but has limited dune-building capability.
We are concerned primarily with Ammophila and Thinopyrum in the present
paper, in particular the ability of these plants to form incipient foredunes and
established foredunes that are resilient to disturbance. The potential for Ammophila
to trap large volumes of sand in massive foredunes has previously been noted (Esler
1970; Heyligers 1985). More recent work has suggested that Ammophila and
Thinopyrum may also inhibit or prevent blowout and transgressive dune develop-
ment (Hilton & Harvey 2002; Hilton et al . 2005). Suppression of blowouts might
have a significant impact on the natural development of sandy coasts in Australia
Impact of Exotic Dune Grass Species 315
and New Zealand. Transgressive dune activity is critical to the physical develop-
ment of coastal barriers on windward coasts in both countries. Transgressive dune
activity is also closely linked with dune habitat and species diversity, since such
activity inevitably leads to topographic and environmental diversity. We examine
the impact of these species on foredune formation through two case studies:
Ammophila invasion in a large transgressive dune system on the west coast of
Stewart Island, New Zealand and, secondly, Thinopyrum invasion along the
Younghusband Peninsula, South Australia. Finally, we examine the proposition
that these species develop incipient foredunes and established foredunes that are
more resilient to environmental disturbance than foredunes associated with
indigenous species.
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Methods
Fieldwork was undertaken at two locations on the Younghusband Peninsula,
South Australia (see Figure 1) and one location at Mason Bay, Stewart Island (see
Figure 2), during 2003 and 2004. The purpose of this work was to describe,
FIGURE 1. The location of the ‘28 Mile Crossing’ and ‘The Granites’ study sites,
Younghusband Peninsula, South Australia. The Younghusband Peninsula is the most recent
in a progradational sequence of Quaternary coastal barriers (after Belperio 1995).
316 M. Hilton et al.
through systematic survey, the morphology of established foredunes associated with
Ammophila and Thinopyrum, as well as the morphology of the adjacent hinterland;
and to map and classify the associated beach-dune flora. Surveying was
accomplished using a Leica 305 total station. Shaded relief plots, contour maps
and profiles were generated for each site using Surfer 3D (Golden Software).
Historic aerial photographs were used to determine the invasion history of these
species, change in vegetation cover and community type and landform develop-
ment. Ground photographs taken by Andy Short and Patrick Hesp during survey
work in 1979 were compared with recent photography.
Foredune morphology and vegetation cover were surveyed at two sites on the
Younghusband Peninsula *‘The Granites’ and ‘28 Mile Crossing’ (see Figure 1).
These sites were chosen to examine the morphology of Thinopyrum incipient
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foredunes on quasi-stable and eroding coastlines, respectively. ‘The Granites’ site is
located within the relatively low energy ‘Coorong III’ morphodynamic class of
Short and Hesp (1980). Compared with ‘28 Mile Crossing’ (Coorong II), this site
is relatively sheltered from the prevailing southwest winds by Cape Jaffa. In
contrast, foredune scarping appears to have occurred frequently over a period of
some years at ‘28 Mile Crossing’. It has not been possible to reconstruct the
invasion history of Thinopyrum along the Younghusband Peninsula, in part because
it established rapidly, in part because it is associated with narrow (5 Á10 m)
incipient foredunes, which are difficult to identify on the earlier aerial photographs.
We do know it established some time after 1979, when Short and Hesp (1980)
undertook a systematic study of the established foredunes along the Younghusband
Peninsula. We utilised a series of ground photographs, made available by Andy
Short, to determine the foredune morphologies that existed prior to Thinopyrum
invasion.
A sequence of aerial photographs records the invasion history of Ammophila at
various sites along the west coast of Stewart Island. Mason Bay has a record of
aerial photography dating from 1958 with subsequent images available for 1978,
1989 and 2002. Ammophila established in the study area in the early 1950s. These
data document (i) the invasion history of Ammophila and indigenous species
displacement; (ii) the physical development and changing morphology of the
Active dune system
STEWART ISLAND
Mason Head
NEW ZEALAND
0 200 km
Mason Bay Island Hill
StewartI s Homestead
Mason Bay Duck Ck
47OS MartinsC k
Parabolic ‘6’
Ernest Islands
0 20 km Kilbride Homestead
0 5 km
168OE
FIGURE 2. Location of Mason Bay study site, Stewart Island, New Zealand.
Impact of Exotic Dune Grass Species 317
established foredune; and (iii) associated coastal progradation. Changes in the
density and extent of Ammophila and Desmoschoenus , the dominant indigenous
foredune species, are mapped for the period 1958 Á2002. Ammophila appears as a
relatively uniform vegetation cover in the aerial photographs, in contrast to the
variable texture of bare sand and scattered Desmoschoenus.
The pre-Ammophila morphology of the established foredune in Mason Bay was
interpreted from photographs dating from the 1930s, descriptions by Leonard
Cockayne, an eminent New Zealand botanist with a particular interest in dune
flora, the 1958 aerial photograph, and small sections of Desmoschoenus foredune
that survive in Mason Bay north of Duck Creek. The elevation and position of
former nabkha associated with Desmoschoenus are indicated by the presence of
rhizomes of this species (as indicated in Figure 5).
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Ammophila arenaria and Thinopyrum junceiforme
Ammophila is a perennial rhizomatous dune grass that forms dense erect tufts with
the aerial shoots taking a tussock form. The plant has an extensive rhizome network
that extends both horizontally and vertically. The primary mode of reproduction of
Ammophila is vegetative (through spreading rhizomes), although seed production is
locally important. Rhizomes are easily broken into fragments by storm waves that
are then transported by currents and washed onshore at new locations (Aptekar &
Rejmanek 2000). Vegetative reproduction can occur from buds attached to the
¨
parent plant or from the long-distance dispersal of rhizome fragments that contain
viable buds (Pavlik 1983; Buell et al . 1995). Rhizome fragments can be dispersed
over long distances by strong winds or sea currents in this way (Baye 1990).
Ammophila is native to the coasts of Europe and North Africa (between 638 N and
308 N latitude), where it is abundant on mobile and semi-fixed dunes (Huiskes
1979).
Ammophila responds positively to burial. The leafy shoots of Ammophila are able
to grow vertically following sand deposition. However, once a burial threshold is
reached for an individual plant, auxiliary buds develop to create vertical shoots with
long internodes (vertical rhizomes). With further growth, the vertical rhizomes
reach the sand surface with the apex becoming a new leafy shoot (Gemmell et al .
1953). Aerial shoots form along the vertical rhizomes creating dense tufts. These
clusters of tillers decrease the wind speed around the plant resulting in increased
sand deposition (Willis et al . 1959; Huiskes 1979). This increased deposition of
sand is counteracted by the rapid production of elongated internodes along the
stems of the vertical rhizomes, resulting in Ammophila being able to tolerate burial
of up to 1 m (Ranwell 1972; Sykes & Wilson 1990). Ammophila not only responds
positively to burial but actively encourages burial by sand for maximum growth and
full completion of its life cycle (Kent et al . 2001).
Ammophila builds high and often steep foredunes across its home range (Doing
1985). Along the coasts of Australia and the Pacific coast of the USA, Ammophila
has been shown to build larger and more continuous dunes than the native sand-
binders (Wiedemann & Pickart 2004). Along parts of the Victorian coast, low and
wide foredunes, characteristic of areas dominated by native grasses, are replaced by
Ammophila foredunes up to 5 m high (Bell 1988). These changes have been
attributed to differences in plant morphology and habit between Ammophila and
the native sand-binders. In New Zealand, Ammophila tends to build higher and
318 M. Hilton et al.
steeper dunes compared to Spinifex or Desmoschoenus. In a study of Manawatu
foredune profiles, Esler (1970) found that dunes formed under Ammophila had
slopes less than 288 and heights over 6 m, whereas dunes formed under
Desmoschoenus had slopes less than 148 and were approximately 0.5 m in height.
These differences were attributed to the ability of the dense Ammophila cover to
trap and bind sand more effectively than the sparse Desmoschoenus cover.
Ammophila can cause major changes to the structure and composition of
indigenous plant communities related to alterations to dune morphology, sand
budgets and nutrient supply (Pickart & Sawyer 1998; Duncan 2001; Hilton et al .
2005). Ammophila excludes other species unable to cope with high rates of burial
(Maun 1998) or burial/erosion during shadow dune development (Hilton et al .
2005). The impact of Ammophila on indigenous dune systems in New Zealand has
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been recognised largely in the context of its potential to displace indigenous species
and degrade habitats. This is reflected in Ammophila being rated as the most
invasive species in New Zealand (Owen 1996). To date, there has been little
consideration of the long-term loss of dune system function as a result of
Ammophila invasion. Ammophila may reduce or prevent phases of instability that
create habitat for specialist dune species (Hilton et al . 2005). These phases of
instability are characteristic of transgressive dune systems and favour the continued
dominance of native dune species. Ammophila -dominated dune systems are
unlikely to respond to climate fluctuations and vegetation stress (such as drought)
in the same way as pre-Ammophila systems (Dixon et al . 2004). Hence, we
postulate that Ammophila is likely to prevent or inhibit episodes of foredune
instability and accelerate vegetation succession, to the detriment of dune system
function.
Thinopyrum is an erect, perennial, rhizomatous grass, native to the Baltic,
Atlantic and western Mediterranean coasts of Europe. It has a wide latitudinal
home range, from Finland (628 N. Lat.) to the Cadiz region of Spain (368 N. Lat.).
It is also known as ‘sea couch’, ‘sand couch-grass’ or ‘Russian wheatgrass’ (in the
USA). It was formerly identified as ‘Elymus farctus’ in Australia, where it was first
observed in Port Phillip Bay, Victoria, in 1933 (Heyligers 1985). The circum-
stances of the introduction of Thinopyrum to Australia (and New Zealand) are
unclear; however, it probably arrived in Australia with ballast water (Heyligers
1985; Mavrinac 1986). Since 1933 Thinopyrum has spread naturally or with human
assistance to South Australia and the northern coasts of Tasmania. It is capable of
dispersal by sea-rafted fragmented rhizome or seed (Harris & Davy 1986).
Thinopyrum grows closer to the sea and lower down the beach than any
indigenous Australian foredunes species. It is exceptionally tolerant of salinity and
occasional tidal inundation (Heyligers 1985). Mavrinac (1986) states that it grows
closer to the sea than any other ‘British’ dune grass. It is capable of rapid tiller and
lateral/oblique rhizome growth and has excellent sand-trapping and dune-forming
abilities. In Europe, Thinopyrum often forms embryonic dunes or the first (frontal)
dune (zone 3 of Doing 1985), which seldom exceed 1 m in height (see Plate 1). The
morphology of Thinopyrum foredunes in Australia has not been described system-
atically. Heyligers (1985) describes Thinopyrum as forming ‘low wide foredunes’ in
low to moderate energy conditions. As wind conditions increase Thinopyrum dunes
become increasingly hummocky. It tends to form a dune with a steep stoss face
when growing in front of a former dune or a terrace if the beach is prograding
Impact of Exotic Dune Grass Species 319
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PLATE 1. Low Thinopyrum incipient foredunes on the southern coast of Texel Island (De
Hors), Netherlands, in a situation of rapid coastal progradation (24 June 2005). Marram
grass occurs on the first established foredune, further inland.
(Heyligers 1985). The vigour of Thinopyrum is much reduced where it grows at
higher elevations on foredunes.
Foredune development following Ammophila invasion, Mason Bay,
Stewart Island
Ammophila was intentionally introduced to Mason Bay during the 1930s to stabilise
dunes on Kilbride farm, at the southern end of the bay (see Figure 2). Subsequent
dispersal probably occurred through the natural spread of rhizome fragments from
north to south (Hilton et al . 2005), although there is anecdotal evidence that some
Ammophila was planted in the vicinity of Duck Creek in the 1950s. By 1958
Ammophila had established between Martins Creek and Duck Creek (hereafter
‘central Mason Bay’), where it occurred in small patches within 400 m of the high
water line. The area dominated by Ammophila , where canopy cover exceeded 50
per cent, was 1.4 ha, representing 0.6 per cent of the area of central Mason Bay (Jul
1998) (see Figure 3).
The area occupied by Ammophila increased exponentially between 1958 and
1978. Ammophila had established across most of the foredune environment
between Martins Creek and Duck Creek by 1978, although the cover was
discontinuous. At this time Ammophila extended up to 750 m inland and
dominated 17.8 ha of the central dunes, an increase of 1137 per cent over 20
years (Jul 1998). A significant increase in both the extent and density of Ammophila
occurred between 1978 and 2000 (see Figure 3). During this period Ammophila
achieved almost total cover between Martins Creek and Duck Creek, eventually
forming a continuous foredune and displacing all Desmoschoenus. By 1998
320 M. Hilton et al.
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FIGURE 3. Pattern of Ammophila invasion north of Martin’s Creek, Mason Bay, Stewart
Island, 1958 Á2000 (Hilton et al . 2005).
Ammophila covered 74.9 ha of the central dunes (Jul 1998). This represents an
increase in area dominated by Ammophila of 5204 per cent in the 40 years between
1958 and 1998.
Foredune development * central Mason Bay
Ammophila invaded the central dune system from south to north and inland from
the established foredune. During this process, the morphology of the foredune
changed from a low, sparsely vegetated and hummocky foredune, discontinuous
alongshore (Stage IV after Hesp 1988), to a relatively massive, densely vegetated,
uniform and continuous alongshore foredune complex (Stage I). We have a good
understanding of the foredune landscape prior to Ammophila invasion. Cockayne
(1909, p. 18) described the foredunes of Mason Bay, prior to the introduction of
Ammophila , as comprising ‘6 Á10 ft (1.8 Á3.0 m) tall, haystack-like dunes’. That is,
the foredune comprised a series of isolated nabkha or shadow dunes (Type 4Á5;
Hesp 1988), formed in association with Desmoschoenus (with some Austrofestuca
littoralis and Euphorbia glauca ). Three dunes of this type still occur in the foredune
environment in Mason Bay, north of Duck Creek (see Plate 2a). Cockayne’s
descriptions accord with ground photographs of the foredune environment near
Duck Creek, taken during the 1930s. The 1958 aerial photograph is of average
quality, but also shows that the vegetation cover at this time was patchy with
numerous nabkha or shadow dunes (see Figure 4a). The irregular alongshore
topography of the Desmoschoenus foredune in 1958 suggests that blowout formation
through the foredune occurred from time to time (see Figure 3).
Ammophila invasion and foredune development progressed rapidly between 1958
and 1998, culminating in a relatively massive, stable, continuous foredune (see
Figure 5). Ammophila had formed a semi-uniform cover by 1978 (see Figure 4f),
creating a continuous, though topographically irregular, foredune (Stage III, Hesp
1988). Development of the foredune during this period occurred through
coalescence of adjacent Ammophila shadow dunes, as the vegetation cover
Impact of Exotic Dune Grass Species 321
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PLATE 2. (a) One of three surviving Desmoschoenus foredunes north of Duck Creek (viewed
from about the high water line, looking inland) and (b) the Ammophila foredune between
Duck Creek and Martins Creek.
increased (Hilton et al . 2005). Desmoschoenus had been displaced from the stoss
face of the established foredune during the period 1958 Á1978, as the new marram
foredune prograded seawards and accreted. Ammophila increased in extent and
density through the backdune environment, with a corresponding decline in the
area of unvegetated and Desmoschoenus-dominated habitat. By 1989, Ammophila
had formed a dense, uniform cover along the stoss face and had become the
322 M. Hilton et al.
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FIGURE 4. A sequence of aerial photographs and vegetation maps highlighting the rapid
spread of Ammophila and concomitant development of foredune and parabolic dune
landforms and vegetation cover. The outline of the parabolic dune (‘parabolic 6’) at each
stage of development is highlighted by the dashed line. The arrow indicates the closure of the
blowout which gave rise to the central parabolic dune. TA: trailing arm; EF: erosional face;
DL: depositional lobe; DS: deflation surface. The location of ‘parabolic 6’ is indicated in
Figure 1.
dominant cover through the lee face of the foredune (see Figure 4g). During the
period 1989 Á2002, the density of Ammophila increased to form an extensive mono-
specific (/80 per cent) cover (see Figure 4h). The contemporary foredune is
Impact of Exotic Dune Grass Species 323
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FIGURE 5. Comparison of the foredune morphology before and after Ammophila invasion,
for the central dune system of Mason Bay. The morphology of the Desmoschoenus -dominated
foredune is derived from the descriptions of Cockayne (1909), field observations of remnant
(dead) Desmoschoenus rhizome and the 1958 aerial photograph.
characterised by a steep stoss face, a broad, relatively flat terrace and a gently
sloping lee face (see Plate 2b and Figure 5) equivalent to Stage I of Hesp (1988).
The overall morphology of the foredune is now very different from the pre-
Ammophila foredune (se Table 1). Lateral growth has occurred primarily through
seaward progradation, averaging 50 m between 1958 and 2002, probably
encouraged by high rates of deposition and stabilisation under Ammophila , rather
than any change to the beach-dune sediment budget.
The stoss face of the massive Ammophila foredune between Duck Creek and
Martin’s Creek appears to be stable. The numerous corridors and depressions that
dissected the foredune at the time of Ammophila invasion have now closed. Two
narrow blowouts adjacent to ‘parabolic 6’ persisted until 1989 (see Figure 4c).
During the initial stages of Ammophila invasion, the width and depth of these
blowouts was enhanced, providing a major pathway for sediment input into the
backdune system. Closure of the blowouts occurred between 1989 and 2002 as the
density of Ammophila increased, stabilising the throat of the blowout and reducing
TABLE 1. Changes to foredune morphology,a 1958 Á2002
1958 2002
Dominant species D. spiralis A. arenaria
Vegetation cover (per cent) 10 Á30 /80
Hesp (1988) foredune stage IV I
Maximum height (m) 3 11
Width (m) 80 150
Area (m2) 240 1650
Volumeb (m3) 4.8 )/105 3.3 )/106
Notes: aDimensions are derived from Figure 5.
b
Assuming a 2 km foredune length.
324 M. Hilton et al.
rates of sediment transport. Narrow channels still occur along the stoss face of the
foredune, but these are ephemeral features, a few metres wide and 20 m or so deep
(Hilton et al . 2005). To date none of these depressions has developed into a
blowout.
Case study I: foredune parabolic dune development, Mason Bay
Parabolic dunes and sand sheets are the most prevalent dune form in the Mason
Bay dune system. Climbing, imbricate forms occur most widely, usually modified
by the underlying bedrock. However, a series of long-walled parabolic dunes
(terminology after Pye 1983) has developed in a relatively flat section of the central
dunes of Mason Bay. The contemporary dune system between Duck Creek and
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Martins Creek comprises six adjacent parabolic dunes transgressing over a low
gradient stonefield. The most southerly of these parabolic dunes (‘parabolic 6’) is
the best defined of these dunes (see Figure 2).
‘Parabolic 6’ evolved from a blowout, some time prior to or coincident with
Ammophila invasion, probably in the late 1940s. The formation of relatively steep-
sided Ammophila shadow dunes may well have caused the development of such
blowouts. In 1958 the blowout lacked trailing arms (see Figure 4a). The
depositional lobe of the incipient parabolic dune would probably have been a
relatively low, sparsely vegetated feature (see Table 2). Ammophila had established a
sparse, patchy cover within the foredune by 1958 and had started to colonise the
more stable walls of the blowout/incipient ‘parabolic 6’ (see Figure 4a). The
characteristic features of a long-walled parabolic dune had formed by 1978.
The northern trailing arm (TA) and a deflation surface (DS) are clearly defined,
along with a sparsely vegetated depositional lobe (DL) (see Figure 4b). Develop-
ment of the deflation surface occurred primarily through elongation, increasing in
length at 7.91 m/year between 1958 and 1978 (see Table 3). Widening of the
surface also occurred, which resulted in an increase in area of the deflation surface
from 0.35 to 1.15 ha between 1958 and 1978, a 226 per cent increase. Ammophila
had formed a near-continuous cover across the foredune by 1978, but it was still
absent from the parabolic dune (see Figure 4b).
‘Parabolic 6’ became increasingly sheltered by the rapidly accreting Ammophila
foredune between 1978 and 1989. It continued to migrate downwind between
1978 and 1989; however, the rate of advance declined (see Table 3). During this
TABLE 2. Temporal changes in the morphology of ‘parabolic 6’
1958a 1978 1989 2002
Total lengthb (m) 176.12 600.71 657.83 664.50
Length of trailing arm (north) (m) Á 308.40 343.70 468.40
Length of trailing arm (south) (m) Á Á 325.60 325.60
Length of deflation surface (m) 92.34 183.74 285.60 452.20
Length of depositional lobe (m) 90.44 355.10 329.39 243.71
Area of deflation surface (ha) 0.35 1.15 2.14 5.20
Area of erosional face (ha) Á 0.68 1.68 0.61
Area of depositional lobe (ha) 0.51 4.35 2.62 2.34
Notes: aBest estimate only, due to poor image quality.
b
Excluding foredune.
Impact of Exotic Dune Grass Species 325
TABLE 3. Calculated migration rates (m/year) of ‘parabolic 6’
1958 Á78 1978 Á89 1989 Á2002
Deflation surface 7.91 6.02 15.60
Erosional face Á 16.29 7.00
Depositional lobe 24.19 5.56 0.79
period the deflation surface became significantly larger, with a corresponding
decline in the length and area of the depositional lobe (see Table 2). The deflation
surface increased in length at a rate of 6.02 m/year between 1978 and 1989, with an
increase in area from 1.15 to 2.14 ha, representing an 86 per cent increase.
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Ammophila had established throughout the parabolic dune by 2002, although
some remnant areas of Desmoschoenus on the northern trailing arm persisted (see
Figure 4f). Between 1989 and 2002 the rate of landward advance of the
depositional lobe was almost negligible (see Table 3). At the same time the
deflation surface increased in length at an average rate of 15.60 m/year and
increased in area to 5.20 ha, representing a 143 per cent increase over 13 years. The
length of the depositional lobe decreased as the eroding face advanced. Develop-
ment of the depositional lobe during this period occurred through vertical
accretion, rather than downwind extension. The depositional lobe now comprises
a series of large Ammophila shadow dunes, 3 Á5 m high.
‘Parabolic 6’ has been relatively stable since Ammophila invasion. The develop-
ment of a massive Ammophila foredune has caused a decline in available sediment
within the parabolic dune system. The development of the deflation surface, and
concomitant reduction in length of the depositional lobe, occurred as the
established foredune evolved. The development of large deflation surfaces of this
type in this context has also been observed in Oregon (Carter et al . 1990).
However, the ongoing formation of nabkha dunes across the seaward half of the
deflation (and former deflation) surface indicates that some sand is still in
circulation. We have observed significant jettation (terminology after Arens 1996)
across the foredune, during strong westerly winds, that provides sand for ongoing
nabkha development across the rear slopes of the foredune and former deflation
zone of the parabolic dune (see Figure 4d).
Ammophila invasion and foredune development may have contributed to the
subsequent stabilisation of ‘parabolic 6’ and the adjacent parabolic dunes. This
foredune has trapped a great deal of sand that might otherwise have contributed to
parabolic dune development and reduced rates of sedimentation in its lee.
Ammophila has also established in almost all elements of the parabolic dunes,
with the exception of the erosional face, forcing stability. However, it may be that
these parabolic dunes were close to completing their life cycle at the time
Ammophila was introduced.
Case study II: sea-wheat grass, Younghusband Peninsula
The coastal plain between the Murray River mouth and Naracoorte in southeast
Australia contains a sequence of at least eight coastal barriers, which have been
stranded and preserved by uplift. The oldest barrier, approximately 800,000 years
326 M. Hilton et al.
old, is situated around 90 km from the coast. The youngest and active barrier, the
Younghusband Peninsula (see Figure 1) is less than 7000 years old (Harvey 1981;
Belperio 1995). This sequence is widely regarded as one of the classic records of
Quaternary sea-level change and barrier development.
The Younghusband Peninsula is a modern analogue for the formation of the
older barriers (Bourman et al . 2000). Short and Hesp (1984) propose a five-stage
model for the Sir Richard and Younghusband Peninsulas (see Figure 6), involving:
(1) the formation of Pleistocene barriers below modern sea level; (2) Holocene sea-
level transgression and formation of the initial Holocene shoreline; (3) barrier
progradation, as foredune ridges; (4) depletion of nearshore sand supplies and
initial dune transgression and shoreline erosion; and (5) continued dune
transgressions and barrier regression. Episodes of transgressive dune development,
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following foredune disturbance were, therefore, central to barrier and dune system
development.
Short and Hesp (1980) describe the morphology of incipient foredunes and
established foredunes between Kingston and the Murray Mouth, in relation to
variations in beachÁsurfzone morphodynamics. At the time of the field survey
FIGURE 6. An interpretation of the genesis of the Younghusband Peninsula (after Short &
Hesp 1984).
Impact of Exotic Dune Grass Species 327
Spinifex sericeus and Austrofestuca littoralis were associated with incipient foredunes.
A wide range of incipient foredune and foredune morphologies were present. In
1979, when Short and Hesp conducted fieldwork, topographically variable and
highly eroded foredune morphologies (types Fd and Fe) occurred widely between
the Murray Mouth and ‘The Granites’, as might be expected with a regressive
barrier. Incipient foredunes were intermittent, particularly in the region extending
between 95 and 123 km south of the Murray Mouth. They observed foredune
stability to increase towards Kingston and transgressive dune development to
decrease, consistent with declining levels of wave and wind energy in the lee of
Cape Jaffa.
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Thinopyum invasion
Thinopyrum established along much of the length of the Younghusband Peninsula
with remarkable rapidity. It was not observed by Short and Hesp (1980) during a
systematic survey of foredune morphology between Kingston and the Murray River
mouth in 1979. Thinopyrum was observed soon after, in 1986, at Butcher Gap
Drain and Blackford Drain near Kingston and the Murray Mouth (Mavrinac
1986). It was also present at this time along sections of the Adelaide coast (Torrens
Outlet and Bungaloo Creek). Thinopyrum had formed a substantial incipient
foredune at each of these sites, 15 Á20 m wide, 1.0 Á1.5 m high, seaward of the
existing Spinifex foredune.
Thinopyrum appears to have occupied the coast between the Murray Mouth and
Kingston, a distance of about 170 km, during the early to mid-1980s. There has
been no work, to date, on processes of Thinopyrum dispersal. There is no evidence
that this species was deliberately planted in South Australia, and it almost certainly
established along the Younghusband Peninsula without assistance. It most likely
spread by fragmented rhizome, which is likely to enter the sea in large quantities
during storm-forced erosion of the incipient foredune. The low elevation of
Thinopyrum foredunes would ensure this process occurs frequently.
Incipient foredune development in conjunction with Thinopyrum ,
Younghusband Peninsula
‘The Granites’
‘The Granites’ site is located in the relatively sheltered ‘Coorong III’ sector of
Short and Hesp (1980). This is a stable site, with no significant trend of
progradation or erosion over the period 1945 to the early 1980s, prior to the
arrival of Thinopyrum. The site is situated 250 m south of the public car park
and beach access point. The incipient foredune at this locality is typical of the
dune for some kilometres north and south of ‘The Granites’. It exhibits a
‘terrace’ form (described in Hesp 2002), with a high cover of Thinopyrum (/75
per cent) (see Figure 7). This feature is 10 m wide and 1 Á2 m high and
continuous alongshore. The backdune comprised a former blowout, now well
vegetated with Leucophyta brownii , Olearia axillaris , Carpobrotus rossii and other
species. The new Thinopyrum foredune partially overlies the stoss face of the
former Spinifex foredune. Spinifex occurs throughout the surveyed area, though
nowhere is it particularly dense (see Figure 7).
328 M. Hilton et al.
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FIGURE 7. ‘The Granites’ *Thinopyrum occurs in a narrow band seaward of the former
Spinifex foredune.
‘28 Mile Crossing’
‘28 Mile Crossing’ is one of several 4-wheel-drive tracks that enable access to the
beach along the southern Coorong. The ‘28 Mile Crossing’ study site is located
approximately 100 m southeast of the beach access of the ‘28 Mile Crossing’ 4-
wheel-drive track. The site was chosen to examine the morphology of incipient
Thinopyrum foredunes on a section of eroding shore. The former Spinifex foredune
comprises a series of remnant knobs (Type 3 Á4 foredune; Hesp 1988), interspersed
with lobes of sand associated with Thinopyrum (see Figure 8). These lobes
constitute sections of incipient foredune, which extend 3Á4 m from the face of the
eroding foredune. They have a ‘ramp’ morphology (after Hesp 2002). Unvegetated
sections of scarped foredune separate these lobes. An extensive deflation surface
lies inland of the foredune, with transverse dunes occurring downwind. The
strandline was 15 Á20 m in front of the foredune on the day of the survey.
Thinopyrum forms a dense patch of vegetation across and extending away from
the eroding face of the foredune and, secondly, a sparse cover over the lee slopes of
the foredune and into the deflation surface (see Figure 8). Spinifex occupies the
remnant knobs and overlaps with Thinopyrum in places. Euphorbia paralias occurs
across the crests of the knobs and across the rear of the foredune. The presence of
various mature backdune shrubs and sedges (Stackhousia spathulata , Olearia
axillaris , Isolepis nodosa and Ozothamnus turbinatus , for example) suggests that
the former Spinifex foredune has been stable for some time, albeit the face of the
foredune is eroding.
Impact of Exotic Dune Grass Species 329
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FIGURE 8. 3D representation of the ‘28 Mile Crossing’ survey area. Thinopyrum has
occupied hollows between remnant knobs of Spinifex and built incipient ramp foredunes 3 Á4
m seaward of the foredune scarp.
Incipient foredune formation in conjunction with Thinopyrum
Thinopyrum has formed a narrow, usually low, incipient foredune along the
Younghusband Peninsula since the early 1980s. The terms ‘foredune’ and
‘incipient foredune’ require some clarification. Foredunes are ‘shore-parallel
dune ridges formed on the top of the backshore by aeolian deposition within
vegetation’ (Hesp 2002, p. 145). ‘Incipient foredunes are new, or developing,
foredunes within pioneer plant communities’ (Hesp 2002, p. 145). The sites
described contain type 2a and 2b incipient foredunes (after Hesp 1989), that is,
they have formed on the backshore by growth of Thinopyrum rhizome growth into
that section of the back-beach that lies between the toe of the foredune and the
driftline. At the two sites described, Thinopyrum has formed a terrace against the
former Spinifex foredune, usually at a slightly lower elevation. We do not know, at
present, whether Thinopyrum is in the process of establishing a more substantial
incipient foredune at ‘28 Mile Crossing’, or whether the environment is preventing
continuous alongshore colonisation.
Thinopyrum invasion has had a significant impact on the morphology of the pre-
existing Spinifex foredune at both sites. It has caused the stoss face of the foredune
to prograde and establish at lower elevations. Thinopyrum tends to occupy low-lying
gaps in eroding foredunes and encourage deposition, resulting in a higher overall
vegetation cover and more uniform topography. Hence, the overall impact of
Thinopyrum is to encourage the formation of wider, more uniform and more
continuous foredunes, at least in situations of low to moderate rates of accretion.
We have seen higher (4Á5 m) Thinopyrum foredunes at the Murray Mouth, where
330 M. Hilton et al.
the rates of accretion may be significantly greater than observed at the sites
reported here.
Thinopyrum grows vigorously on the stoss face of the foredune, and is clearly
tolerant of more frequent sea-water inundation and soil salinity. Our surveys
demonstrate that it also survives in backdune environments, albeit in a semi-
moribund form. It is not, therefore, vulnerable to catastrophic removal during
episodes of severe foredune scarping. Backdune plants of Thinopyrum are probably
capable of rapid growth when erosion exposes them to higher rates of sedimenta-
tion and higher nutrient levels.
Spinifex has been displaced from the front face of the foredune along the
Younghusband Peninsula. This will greatly reduce the distribution of Spinifex and
associated indigenous species, but it may have significant ecological implications for
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other flora and fauna. It is inevitable, for example, that the habitat of shore birds
such as the hooded plover will decline, as Thinopyrum colonises gaps in the
foredune and blowouts, and occupies areas of the backbeach that would not
normally be colonised by Spinifex .
The potential impact of Ammophila and Thinopyrum on blowout
formation
Ammophila and Thinopyrum have produced new dune morphologies in the dune
systems described. A massive foredune complex established at Mason Bay
following Ammophila invasion. The landscape associated with Desmoschoenus has
been buried beneath an evenly vegetated, continuous established foredune, 150 Á
200 m wide and 10 Á12 m high. Large quantities of sand, which would otherwise
have entered the dune system, are now trapped in this foredune complex.
Ammophila has also invaded the hinterland of the dune system, with resulting
loss of dune flora and transgressive dune mobility. Thinopyrum occupies a more
limited range *it is primarily a species of the back beach. It has also established
continuous incipient foredunes along most of the length of the Younghusband
Peninsula, and by the mid-1980s occupied approximately 170 km of coast between
the Murray Mouth and Kingston. In both cases irregular foredunes have been
replaced by regular, continuous-alongshore foredunes. Thinopyrum and Ammophila
have also had a major impact on the indigenous flora of the two study sites.
Thinopyrum has displaced Spinifex from the stoss face of the foredune along the
length of the Younghusband Peninsula. The range of Spinifex may be reduced over
time, since it is likely that the crest and rear of the foredune will experience less
sediment input, greater stability and (possibly) accelerated vegetation succession.
Ammophila has had an overwhelmingly adverse impact on the indigenous dune
flora of Mason Bay. The indigenous species associated with the pre-Ammophila
foredune have been totally displaced.
Thinopyrum and Ammophila have produced new dune landscapes and new dune
ecosystems. Here we will consider whether these grasses are likely to reduce the
frequency or intensity of blowout development. This impact would have significant
implications for the long-term development of Mason Bay and Younghusband
Peninsula dune systems, by reducing the incidence of transgressive dune develop-
ment. This would, in turn, impact on the diversity of habitats within the dune
systems.
Impact of Exotic Dune Grass Species 331
The processes that lead to blowout development are well documented. A
blowout is a saucer-, cup- or trough-shaped depression or hollow formed by wind-
forced erosion in a pre-existing deposit of sand. They may be initiated as a result
of: wave erosion along the seaward face of the foredune; topographic acceleration
of airflow over the dune crest; climate change; vegetation variation through space
or change through time; water erosion; high velocity wind erosion, sand inunda-
tion and burial; and human activities (for a review of these processes see Hesp
2002).
There is some evidence that Ammophila and Thinopyrum respond differently to
these processes compared with indigenous foredune species. Both species form
uniform, continuous, Type I foredunes (or contribute to this character along the
stoss face of pre-existing Spinifex foredunes). The potential for blowout develop-
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ment through topographic acceleration of airflow is, therefore, likely to be reduced.
Ammophila and Thinopyrum occur on a range of coasts, including those with dry
Mediterranean climates. Ammophila has a number of physiological and morpho-
logical adaptations that reduce the impact of drought stress. In a series of
glasshouse experiments, Dixon et al . (2004) compared the tolerance of Ammophila
and Desmoschoenus to drought conditions. Desmoschoenus showed signs of water
stress after 8 days in glasshouse drought conditions, whereas Ammophila showed no
signs of stress until day 18. At the completion of the drought trial, 5 per cent of
Desmoschoenus and 80 per cent of the Ammophila recovered. These results accord
with field observations and the results of earlier glasshouse experiments (Huiskes
1979). The relative tolerance of Spinifex and Thinopyrum and Spinifex and
Ammophila to drought has not been ascertained.
Both Ammophila and Thinopyrum species are rhizomatous grasses capable of
trapping and binding sand. Their leaves are capable of ‘lodging’ and so are able to
withstand strong winds and maintain a uniform vegetation cover. Established
foredunes associated with both species are vulnerable to scarping by storm wave
swash; however, gaps or low points are likely to be rapidly repaired by post-storm
growth. Our observations at ‘28 Mile Crossing’ on the Younghusband Peninsula
indicate that Thinopyrum is able to establish cover and form incipient foredunes on
an eroding coastline. Burial may pose a significant threat to Thinopyrum ; however,
like Ammophila , it is possibly tolerant of darkness and may be able to emerge when
buried. The incipient foredunes of both species are scarped during episodes of
storm wave activity and storm surge. We suspect that in both cases incipient
foredunes are rapidly repaired by, first, mass failure of the upper sections of the
scarp and then rapid elongation of lateral rhizomes. The toe of foredunes of both
species must be in constant flux, even on prograding coasts, given their proximity to
the sea. Finally, Thinopyrum is known to be exceptionally tolerant of salt during
episodes of elevated sea level associated with storms. Ammophila is not tolerant of
salt in the root zone, but avoids the problem by building relatively high, massive
foredunes.
Conclusions
In conclusion, it has been demonstrated that two exotic dune grasses, namely
Thinopyrum junceiforme and Ammophila arenaria , have been introduced into dune
environments in Australia and New Zealand, respectively, where they have:
332 M. Hilton et al.
(1) replaced irregular, sparsely vegetated, established foredunes with continuous
incipient foredunes;
(2) encouraged accretion and progradation;
(3) increased the extent and evenness of vegetation cover;
(4) rapidly displaced native species; and
(5) altered dune habitat for indigenous fauna and flora.
These impacts have occurred over the last few decades. We have raised the question
of the long-term impact of these species on barrier and dune system development.
Are these species likely to reduce the incidence or extent of blowout development?
If so, could a reduction in transgressive dune development affect the natural
development of coastal barriers along coasts occupied by Ammophila and
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Thinopyrum? These processes are an essential part of the geomorphic and ecologic
character of temperate sandy coasts. Both study sites are located in national parks
which have been designated to conserve natural values.
A substantial programme of research is required to resolve this matter. Such a
programme would need to compare the relative response of indigenous and exotic
species to the environmental stresses identified above, both in isolation and in
combination. We already know that Ammophila is more tolerant of burial (Sykes &
Wilson 1990) and more tolerant of drought (Dixon et al . 2004) than Desmoschoe-
nus. Relatively little is known about the response of Thinopyrum to environmental
stress on temperate coasts, in part because this species has attracted virtually no
attention in the short period it has been in Australia.
There is clearly a need for a management response to this issue. In New Zealand
the Department of Conservation (DoC) has commenced a programme of
Ammophila eradication in Fiordland and Rakiura National Parks (Stewart Island)
in southern New Zealand. The DoC has employed helicopters, all-terrain vehicles
and back packs to apply selective herbicides in these parks. Much smaller
operations, targeting Ammophila , are occurring elsewhere in New Zealand. The
Stewart Island operations commenced in 1987 and are likely to run for at least
another 15 years. The question remains as to whether the DoC, local authorities or
private landowners will undertake Ammophila eradication in significant conserva-
tion areas outside national parks. At least one NGO, the Yellow-eyed Penguin
Trust, based in Dunedin, has initiated marram-control operations on private land
to improve penguin habitat and restore indigenous vegetation. In contrast, there
has been little extensive control of Ammophila in Australia, apart from work by the
Department of Primary Industry, Water and the Environment (DIPWE) in
southwest Tasmania. No control of Thinopyrum has yet occurred in Australia.
This may be due to the recent focus on invasive species of backdune and established
foredunes such as Bitou Bush (Chrysanthemoides monilifera ), but this inaction
probably also reflects low levels of awareness of the impact of introduced foredune
species. The recent release of the Tasmanian Beach Weed Strategy (Rudman 2003)
and initiation of an investigation of the impact of South African Pyp Grass
(Ehrharta villosa ) in Coorong National Park by the Department of Environment
and Heritage indicate a developing awareness of exotic dune weeds.
Correspondence: Mike Hilton, Department of Geography, University of Otago, PO
Box 56, Dunedin, New Zealand. E-mail: mjh@geography.otago.ac.nz
Impact of Exotic Dune Grass Species 333
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The Impact of Exotic Dune Grass Species on Foredune Development in
Australia and New Zealand: a case study of Ammophila arenaria and
Thinopyrum junceiforme
Mike Hilton a; Nick Harvey a; Andrew Hart a; Kris James a; Chris Arbuckle a
a
University of OtagoUniversity of AdelaideUniversity of OtagoUniversity of AdelaideOtago Regional Council,
Dunedin, New Zealand, Australia, New Zealand, Australia, New Zealand
Online Publication Date: 01 November 2006
To cite this Article Hilton, Mike, Harvey, Nick, Hart, Andrew, James, Kris and Arbuckle, Chris(2006)'The Impact of Exotic Dune Grass
Species on Foredune Development in Australia and New Zealand: a case study of Ammophila arenaria and Thinopyrum
junceiforme',Australian Geographer,37:3,313 — 334
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Australian Geographer, Vol. 37, No. 3,
pp. 313 Á334, November 2006
The Impact of Exotic Dune Grass Species on
Foredune Development in Australia and
New Zealand: a case study of Ammophila
arenaria and Thinopyrum junceiforme
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MIKE HILTON, NICK HARVEY, ANDREW HART, KRIS JAMES
& CHRIS ARBUCKLE, University of Otago, New Zealand; University of
Adelaide, Australia; University of Otago, New Zealand; University of Adelaide,
Australia; Otago Regional Council, Dunedin, New Zealand
ABSTRACT Marram grass (Ammophila arenaria) and sea-wheat grass ( Thino-
pyrum junceiforme) have been introduced to Australia and New Zealand. This study
examines the morphology of incipient foredunes and established foredunes associated with
these species at two sites, Mason Bay in southern New Zealand, and the Younghusband
Peninsula in South Australia. Both species invaded the existing foredunes very rapidly. In
both cases the antecedent topography comprised relatively sparsely vegetated, irregular
foredunes. Invasion resulted in continuous, regular, evenly vegetated foredunes. At Mason
Bay a massive foredune has formed since 1958, in conjunction with Ammophila.
Thinopyrum has formed an incipient foredune, with a ramp or terrace morphology, along
the Younghusband Peninsula, South Australia. In both cases gaps in the former foredune
have been closed and the indigenous foredune vegetation has been displaced. Both species
may decrease the frequency and severity of blowout development. They are likely to be
resilient to aeolian processes of sedimentation compared with dunes formed by indigenous
species. Ammophila survives burial, is tolerant of drought and is resistant to erosion
associated with storm surge and high waves. Thinopyrum is very tolerant of salinity.
These species may adversely affect the long-term development of coastal barriers by
inhibiting transgressive dune development.
KEY WORDS Marram grass; sea-wheat grass; invasive species impacts; coastal dune
development; Australia; New Zealand.
Introduction
The development and morphology of established foredunes has been well
documented in relation to climate, sediment supply and type, wave climate and
onshore winds (see Hesp 2002 for a review), but less attention has been paid to the
important role of sand-binding grasses, sedges and herbs. Dense, tall, erect grasses
ISSN 0004-9182 print/ISSN 1465-3311 online/06/030313-22 # 2006 Geographical Society of New South Wales Inc.
DOI: 10.1080/00049180600954765
314 M. Hilton et al.
are associated with high, steep-sided, asymmetric foredunes (Hesp 1983, 1989;
Van Dijk et al . 1999). In contrast, species with a lower, more spreading rhizomatous
growth form tend to produce lower, less hummocky dunes (Hesp 2002). The
alongshore morphology of established foredunes is likewise influenced by vegeta-
tion cover, which may be more or less uniform, depending on patterns of species
type, vigour or cover (Carter 1988; Hesp 1999). Over the last 50 years there is
evidence for the global redistribution of dune grass species (Hilton & Harvey 2005)
which may change the dominant foredune grass species and hence affect the dune
morphology. This paper explores such changes using examples from Australia and
New Zealand, and considers the long-term implications of these species for dune
system and barrier genesis, in particular the impact of these grasses on transgressive
dune systems.
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The beaches and dunes of the temperate coasts of Australia and New Zealand
contained only a handful of marine-dispersed species capable of colonising and
occupying the back-beach, between the strandline and the established foredune:
Spinifex sericeus , in both countries; Atriplex billardierei in Australia; and Atriplex
hollowayi (a crystalwort) in New Zealand. In contrast, the established foredune and
back dune floras of both countries contain a rich diversity of species. Of those
indigenous species capable of forming incipient foredunes and established
foredunes, just four are widespread. Spinifex sericeus (spinifex) and Austrofestuca
littoralis (sand tussock) occur in both countries. Desmoschoenus spiralis (pingao or
pikao), a sedge, is endemic to New Zealand. Spinifex longifolia is dominant in
Western Australia. In addition, Atriplex cinerea (grey salt bush), is locally important
in southeast Australia.
The dune flora of Australia and New Zealand changed rapidly over a 50-year
period, from the late 1800s, with the introduction of European, American and
South African dune plants (Heyligers 1985; Hilton & Harvey 2005). Sea spurge
(Euphorbia paralias ), European sea rocket (Cakile maritima ) and American sea
rocket (Cakile edentula ) were accidentally introduced in the ballast of ships. The
seeds of these species apparently survive in sea water for long periods. Sea wheat-
grass (Thinopyrum junceiforme ) and marram grass (Ammophila arenaria ) were
deliberately introduced to New Zealand and Australia to stabilise mobile dunes.
Two South African species, bitou bush (Chrysanthemoides monilifera ) and pyp grass
(Ehrharta villosa ) were also introduced for this purpose. Arctotheca populifolia
(beach daisy), another South African plant, was probably brought to Australia as an
ornamental. These introductions contributed four new species to the strandline Á
back-beach community in southeast Australia (the two sea rockets, beach daisy and
sea spurge); two new species capable of forming incipient and established foredunes
(Ammophila and Thinopyrum ) and two species more commonly associated with
backdune environments or established foredunes (bitou bush and pyp grass). Sea
spurge occurs in all environments, but has limited dune-building capability.
We are concerned primarily with Ammophila and Thinopyrum in the present
paper, in particular the ability of these plants to form incipient foredunes and
established foredunes that are resilient to disturbance. The potential for Ammophila
to trap large volumes of sand in massive foredunes has previously been noted (Esler
1970; Heyligers 1985). More recent work has suggested that Ammophila and
Thinopyrum may also inhibit or prevent blowout and transgressive dune develop-
ment (Hilton & Harvey 2002; Hilton et al . 2005). Suppression of blowouts might
have a significant impact on the natural development of sandy coasts in Australia
Impact of Exotic Dune Grass Species 315
and New Zealand. Transgressive dune activity is critical to the physical develop-
ment of coastal barriers on windward coasts in both countries. Transgressive dune
activity is also closely linked with dune habitat and species diversity, since such
activity inevitably leads to topographic and environmental diversity. We examine
the impact of these species on foredune formation through two case studies:
Ammophila invasion in a large transgressive dune system on the west coast of
Stewart Island, New Zealand and, secondly, Thinopyrum invasion along the
Younghusband Peninsula, South Australia. Finally, we examine the proposition
that these species develop incipient foredunes and established foredunes that are
more resilient to environmental disturbance than foredunes associated with
indigenous species.
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Methods
Fieldwork was undertaken at two locations on the Younghusband Peninsula,
South Australia (see Figure 1) and one location at Mason Bay, Stewart Island (see
Figure 2), during 2003 and 2004. The purpose of this work was to describe,
FIGURE 1. The location of the ‘28 Mile Crossing’ and ‘The Granites’ study sites,
Younghusband Peninsula, South Australia. The Younghusband Peninsula is the most recent
in a progradational sequence of Quaternary coastal barriers (after Belperio 1995).
316 M. Hilton et al.
through systematic survey, the morphology of established foredunes associated with
Ammophila and Thinopyrum, as well as the morphology of the adjacent hinterland;
and to map and classify the associated beach-dune flora. Surveying was
accomplished using a Leica 305 total station. Shaded relief plots, contour maps
and profiles were generated for each site using Surfer 3D (Golden Software).
Historic aerial photographs were used to determine the invasion history of these
species, change in vegetation cover and community type and landform develop-
ment. Ground photographs taken by Andy Short and Patrick Hesp during survey
work in 1979 were compared with recent photography.
Foredune morphology and vegetation cover were surveyed at two sites on the
Younghusband Peninsula *‘The Granites’ and ‘28 Mile Crossing’ (see Figure 1).
These sites were chosen to examine the morphology of Thinopyrum incipient
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foredunes on quasi-stable and eroding coastlines, respectively. ‘The Granites’ site is
located within the relatively low energy ‘Coorong III’ morphodynamic class of
Short and Hesp (1980). Compared with ‘28 Mile Crossing’ (Coorong II), this site
is relatively sheltered from the prevailing southwest winds by Cape Jaffa. In
contrast, foredune scarping appears to have occurred frequently over a period of
some years at ‘28 Mile Crossing’. It has not been possible to reconstruct the
invasion history of Thinopyrum along the Younghusband Peninsula, in part because
it established rapidly, in part because it is associated with narrow (5 Á10 m)
incipient foredunes, which are difficult to identify on the earlier aerial photographs.
We do know it established some time after 1979, when Short and Hesp (1980)
undertook a systematic study of the established foredunes along the Younghusband
Peninsula. We utilised a series of ground photographs, made available by Andy
Short, to determine the foredune morphologies that existed prior to Thinopyrum
invasion.
A sequence of aerial photographs records the invasion history of Ammophila at
various sites along the west coast of Stewart Island. Mason Bay has a record of
aerial photography dating from 1958 with subsequent images available for 1978,
1989 and 2002. Ammophila established in the study area in the early 1950s. These
data document (i) the invasion history of Ammophila and indigenous species
displacement; (ii) the physical development and changing morphology of the
Active dune system
STEWART ISLAND
Mason Head
NEW ZEALAND
0 200 km
Mason Bay Island Hill
StewartI s Homestead
Mason Bay Duck Ck
47OS MartinsC k
Parabolic ‘6’
Ernest Islands
0 20 km Kilbride Homestead
0 5 km
168OE
FIGURE 2. Location of Mason Bay study site, Stewart Island, New Zealand.
Impact of Exotic Dune Grass Species 317
established foredune; and (iii) associated coastal progradation. Changes in the
density and extent of Ammophila and Desmoschoenus , the dominant indigenous
foredune species, are mapped for the period 1958 Á2002. Ammophila appears as a
relatively uniform vegetation cover in the aerial photographs, in contrast to the
variable texture of bare sand and scattered Desmoschoenus.
The pre-Ammophila morphology of the established foredune in Mason Bay was
interpreted from photographs dating from the 1930s, descriptions by Leonard
Cockayne, an eminent New Zealand botanist with a particular interest in dune
flora, the 1958 aerial photograph, and small sections of Desmoschoenus foredune
that survive in Mason Bay north of Duck Creek. The elevation and position of
former nabkha associated with Desmoschoenus are indicated by the presence of
rhizomes of this species (as indicated in Figure 5).
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Ammophila arenaria and Thinopyrum junceiforme
Ammophila is a perennial rhizomatous dune grass that forms dense erect tufts with
the aerial shoots taking a tussock form. The plant has an extensive rhizome network
that extends both horizontally and vertically. The primary mode of reproduction of
Ammophila is vegetative (through spreading rhizomes), although seed production is
locally important. Rhizomes are easily broken into fragments by storm waves that
are then transported by currents and washed onshore at new locations (Aptekar &
Rejmanek 2000). Vegetative reproduction can occur from buds attached to the
¨
parent plant or from the long-distance dispersal of rhizome fragments that contain
viable buds (Pavlik 1983; Buell et al . 1995). Rhizome fragments can be dispersed
over long distances by strong winds or sea currents in this way (Baye 1990).
Ammophila is native to the coasts of Europe and North Africa (between 638 N and
308 N latitude), where it is abundant on mobile and semi-fixed dunes (Huiskes
1979).
Ammophila responds positively to burial. The leafy shoots of Ammophila are able
to grow vertically following sand deposition. However, once a burial threshold is
reached for an individual plant, auxiliary buds develop to create vertical shoots with
long internodes (vertical rhizomes). With further growth, the vertical rhizomes
reach the sand surface with the apex becoming a new leafy shoot (Gemmell et al .
1953). Aerial shoots form along the vertical rhizomes creating dense tufts. These
clusters of tillers decrease the wind speed around the plant resulting in increased
sand deposition (Willis et al . 1959; Huiskes 1979). This increased deposition of
sand is counteracted by the rapid production of elongated internodes along the
stems of the vertical rhizomes, resulting in Ammophila being able to tolerate burial
of up to 1 m (Ranwell 1972; Sykes & Wilson 1990). Ammophila not only responds
positively to burial but actively encourages burial by sand for maximum growth and
full completion of its life cycle (Kent et al . 2001).
Ammophila builds high and often steep foredunes across its home range (Doing
1985). Along the coasts of Australia and the Pacific coast of the USA, Ammophila
has been shown to build larger and more continuous dunes than the native sand-
binders (Wiedemann & Pickart 2004). Along parts of the Victorian coast, low and
wide foredunes, characteristic of areas dominated by native grasses, are replaced by
Ammophila foredunes up to 5 m high (Bell 1988). These changes have been
attributed to differences in plant morphology and habit between Ammophila and
the native sand-binders. In New Zealand, Ammophila tends to build higher and
318 M. Hilton et al.
steeper dunes compared to Spinifex or Desmoschoenus. In a study of Manawatu
foredune profiles, Esler (1970) found that dunes formed under Ammophila had
slopes less than 288 and heights over 6 m, whereas dunes formed under
Desmoschoenus had slopes less than 148 and were approximately 0.5 m in height.
These differences were attributed to the ability of the dense Ammophila cover to
trap and bind sand more effectively than the sparse Desmoschoenus cover.
Ammophila can cause major changes to the structure and composition of
indigenous plant communities related to alterations to dune morphology, sand
budgets and nutrient supply (Pickart & Sawyer 1998; Duncan 2001; Hilton et al .
2005). Ammophila excludes other species unable to cope with high rates of burial
(Maun 1998) or burial/erosion during shadow dune development (Hilton et al .
2005). The impact of Ammophila on indigenous dune systems in New Zealand has
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been recognised largely in the context of its potential to displace indigenous species
and degrade habitats. This is reflected in Ammophila being rated as the most
invasive species in New Zealand (Owen 1996). To date, there has been little
consideration of the long-term loss of dune system function as a result of
Ammophila invasion. Ammophila may reduce or prevent phases of instability that
create habitat for specialist dune species (Hilton et al . 2005). These phases of
instability are characteristic of transgressive dune systems and favour the continued
dominance of native dune species. Ammophila -dominated dune systems are
unlikely to respond to climate fluctuations and vegetation stress (such as drought)
in the same way as pre-Ammophila systems (Dixon et al . 2004). Hence, we
postulate that Ammophila is likely to prevent or inhibit episodes of foredune
instability and accelerate vegetation succession, to the detriment of dune system
function.
Thinopyrum is an erect, perennial, rhizomatous grass, native to the Baltic,
Atlantic and western Mediterranean coasts of Europe. It has a wide latitudinal
home range, from Finland (628 N. Lat.) to the Cadiz region of Spain (368 N. Lat.).
It is also known as ‘sea couch’, ‘sand couch-grass’ or ‘Russian wheatgrass’ (in the
USA). It was formerly identified as ‘Elymus farctus’ in Australia, where it was first
observed in Port Phillip Bay, Victoria, in 1933 (Heyligers 1985). The circum-
stances of the introduction of Thinopyrum to Australia (and New Zealand) are
unclear; however, it probably arrived in Australia with ballast water (Heyligers
1985; Mavrinac 1986). Since 1933 Thinopyrum has spread naturally or with human
assistance to South Australia and the northern coasts of Tasmania. It is capable of
dispersal by sea-rafted fragmented rhizome or seed (Harris & Davy 1986).
Thinopyrum grows closer to the sea and lower down the beach than any
indigenous Australian foredunes species. It is exceptionally tolerant of salinity and
occasional tidal inundation (Heyligers 1985). Mavrinac (1986) states that it grows
closer to the sea than any other ‘British’ dune grass. It is capable of rapid tiller and
lateral/oblique rhizome growth and has excellent sand-trapping and dune-forming
abilities. In Europe, Thinopyrum often forms embryonic dunes or the first (frontal)
dune (zone 3 of Doing 1985), which seldom exceed 1 m in height (see Plate 1). The
morphology of Thinopyrum foredunes in Australia has not been described system-
atically. Heyligers (1985) describes Thinopyrum as forming ‘low wide foredunes’ in
low to moderate energy conditions. As wind conditions increase Thinopyrum dunes
become increasingly hummocky. It tends to form a dune with a steep stoss face
when growing in front of a former dune or a terrace if the beach is prograding
Impact of Exotic Dune Grass Species 319
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PLATE 1. Low Thinopyrum incipient foredunes on the southern coast of Texel Island (De
Hors), Netherlands, in a situation of rapid coastal progradation (24 June 2005). Marram
grass occurs on the first established foredune, further inland.
(Heyligers 1985). The vigour of Thinopyrum is much reduced where it grows at
higher elevations on foredunes.
Foredune development following Ammophila invasion, Mason Bay,
Stewart Island
Ammophila was intentionally introduced to Mason Bay during the 1930s to stabilise
dunes on Kilbride farm, at the southern end of the bay (see Figure 2). Subsequent
dispersal probably occurred through the natural spread of rhizome fragments from
north to south (Hilton et al . 2005), although there is anecdotal evidence that some
Ammophila was planted in the vicinity of Duck Creek in the 1950s. By 1958
Ammophila had established between Martins Creek and Duck Creek (hereafter
‘central Mason Bay’), where it occurred in small patches within 400 m of the high
water line. The area dominated by Ammophila , where canopy cover exceeded 50
per cent, was 1.4 ha, representing 0.6 per cent of the area of central Mason Bay (Jul
1998) (see Figure 3).
The area occupied by Ammophila increased exponentially between 1958 and
1978. Ammophila had established across most of the foredune environment
between Martins Creek and Duck Creek by 1978, although the cover was
discontinuous. At this time Ammophila extended up to 750 m inland and
dominated 17.8 ha of the central dunes, an increase of 1137 per cent over 20
years (Jul 1998). A significant increase in both the extent and density of Ammophila
occurred between 1978 and 2000 (see Figure 3). During this period Ammophila
achieved almost total cover between Martins Creek and Duck Creek, eventually
forming a continuous foredune and displacing all Desmoschoenus. By 1998
320 M. Hilton et al.
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FIGURE 3. Pattern of Ammophila invasion north of Martin’s Creek, Mason Bay, Stewart
Island, 1958 Á2000 (Hilton et al . 2005).
Ammophila covered 74.9 ha of the central dunes (Jul 1998). This represents an
increase in area dominated by Ammophila of 5204 per cent in the 40 years between
1958 and 1998.
Foredune development * central Mason Bay
Ammophila invaded the central dune system from south to north and inland from
the established foredune. During this process, the morphology of the foredune
changed from a low, sparsely vegetated and hummocky foredune, discontinuous
alongshore (Stage IV after Hesp 1988), to a relatively massive, densely vegetated,
uniform and continuous alongshore foredune complex (Stage I). We have a good
understanding of the foredune landscape prior to Ammophila invasion. Cockayne
(1909, p. 18) described the foredunes of Mason Bay, prior to the introduction of
Ammophila , as comprising ‘6 Á10 ft (1.8 Á3.0 m) tall, haystack-like dunes’. That is,
the foredune comprised a series of isolated nabkha or shadow dunes (Type 4Á5;
Hesp 1988), formed in association with Desmoschoenus (with some Austrofestuca
littoralis and Euphorbia glauca ). Three dunes of this type still occur in the foredune
environment in Mason Bay, north of Duck Creek (see Plate 2a). Cockayne’s
descriptions accord with ground photographs of the foredune environment near
Duck Creek, taken during the 1930s. The 1958 aerial photograph is of average
quality, but also shows that the vegetation cover at this time was patchy with
numerous nabkha or shadow dunes (see Figure 4a). The irregular alongshore
topography of the Desmoschoenus foredune in 1958 suggests that blowout formation
through the foredune occurred from time to time (see Figure 3).
Ammophila invasion and foredune development progressed rapidly between 1958
and 1998, culminating in a relatively massive, stable, continuous foredune (see
Figure 5). Ammophila had formed a semi-uniform cover by 1978 (see Figure 4f),
creating a continuous, though topographically irregular, foredune (Stage III, Hesp
1988). Development of the foredune during this period occurred through
coalescence of adjacent Ammophila shadow dunes, as the vegetation cover
Impact of Exotic Dune Grass Species 321
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PLATE 2. (a) One of three surviving Desmoschoenus foredunes north of Duck Creek (viewed
from about the high water line, looking inland) and (b) the Ammophila foredune between
Duck Creek and Martins Creek.
increased (Hilton et al . 2005). Desmoschoenus had been displaced from the stoss
face of the established foredune during the period 1958 Á1978, as the new marram
foredune prograded seawards and accreted. Ammophila increased in extent and
density through the backdune environment, with a corresponding decline in the
area of unvegetated and Desmoschoenus-dominated habitat. By 1989, Ammophila
had formed a dense, uniform cover along the stoss face and had become the
322 M. Hilton et al.
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FIGURE 4. A sequence of aerial photographs and vegetation maps highlighting the rapid
spread of Ammophila and concomitant development of foredune and parabolic dune
landforms and vegetation cover. The outline of the parabolic dune (‘parabolic 6’) at each
stage of development is highlighted by the dashed line. The arrow indicates the closure of the
blowout which gave rise to the central parabolic dune. TA: trailing arm; EF: erosional face;
DL: depositional lobe; DS: deflation surface. The location of ‘parabolic 6’ is indicated in
Figure 1.
dominant cover through the lee face of the foredune (see Figure 4g). During the
period 1989 Á2002, the density of Ammophila increased to form an extensive mono-
specific (/80 per cent) cover (see Figure 4h). The contemporary foredune is
Impact of Exotic Dune Grass Species 323
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FIGURE 5. Comparison of the foredune morphology before and after Ammophila invasion,
for the central dune system of Mason Bay. The morphology of the Desmoschoenus -dominated
foredune is derived from the descriptions of Cockayne (1909), field observations of remnant
(dead) Desmoschoenus rhizome and the 1958 aerial photograph.
characterised by a steep stoss face, a broad, relatively flat terrace and a gently
sloping lee face (see Plate 2b and Figure 5) equivalent to Stage I of Hesp (1988).
The overall morphology of the foredune is now very different from the pre-
Ammophila foredune (se Table 1). Lateral growth has occurred primarily through
seaward progradation, averaging 50 m between 1958 and 2002, probably
encouraged by high rates of deposition and stabilisation under Ammophila , rather
than any change to the beach-dune sediment budget.
The stoss face of the massive Ammophila foredune between Duck Creek and
Martin’s Creek appears to be stable. The numerous corridors and depressions that
dissected the foredune at the time of Ammophila invasion have now closed. Two
narrow blowouts adjacent to ‘parabolic 6’ persisted until 1989 (see Figure 4c).
During the initial stages of Ammophila invasion, the width and depth of these
blowouts was enhanced, providing a major pathway for sediment input into the
backdune system. Closure of the blowouts occurred between 1989 and 2002 as the
density of Ammophila increased, stabilising the throat of the blowout and reducing
TABLE 1. Changes to foredune morphology,a 1958 Á2002
1958 2002
Dominant species D. spiralis A. arenaria
Vegetation cover (per cent) 10 Á30 /80
Hesp (1988) foredune stage IV I
Maximum height (m) 3 11
Width (m) 80 150
Area (m2) 240 1650
Volumeb (m3) 4.8 )/105 3.3 )/106
Notes: aDimensions are derived from Figure 5.
b
Assuming a 2 km foredune length.
324 M. Hilton et al.
rates of sediment transport. Narrow channels still occur along the stoss face of the
foredune, but these are ephemeral features, a few metres wide and 20 m or so deep
(Hilton et al . 2005). To date none of these depressions has developed into a
blowout.
Case study I: foredune parabolic dune development, Mason Bay
Parabolic dunes and sand sheets are the most prevalent dune form in the Mason
Bay dune system. Climbing, imbricate forms occur most widely, usually modified
by the underlying bedrock. However, a series of long-walled parabolic dunes
(terminology after Pye 1983) has developed in a relatively flat section of the central
dunes of Mason Bay. The contemporary dune system between Duck Creek and
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Martins Creek comprises six adjacent parabolic dunes transgressing over a low
gradient stonefield. The most southerly of these parabolic dunes (‘parabolic 6’) is
the best defined of these dunes (see Figure 2).
‘Parabolic 6’ evolved from a blowout, some time prior to or coincident with
Ammophila invasion, probably in the late 1940s. The formation of relatively steep-
sided Ammophila shadow dunes may well have caused the development of such
blowouts. In 1958 the blowout lacked trailing arms (see Figure 4a). The
depositional lobe of the incipient parabolic dune would probably have been a
relatively low, sparsely vegetated feature (see Table 2). Ammophila had established a
sparse, patchy cover within the foredune by 1958 and had started to colonise the
more stable walls of the blowout/incipient ‘parabolic 6’ (see Figure 4a). The
characteristic features of a long-walled parabolic dune had formed by 1978.
The northern trailing arm (TA) and a deflation surface (DS) are clearly defined,
along with a sparsely vegetated depositional lobe (DL) (see Figure 4b). Develop-
ment of the deflation surface occurred primarily through elongation, increasing in
length at 7.91 m/year between 1958 and 1978 (see Table 3). Widening of the
surface also occurred, which resulted in an increase in area of the deflation surface
from 0.35 to 1.15 ha between 1958 and 1978, a 226 per cent increase. Ammophila
had formed a near-continuous cover across the foredune by 1978, but it was still
absent from the parabolic dune (see Figure 4b).
‘Parabolic 6’ became increasingly sheltered by the rapidly accreting Ammophila
foredune between 1978 and 1989. It continued to migrate downwind between
1978 and 1989; however, the rate of advance declined (see Table 3). During this
TABLE 2. Temporal changes in the morphology of ‘parabolic 6’
1958a 1978 1989 2002
Total lengthb (m) 176.12 600.71 657.83 664.50
Length of trailing arm (north) (m) Á 308.40 343.70 468.40
Length of trailing arm (south) (m) Á Á 325.60 325.60
Length of deflation surface (m) 92.34 183.74 285.60 452.20
Length of depositional lobe (m) 90.44 355.10 329.39 243.71
Area of deflation surface (ha) 0.35 1.15 2.14 5.20
Area of erosional face (ha) Á 0.68 1.68 0.61
Area of depositional lobe (ha) 0.51 4.35 2.62 2.34
Notes: aBest estimate only, due to poor image quality.
b
Excluding foredune.
Impact of Exotic Dune Grass Species 325
TABLE 3. Calculated migration rates (m/year) of ‘parabolic 6’
1958 Á78 1978 Á89 1989 Á2002
Deflation surface 7.91 6.02 15.60
Erosional face Á 16.29 7.00
Depositional lobe 24.19 5.56 0.79
period the deflation surface became significantly larger, with a corresponding
decline in the length and area of the depositional lobe (see Table 2). The deflation
surface increased in length at a rate of 6.02 m/year between 1978 and 1989, with an
increase in area from 1.15 to 2.14 ha, representing an 86 per cent increase.
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Ammophila had established throughout the parabolic dune by 2002, although
some remnant areas of Desmoschoenus on the northern trailing arm persisted (see
Figure 4f). Between 1989 and 2002 the rate of landward advance of the
depositional lobe was almost negligible (see Table 3). At the same time the
deflation surface increased in length at an average rate of 15.60 m/year and
increased in area to 5.20 ha, representing a 143 per cent increase over 13 years. The
length of the depositional lobe decreased as the eroding face advanced. Develop-
ment of the depositional lobe during this period occurred through vertical
accretion, rather than downwind extension. The depositional lobe now comprises
a series of large Ammophila shadow dunes, 3 Á5 m high.
‘Parabolic 6’ has been relatively stable since Ammophila invasion. The develop-
ment of a massive Ammophila foredune has caused a decline in available sediment
within the parabolic dune system. The development of the deflation surface, and
concomitant reduction in length of the depositional lobe, occurred as the
established foredune evolved. The development of large deflation surfaces of this
type in this context has also been observed in Oregon (Carter et al . 1990).
However, the ongoing formation of nabkha dunes across the seaward half of the
deflation (and former deflation) surface indicates that some sand is still in
circulation. We have observed significant jettation (terminology after Arens 1996)
across the foredune, during strong westerly winds, that provides sand for ongoing
nabkha development across the rear slopes of the foredune and former deflation
zone of the parabolic dune (see Figure 4d).
Ammophila invasion and foredune development may have contributed to the
subsequent stabilisation of ‘parabolic 6’ and the adjacent parabolic dunes. This
foredune has trapped a great deal of sand that might otherwise have contributed to
parabolic dune development and reduced rates of sedimentation in its lee.
Ammophila has also established in almost all elements of the parabolic dunes,
with the exception of the erosional face, forcing stability. However, it may be that
these parabolic dunes were close to completing their life cycle at the time
Ammophila was introduced.
Case study II: sea-wheat grass, Younghusband Peninsula
The coastal plain between the Murray River mouth and Naracoorte in southeast
Australia contains a sequence of at least eight coastal barriers, which have been
stranded and preserved by uplift. The oldest barrier, approximately 800,000 years
326 M. Hilton et al.
old, is situated around 90 km from the coast. The youngest and active barrier, the
Younghusband Peninsula (see Figure 1) is less than 7000 years old (Harvey 1981;
Belperio 1995). This sequence is widely regarded as one of the classic records of
Quaternary sea-level change and barrier development.
The Younghusband Peninsula is a modern analogue for the formation of the
older barriers (Bourman et al . 2000). Short and Hesp (1984) propose a five-stage
model for the Sir Richard and Younghusband Peninsulas (see Figure 6), involving:
(1) the formation of Pleistocene barriers below modern sea level; (2) Holocene sea-
level transgression and formation of the initial Holocene shoreline; (3) barrier
progradation, as foredune ridges; (4) depletion of nearshore sand supplies and
initial dune transgression and shoreline erosion; and (5) continued dune
transgressions and barrier regression. Episodes of transgressive dune development,
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following foredune disturbance were, therefore, central to barrier and dune system
development.
Short and Hesp (1980) describe the morphology of incipient foredunes and
established foredunes between Kingston and the Murray Mouth, in relation to
variations in beachÁsurfzone morphodynamics. At the time of the field survey
FIGURE 6. An interpretation of the genesis of the Younghusband Peninsula (after Short &
Hesp 1984).
Impact of Exotic Dune Grass Species 327
Spinifex sericeus and Austrofestuca littoralis were associated with incipient foredunes.
A wide range of incipient foredune and foredune morphologies were present. In
1979, when Short and Hesp conducted fieldwork, topographically variable and
highly eroded foredune morphologies (types Fd and Fe) occurred widely between
the Murray Mouth and ‘The Granites’, as might be expected with a regressive
barrier. Incipient foredunes were intermittent, particularly in the region extending
between 95 and 123 km south of the Murray Mouth. They observed foredune
stability to increase towards Kingston and transgressive dune development to
decrease, consistent with declining levels of wave and wind energy in the lee of
Cape Jaffa.
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Thinopyum invasion
Thinopyrum established along much of the length of the Younghusband Peninsula
with remarkable rapidity. It was not observed by Short and Hesp (1980) during a
systematic survey of foredune morphology between Kingston and the Murray River
mouth in 1979. Thinopyrum was observed soon after, in 1986, at Butcher Gap
Drain and Blackford Drain near Kingston and the Murray Mouth (Mavrinac
1986). It was also present at this time along sections of the Adelaide coast (Torrens
Outlet and Bungaloo Creek). Thinopyrum had formed a substantial incipient
foredune at each of these sites, 15 Á20 m wide, 1.0 Á1.5 m high, seaward of the
existing Spinifex foredune.
Thinopyrum appears to have occupied the coast between the Murray Mouth and
Kingston, a distance of about 170 km, during the early to mid-1980s. There has
been no work, to date, on processes of Thinopyrum dispersal. There is no evidence
that this species was deliberately planted in South Australia, and it almost certainly
established along the Younghusband Peninsula without assistance. It most likely
spread by fragmented rhizome, which is likely to enter the sea in large quantities
during storm-forced erosion of the incipient foredune. The low elevation of
Thinopyrum foredunes would ensure this process occurs frequently.
Incipient foredune development in conjunction with Thinopyrum ,
Younghusband Peninsula
‘The Granites’
‘The Granites’ site is located in the relatively sheltered ‘Coorong III’ sector of
Short and Hesp (1980). This is a stable site, with no significant trend of
progradation or erosion over the period 1945 to the early 1980s, prior to the
arrival of Thinopyrum. The site is situated 250 m south of the public car park
and beach access point. The incipient foredune at this locality is typical of the
dune for some kilometres north and south of ‘The Granites’. It exhibits a
‘terrace’ form (described in Hesp 2002), with a high cover of Thinopyrum (/75
per cent) (see Figure 7). This feature is 10 m wide and 1 Á2 m high and
continuous alongshore. The backdune comprised a former blowout, now well
vegetated with Leucophyta brownii , Olearia axillaris , Carpobrotus rossii and other
species. The new Thinopyrum foredune partially overlies the stoss face of the
former Spinifex foredune. Spinifex occurs throughout the surveyed area, though
nowhere is it particularly dense (see Figure 7).
328 M. Hilton et al.
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FIGURE 7. ‘The Granites’ *Thinopyrum occurs in a narrow band seaward of the former
Spinifex foredune.
‘28 Mile Crossing’
‘28 Mile Crossing’ is one of several 4-wheel-drive tracks that enable access to the
beach along the southern Coorong. The ‘28 Mile Crossing’ study site is located
approximately 100 m southeast of the beach access of the ‘28 Mile Crossing’ 4-
wheel-drive track. The site was chosen to examine the morphology of incipient
Thinopyrum foredunes on a section of eroding shore. The former Spinifex foredune
comprises a series of remnant knobs (Type 3 Á4 foredune; Hesp 1988), interspersed
with lobes of sand associated with Thinopyrum (see Figure 8). These lobes
constitute sections of incipient foredune, which extend 3Á4 m from the face of the
eroding foredune. They have a ‘ramp’ morphology (after Hesp 2002). Unvegetated
sections of scarped foredune separate these lobes. An extensive deflation surface
lies inland of the foredune, with transverse dunes occurring downwind. The
strandline was 15 Á20 m in front of the foredune on the day of the survey.
Thinopyrum forms a dense patch of vegetation across and extending away from
the eroding face of the foredune and, secondly, a sparse cover over the lee slopes of
the foredune and into the deflation surface (see Figure 8). Spinifex occupies the
remnant knobs and overlaps with Thinopyrum in places. Euphorbia paralias occurs
across the crests of the knobs and across the rear of the foredune. The presence of
various mature backdune shrubs and sedges (Stackhousia spathulata , Olearia
axillaris , Isolepis nodosa and Ozothamnus turbinatus , for example) suggests that
the former Spinifex foredune has been stable for some time, albeit the face of the
foredune is eroding.
Impact of Exotic Dune Grass Species 329
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FIGURE 8. 3D representation of the ‘28 Mile Crossing’ survey area. Thinopyrum has
occupied hollows between remnant knobs of Spinifex and built incipient ramp foredunes 3 Á4
m seaward of the foredune scarp.
Incipient foredune formation in conjunction with Thinopyrum
Thinopyrum has formed a narrow, usually low, incipient foredune along the
Younghusband Peninsula since the early 1980s. The terms ‘foredune’ and
‘incipient foredune’ require some clarification. Foredunes are ‘shore-parallel
dune ridges formed on the top of the backshore by aeolian deposition within
vegetation’ (Hesp 2002, p. 145). ‘Incipient foredunes are new, or developing,
foredunes within pioneer plant communities’ (Hesp 2002, p. 145). The sites
described contain type 2a and 2b incipient foredunes (after Hesp 1989), that is,
they have formed on the backshore by growth of Thinopyrum rhizome growth into
that section of the back-beach that lies between the toe of the foredune and the
driftline. At the two sites described, Thinopyrum has formed a terrace against the
former Spinifex foredune, usually at a slightly lower elevation. We do not know, at
present, whether Thinopyrum is in the process of establishing a more substantial
incipient foredune at ‘28 Mile Crossing’, or whether the environment is preventing
continuous alongshore colonisation.
Thinopyrum invasion has had a significant impact on the morphology of the pre-
existing Spinifex foredune at both sites. It has caused the stoss face of the foredune
to prograde and establish at lower elevations. Thinopyrum tends to occupy low-lying
gaps in eroding foredunes and encourage deposition, resulting in a higher overall
vegetation cover and more uniform topography. Hence, the overall impact of
Thinopyrum is to encourage the formation of wider, more uniform and more
continuous foredunes, at least in situations of low to moderate rates of accretion.
We have seen higher (4Á5 m) Thinopyrum foredunes at the Murray Mouth, where
330 M. Hilton et al.
the rates of accretion may be significantly greater than observed at the sites
reported here.
Thinopyrum grows vigorously on the stoss face of the foredune, and is clearly
tolerant of more frequent sea-water inundation and soil salinity. Our surveys
demonstrate that it also survives in backdune environments, albeit in a semi-
moribund form. It is not, therefore, vulnerable to catastrophic removal during
episodes of severe foredune scarping. Backdune plants of Thinopyrum are probably
capable of rapid growth when erosion exposes them to higher rates of sedimenta-
tion and higher nutrient levels.
Spinifex has been displaced from the front face of the foredune along the
Younghusband Peninsula. This will greatly reduce the distribution of Spinifex and
associated indigenous species, but it may have significant ecological implications for
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other flora and fauna. It is inevitable, for example, that the habitat of shore birds
such as the hooded plover will decline, as Thinopyrum colonises gaps in the
foredune and blowouts, and occupies areas of the backbeach that would not
normally be colonised by Spinifex .
The potential impact of Ammophila and Thinopyrum on blowout
formation
Ammophila and Thinopyrum have produced new dune morphologies in the dune
systems described. A massive foredune complex established at Mason Bay
following Ammophila invasion. The landscape associated with Desmoschoenus has
been buried beneath an evenly vegetated, continuous established foredune, 150 Á
200 m wide and 10 Á12 m high. Large quantities of sand, which would otherwise
have entered the dune system, are now trapped in this foredune complex.
Ammophila has also invaded the hinterland of the dune system, with resulting
loss of dune flora and transgressive dune mobility. Thinopyrum occupies a more
limited range *it is primarily a species of the back beach. It has also established
continuous incipient foredunes along most of the length of the Younghusband
Peninsula, and by the mid-1980s occupied approximately 170 km of coast between
the Murray Mouth and Kingston. In both cases irregular foredunes have been
replaced by regular, continuous-alongshore foredunes. Thinopyrum and Ammophila
have also had a major impact on the indigenous flora of the two study sites.
Thinopyrum has displaced Spinifex from the stoss face of the foredune along the
length of the Younghusband Peninsula. The range of Spinifex may be reduced over
time, since it is likely that the crest and rear of the foredune will experience less
sediment input, greater stability and (possibly) accelerated vegetation succession.
Ammophila has had an overwhelmingly adverse impact on the indigenous dune
flora of Mason Bay. The indigenous species associated with the pre-Ammophila
foredune have been totally displaced.
Thinopyrum and Ammophila have produced new dune landscapes and new dune
ecosystems. Here we will consider whether these grasses are likely to reduce the
frequency or intensity of blowout development. This impact would have significant
implications for the long-term development of Mason Bay and Younghusband
Peninsula dune systems, by reducing the incidence of transgressive dune develop-
ment. This would, in turn, impact on the diversity of habitats within the dune
systems.
Impact of Exotic Dune Grass Species 331
The processes that lead to blowout development are well documented. A
blowout is a saucer-, cup- or trough-shaped depression or hollow formed by wind-
forced erosion in a pre-existing deposit of sand. They may be initiated as a result
of: wave erosion along the seaward face of the foredune; topographic acceleration
of airflow over the dune crest; climate change; vegetation variation through space
or change through time; water erosion; high velocity wind erosion, sand inunda-
tion and burial; and human activities (for a review of these processes see Hesp
2002).
There is some evidence that Ammophila and Thinopyrum respond differently to
these processes compared with indigenous foredune species. Both species form
uniform, continuous, Type I foredunes (or contribute to this character along the
stoss face of pre-existing Spinifex foredunes). The potential for blowout develop-
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ment through topographic acceleration of airflow is, therefore, likely to be reduced.
Ammophila and Thinopyrum occur on a range of coasts, including those with dry
Mediterranean climates. Ammophila has a number of physiological and morpho-
logical adaptations that reduce the impact of drought stress. In a series of
glasshouse experiments, Dixon et al . (2004) compared the tolerance of Ammophila
and Desmoschoenus to drought conditions. Desmoschoenus showed signs of water
stress after 8 days in glasshouse drought conditions, whereas Ammophila showed no
signs of stress until day 18. At the completion of the drought trial, 5 per cent of
Desmoschoenus and 80 per cent of the Ammophila recovered. These results accord
with field observations and the results of earlier glasshouse experiments (Huiskes
1979). The relative tolerance of Spinifex and Thinopyrum and Spinifex and
Ammophila to drought has not been ascertained.
Both Ammophila and Thinopyrum species are rhizomatous grasses capable of
trapping and binding sand. Their leaves are capable of ‘lodging’ and so are able to
withstand strong winds and maintain a uniform vegetation cover. Established
foredunes associated with both species are vulnerable to scarping by storm wave
swash; however, gaps or low points are likely to be rapidly repaired by post-storm
growth. Our observations at ‘28 Mile Crossing’ on the Younghusband Peninsula
indicate that Thinopyrum is able to establish cover and form incipient foredunes on
an eroding coastline. Burial may pose a significant threat to Thinopyrum ; however,
like Ammophila , it is possibly tolerant of darkness and may be able to emerge when
buried. The incipient foredunes of both species are scarped during episodes of
storm wave activity and storm surge. We suspect that in both cases incipient
foredunes are rapidly repaired by, first, mass failure of the upper sections of the
scarp and then rapid elongation of lateral rhizomes. The toe of foredunes of both
species must be in constant flux, even on prograding coasts, given their proximity to
the sea. Finally, Thinopyrum is known to be exceptionally tolerant of salt during
episodes of elevated sea level associated with storms. Ammophila is not tolerant of
salt in the root zone, but avoids the problem by building relatively high, massive
foredunes.
Conclusions
In conclusion, it has been demonstrated that two exotic dune grasses, namely
Thinopyrum junceiforme and Ammophila arenaria , have been introduced into dune
environments in Australia and New Zealand, respectively, where they have:
332 M. Hilton et al.
(1) replaced irregular, sparsely vegetated, established foredunes with continuous
incipient foredunes;
(2) encouraged accretion and progradation;
(3) increased the extent and evenness of vegetation cover;
(4) rapidly displaced native species; and
(5) altered dune habitat for indigenous fauna and flora.
These impacts have occurred over the last few decades. We have raised the question
of the long-term impact of these species on barrier and dune system development.
Are these species likely to reduce the incidence or extent of blowout development?
If so, could a reduction in transgressive dune development affect the natural
development of coastal barriers along coasts occupied by Ammophila and
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Thinopyrum? These processes are an essential part of the geomorphic and ecologic
character of temperate sandy coasts. Both study sites are located in national parks
which have been designated to conserve natural values.
A substantial programme of research is required to resolve this matter. Such a
programme would need to compare the relative response of indigenous and exotic
species to the environmental stresses identified above, both in isolation and in
combination. We already know that Ammophila is more tolerant of burial (Sykes &
Wilson 1990) and more tolerant of drought (Dixon et al . 2004) than Desmoschoe-
nus. Relatively little is known about the response of Thinopyrum to environmental
stress on temperate coasts, in part because this species has attracted virtually no
attention in the short period it has been in Australia.
There is clearly a need for a management response to this issue. In New Zealand
the Department of Conservation (DoC) has commenced a programme of
Ammophila eradication in Fiordland and Rakiura National Parks (Stewart Island)
in southern New Zealand. The DoC has employed helicopters, all-terrain vehicles
and back packs to apply selective herbicides in these parks. Much smaller
operations, targeting Ammophila , are occurring elsewhere in New Zealand. The
Stewart Island operations commenced in 1987 and are likely to run for at least
another 15 years. The question remains as to whether the DoC, local authorities or
private landowners will undertake Ammophila eradication in significant conserva-
tion areas outside national parks. At least one NGO, the Yellow-eyed Penguin
Trust, based in Dunedin, has initiated marram-control operations on private land
to improve penguin habitat and restore indigenous vegetation. In contrast, there
has been little extensive control of Ammophila in Australia, apart from work by the
Department of Primary Industry, Water and the Environment (DIPWE) in
southwest Tasmania. No control of Thinopyrum has yet occurred in Australia.
This may be due to the recent focus on invasive species of backdune and established
foredunes such as Bitou Bush (Chrysanthemoides monilifera ), but this inaction
probably also reflects low levels of awareness of the impact of introduced foredune
species. The recent release of the Tasmanian Beach Weed Strategy (Rudman 2003)
and initiation of an investigation of the impact of South African Pyp Grass
(Ehrharta villosa ) in Coorong National Park by the Department of Environment
and Heritage indicate a developing awareness of exotic dune weeds.
Correspondence: Mike Hilton, Department of Geography, University of Otago, PO
Box 56, Dunedin, New Zealand. E-mail: mjh@geography.otago.ac.nz
Impact of Exotic Dune Grass Species 333
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