Northern Hemisphere Coastal Vegetation

7.2.1 Foreshore plant communities

The foreshore flora, although at the mercy of the sea, nevertheless derives from this perilous situation the advantage of oceanic seed dispersal. Consequently, beaches on either side of the North Atlantic from

Quebec, Newfoundland and Labrador, to the northwestern shores of Europe have very similar strand-line plant communities dominated in summer by annual species such as orache (Atriplex spp.), sea rocket (Cakile maritima), and sea sandwort (Honckenya peploides) as a result of transatlantic seed dispersal (Figs. 7.13-7.14). Some coastal species are notable for their widespread distribution. Scots lovage (Ligusticum scoticum) has a disjunct distribution and can be seen on the cliffs from Norway's Nordkapp (71° N) to southern Scotland (55° N) and from western Greenland (67° N) to Newfoundland (Fig. 7.14), Nova Scotia and New England (42° N).

Seedling establishment on the upper beach is extremely hazardous. With the ever-present dangers of erosion, burial, overheating and desiccation, as well as inundation by seawater, it is not surprising that much of the foreshore plant cover in summer is composed of annual species. Colonization of this zone is highly dependent on the shelter and nutrition that can be provided by the flotsam and jetsam that washes up on the strand line. Shorelines with a good supply of detritus can develop a luxuriant growth of annual plants in summer (Fig. 7.12). Seaweed is one of the best

Seaweed Strand

Fig. 7.13 Strand vegetation flourishing on a shore in Holandsfjorden, Arctic Norway, with copious deposits of seaweed, mainly bladder wrack (Fucus vesiculosus). The large green patches are sea sandwort (Honckenya peploides). (Inset) Detail from strand vegetation shown above with orache (Atriplex sp.) flourishing in a bed of washed-up bladder wrack.

Fig. 7.13 Strand vegetation flourishing on a shore in Holandsfjorden, Arctic Norway, with copious deposits of seaweed, mainly bladder wrack (Fucus vesiculosus). The large green patches are sea sandwort (Honckenya peploides). (Inset) Detail from strand vegetation shown above with orache (Atriplex sp.) flourishing in a bed of washed-up bladder wrack.

ameliorators of environmental extremes for young foreshore plants as it provides not only shelter and anchorage but also nutrition.

The strand line has always been a natural repository for seaweed and timber even before human activity contributed to the rubbish that is now found on most shores. Modern detritus may be unsightly but some litter either natural or artificial is essential for diminishing sand movement, reducing excessive temperature fluctuations, conserving moisture, and fostering the development of the all-important embryo dunes. Flotsam and jetsam also provide a habitat for the

Fig. 7.14 Ligusticum scoticum growing on the south-west shore of Newfoundland: an example of an amphi-Atlantic coastal species. In Quebec this species is commonly called the 'Liveche Ecossaise'. (Inset) North-Atlantic distribution of Ligusticum scoticum. (Reproduced with permission from Hulten & Fries, 1986.)

Fig. 7.14 Ligusticum scoticum growing on the south-west shore of Newfoundland: an example of an amphi-Atlantic coastal species. In Quebec this species is commonly called the 'Liveche Ecossaise'. (Inset) North-Atlantic distribution of Ligusticum scoticum. (Reproduced with permission from Hulten & Fries, 1986.)

invertebrate fauna that is sought by the many different bird species that feed along the strand line (Llewellyn & Shackley, 1996). All too often a short-sighted environmental policy leads certain local government authorities, anxious to promote the touristic attraction of their beaches, to remove this detritus with regular raking. Although the desire to remove some of the more objectionable items of rubbish is understandable, it is unfortunate that regular beach raking is often so thorough that it removes completely the first line of plant colonization and an important bird feeding area. Sadly, the economic desire to gain prestigious awards for clean beaches, such as the European Blue Flag, can result in the disappearance of the foreshore vegetation and expose the front dunes to erosion by wind and tide (Fig. 7.11).

The annual vegetation cover on the foreshore is only ephemeral, but during its brief summer existence it reduces the steepness of the shore profile with accreted sand (Figs. 7.15-7.16). Once the summer growing season is over the annual plants die, the upper shore profile again steepens, and the perennial rhizomatous species are left to hold the foredunes against the winter storms. The buried rhizomatous network most commonly

Fig. 7.15 Sloping foredune in midsummer on the island of Sanday, Orkney. The band of annual species colonizing the foreshore immediately in front of the dune reduces the steepness of the dune profile. At the end of summer the gradient of the dune profile gradually increases in the face of the winter storms.

Fig. 7.15 Sloping foredune in midsummer on the island of Sanday, Orkney. The band of annual species colonizing the foreshore immediately in front of the dune reduces the steepness of the dune profile. At the end of summer the gradient of the dune profile gradually increases in the face of the winter storms.

created by graminoid species can hold the young embryo dunes in place and through their carbohydrate reserves produce new shoots in spring even though they may be buried to considerable depths.

One of the commonest rhizomatous grasses holding in place the first embryo dunes in northern and western Europe is sand couch-grass (Elytrigia juncea; Fig. 7.17). Once an initial reserve of tillers is established the oblique growing stolons of this species can emerge from burial below 30 cm of sand. Tiller fragments are also easily spread, but the young plants in newly formed sand accretions produce little seed. Thus, seed for spreading the species and aiding the formation of new embryo dunes is dependent largely on the state of the yellow dunes to landward, which provide the main source of seed.

When the shore is denuded by erosion or biologically impoverished, as when concrete walls replace the yellow dunes, recolonization is hindered due to the lack of propagules of this important pioneer species. Once the embryo dune is destroyed, the main front dunes become vulnerable and inevitably erode (Fig. 7.18). There is therefore a mutualistic relation ship between young foredunes and the first line of yellow or mobile dunes on their landward side, with the embryo dunes being dependent on the mature dunes for seed regeneration, and the mature dunes dependent on the younger dunes for protection against erosion.

7.2.2 Dune systems of the North Atlantic

In the northern hemisphere dune systems are mainly anchored by grass species. Grasses produce their leaves sequentially, which means a considerable portion of the growing season has passed before they develop their full photosynthetic potential. Consequently, as growing seasons shorten at higher latitudes, grasses tend to be disadvantaged due to their failure to maximize their full leaf-area potential early in a short growing season. Marram (Ammophila arenaria) is the principal European dune-forming grass, and reaches a northern limit at Faerna (63° N) on the Norwegian coast, only just north of the limit for sand couch-grass (Elytrigia juncea; Fig. 7.17) at 62° N (near Alesund, Norway).

Along the east coast of North America, the St Lawrence and the Great Lakes, the vicarious

Fig. 7.16 Sea rocket (Cakile maritima) growing on the Orkney sand dune in Fig. 7.15. This is a common and variable species that colonizes drift-line communities on sand and shingle. When various vicarious and subspecies are included it is one of the most widespread species around the shores of the North Atlantic and Mediterranean Sea. Cakile islandica was the first species to be recorded when the island of Surtsey was created by submarine volcanic eruptions between 1963 and 1967.

Fig. 7.16 Sea rocket (Cakile maritima) growing on the Orkney sand dune in Fig. 7.15. This is a common and variable species that colonizes drift-line communities on sand and shingle. When various vicarious and subspecies are included it is one of the most widespread species around the shores of the North Atlantic and Mediterranean Sea. Cakile islandica was the first species to be recorded when the island of Surtsey was created by submarine volcanic eruptions between 1963 and 1967.

Ammophila species, American beach grass (A. brevigu-lata) is the dominant primary sand dune stabilizer, as far north as Newfoundland and Labrador. However, the American beach grass - like the vicarious European species (A. arenaria) - does not extend into the Arctic. Both Ammophila species are well adapted to sand burial, wind, lack of moisture and nutrients. In both species growth is even stimulated by burial with wind-blown sand, and new shoots can emerge from burial when covered to a depth of one metre. The leaves are also rich in silica, which enables them to withstand sand blasting.

In the more southern regions of the European Arctic, lyme grass (Leymus arenarius) is the major species as far north as Norway's North Cape (71 ° N). The vicarious North America dune-grass (Leymus mollis) is equally dominant on arctic shores from Alaska and northern Canada to Greenland. A study of this species carried out on the east coast of the Hudson Bay (northern Quebec) found that populations of L. mollis have different phenotypic responses depending on whether they are growing on the low foreshore or on the stabilized dune. On the latter, Leymus mollis ramets tend to have a lower net carbon assimilation rate and water use efficiency, and a higher substomatal CO2 concentration than on the foredune. However, under controlled conditions the differences observed in the field disappear, suggesting that these are not genetic but determined by environmental changes along the

Sand Dunes Embryo
Fig. 7.17 Sand couch grass (Elytrigia juncea) — one of the most important perennial species in forming embryo dunes which are essential for the protection and renewal of the main dune system.
Fig. 7.18 Eroding sand dune at Tres Ness Point in Sanday (Orkney, UK). The once thriving arable farm is now in danger of disappearing due to coastal erosion.

foredune-stabilized dune gradient. It has been suggested that the higher net carbon assimilation rate on the foredune might be related to higher sink strength in relation to the growth-stimulating effect of sand burial (Imbert & Houle, 2000).

7.2.3 Arctic shores

The most marginal coastal sites for plant colonization are the shoreline fringes of the ice-covered lands of Antarctica and the continents and islands bordering the

Arctic Ocean. Each summer the ice and snow retreat sufficiently for plants that inhabit these shores to have a growing season that can vary from as little as 6 weeks to up to 3 months. It is on the low shores nearest to the sea that the shortest polar growing seasons are found. In addition, the beach is reworked annually by the on shore movement of winter sea ice creating a gravel beach ridge (Figs. 7.19-7.20). This ice scouring leaves behind a low-lying flood-prone plain bordered to landward by the displaced material. The polar shores therefore have two distinct habitats: a low coastal plain and a beach ridge. In the low coastal plain, proximity to the cold sea causes snow and ice to persist longer in early summer than on the adjacent beach ridge. Even during the height of the growing season the soil and air temperatures down on the low foreshore are lower than on the beach ridge (Fig. 7.21). It is one of the botanical marvels of high-latitude ecology that there are so many species of flowering plants that succeed in living in these low-lying shores.

Arctic shorelines do not provide the quantities of sand that are needed to form dunes. The tidal amplitude is low, often only 0.2-0.3 m. Salinity is low in arctic coastal waters, which allows many non-halophytic species to colonize foreshore habitats. The plants that survive on polar shores are therefore a highly varied group of pioneer species that can be found in many other habitats throughout the Arctic.

In continental areas such as northern Canada and Greenland the upper shore is drought prone due to the lack of summer precipitation. Despite the absence of dunes there are nevertheless a number of characteristic arctic grass species that are widespread along arctic shorelines. Some of the most widespread grasses throughout the Arctic are the arctic salt marsh grasses (Puccinellia spp.), of which the most widespread species is the creeping salt marsh grass (P. phryganodes), which has a circumpolar distribution throughout the High Arctic with the exception of the extreme north of Greenland (see also Section 6.7).

The drier parts of the arctic shore are not very different physically from open land anywhere else at high latitudes. One of the more successful species on the low arctic shore, and also on exposed mountain slopes and screes, is the purple saxifrage (Saxifraga oppositifolia; Section 6.4.2).

The arctic shore does, however, differ due to the colder conditions and shorter growing seasons found here than on sun-trapping mountain slopes. In many years the melting of the residual snow banks, and therefore the onset of growing season on the shore, can be delayed. Although the purple saxifrage may flower and even produce pollen, the shortness of the growing season often results in little viable seed being produced (see Chapter 4). However, the proximity of the warmer and drier beach ridge provides an adjacent population (a different ecotype) that can be fertilized by the plants on the low shore and thus preserve the genetic characteristics of the shore ecotype. As already mentioned (Chapter 4) the warmer dry ridge and the cold shore support different ecotypes of the purple saxifrage and this can replenish the populations on the low shore with hybrid seed. In some areas the shore is now enjoying an earlier beginning to the growing season and producing remarkable flowering displays (Fig. 6.6).

Other species capable of surviving in this habitat include moss campion (Silene acaulis) and the alpine bistort (Polygonum viviparum). Moss campion being a cushion plant maximizes internal tissue temperatures with a minimal surface area in reaction to the body mass of the plant. The viviparous bistort (Polygonum vivi-parum) despite the fact that it reproduces mainly vegetatively with bulbils nevertheless retains considerable genetic diversity, as can be observed in different enzyme phenotypes and bulbil colours (Fig. 7.22). The differentiation ofecotypes and medium to high levels of genetic diversity in arctic and alpine populations is thought to be the result of occasional sexual reproduction (Bauert, 1996).

Notwithstanding the short growing season some polar shores are highly productive and are sought out as feeding places by various species of migrating geese. In the more sheltered bays and fiords salt marshes develop and the sedges and grasses of these coastal flats are grazed by barnacle geese (Branta leucopsis; Figs. 7.23-7.24) that fly from the Solway Firth in southern Scotland (55° N) as far north as Spitsbergen (79° N). In North America, the lesser snow goose (Anser caerulescens) undertakes an even longer migration. Birds that winter on the coast of the Gulf of Mexico (30° N) fly up the Mississippi and Missouri rivers and can reach the Hudson Bay coast (60° N) and further north into the eastern Arctic (70° N). The mutual nutritional interactions between goose grazing and arctic salt marsh ecology during the long growing

Fig. 7.19 Arctic shore and beach ridge at Ny Alesund, Spitsbergen, as viewed from the air with the beach ridge and the drier land behind it to the left of the picture. The position of the beach ridge where it adjoins the low shore is indicated by a red arrow.

Figs. 7.20 Arctic shore and beach ridges at Ny Alesund, Spitsbergen. Note the heterogeneous nature of the arctic shores with dry ridges, wet hollows, and late-lying snow patches.

days of the arctic summer have already been discussed in Section 2.4.1 where sugar-rich salt marsh grasses and sedges, fertilized by goose droppings, provide nutritious feeding (Fig. 7.24). The advantages to the geese of these polar pastures are demonstrated in the speed with which the goslings which are hatched in July grow and become sufficiently strong to begin their long migration south. In August, barnacle geese leave Spitsbergen

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Fig. 7.21 Comparison of (a) leaf (Saxífraga oppositifolia), (b) soil and (c) air temperatures over a continuous 60 h period from 12 noon on 25 July 1994 on adjacent ridge and shore sites as shown in Fig. 7.19 at Ny Alesund, Spitsbergen. (For further experimental detail see Crawford et al., 1995.)

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Fig. 7.21 Comparison of (a) leaf (Saxífraga oppositifolia), (b) soil and (c) air temperatures over a continuous 60 h period from 12 noon on 25 July 1994 on adjacent ridge and shore sites as shown in Fig. 7.19 at Ny Alesund, Spitsbergen. (For further experimental detail see Crawford et al., 1995.)

flying first to Bear Island (74o N), and then continue to the British Isles, arriving in the Solway Firth (55o N) between the end of September and early October.

Despite the extreme thermal limitations of the environment, the plant species that inhabit northern shores even into the High Arctic do not lack for genetic diversity (see Chapter 2). The reserves of genetic diversity found at these high latitudes probably reflect two important aspects of these unique shores. The first is the antiquity of the habitat. During the Pleistocene, when sea levels were as much as 140 m lower than at present, the shore area and polar desert hinterland north of the major ice sheets would have been more extensive and would therefore have provided a sufficiently open habitat for plant survival. Secondly, the arctic shores are not geographically isolated from one another. The circumpolar coastlines have never at any time been entirely encased in ice and plant migrations will have continued in different areas throughout most of the Pleistocene. The evidence for the possibility of this remarkable survival has been much debated.

Fig. 7.22 The alpine bistort (Polygonum viviparum), a widespread species in arctic and alpine habitats that produces mainly vegetatively with bulbils. Occasionally the flower sets fertile seeds which may account for the considerable genetic variability found in both arctic and alpine populations of this species (see text).

Fig. 7.22 The alpine bistort (Polygonum viviparum), a widespread species in arctic and alpine habitats that produces mainly vegetatively with bulbils. Occasionally the flower sets fertile seeds which may account for the considerable genetic variability found in both arctic and alpine populations of this species (see text).

However, the study of chloroplast DNA in present-day populations of the purple saxifrage (Saxifraga opposi-tifolia) has substantiated this view of the antiquity of at least this species (Abbott & Comes, 2004) and by implication other high arctic species with similar distributions (see Section 6.4.2 for details).

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Fig. 7.24 A coastal marsh much visited by geese in Kongsfjord (Spitsbergen) with clumps ofthe anoxia-tolerant sedge Carex misandra, a favoured fodder for barnacle geese.

7.3 SOUTHERN HEMISPHERE SHORES 7.3.1 Antarctic shores

The ultimate marginal coastal environment for the survival of flowering plants is without doubt the Antarctic Peninsula and adjacent islands where the flora consists of 380 species of lichens, 130 species of bryo-phytes but only two species of flowering plants (Alberdi et al., 2002). The Antarctic pearlwort (Colobanthus quitensis) and the Antarctic hairgrass (Deschampsia antarctica) are the only two angiosperm species to survive south of 56° S and occur in small clumps near the shore on the west coast of the Antarctic Peninsula (Fig. 7.25-7.27). It is in this area of maritime Antarctica, where mean air temperature tends to be above zero during the summer, that most of the Antarctic vegetation is found. Climatic warming in Antarctica has prompted a number of studies in recent years into these two remarkable flowering plants. This low number of species, compared with the Arctic, may be due to a long history of low temperature and isolation from sources of propagules. The degree of isolation for plant populations in Antarctica is not just from other continents, but also between different regions of the Antarctic Peninsula. This can be seen in Deschampsia antarctica where two distinct populations, one from the maritime Antarctic, namely Signy Island in the South Orkney Islands, and the other from the Leonie Islands 1350 km further south (67° S) were found to be genetically distinct from each other with low levels of historical gene flow between them. Their genetic structure suggests that new populations of D. antarctica are founded by one or just a few individuals and that vegetative reproduction and selfing are therefore likely to have been key factors

Fig. 7.26 A coastal colony on the Antarctic Peninsula with a plant community containing both Deschampsia antarctica and Colobanthus quitensis. (Photo Dr T. A. Day.)
Fig. 7.27 Details of the only two flowering plants native to Antarctica. (Left) The antarctic pearlwort (Colobanthus quitensis). (Right) The antarctic hair grass (Deschamspsia antarctica). (Photos Dr T. A. Day.)

in the establishment of D. antarctica at new sites in the Antarctic during recent years (Holderegger et al., 2003).

It is of interest that despite the harsh conditions of the environment both D. antarctica and Colobanthus quitensis succeed in establishing significant seed banks of between 107 and 1648 seeds m~2, which are comparable in size to arctic and alpine species (McGraw & Day, 1997). Physiologically, the existence of these plants in such a permanently harsh environment makes them of particular interest for the study of adaptations to cold environments and mechanisms of cold resistance in plants. Both species have a high resistance to freezing and can show a high photosyn-thetic capacity at low temperatures (Alberdi et al., 2002). Despite the fact that these two species share closely the same habitat the nature of their cold resistance differs. Comparisons of the thermal properties of leaves and the lethal freezing temperatures (LT50 - the temperature required to induce a 50% mortality in the leaf tissues) have shown that the grass D. antarctica was able to tolerate freezing to a lower temperature than C. quitensis. Super cooling (cooling of a liquid below its freezing point without freezing) was found to be the main freezing resistance mechanism for C. quitensis. Thus, the grass D. antarctica is mainly a freezing-tolerant species, while C. quitensis avoids freezing (Bravo et al., 2001). The species also differ in the nature of their foliage. D. antarctica is highly sclerophyllous while C. quitensis only has sclerenchymatic tissues and thin cuticles (Mantovani & Vieira, 2000). Thus, as well as their hardiness in sharing the same habitat, these species show that even in these minimal conditions it is possible for plants to differ in how they adapt to such a marginal situation.

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica have also been found to differ and to be more efficient at low than high temperatures (Xiong et al., 1999). Measurements of whole-canopy CO2 gas exchange and chlorophyll fluorescence of plants growing near Palmer Station along the Antarctic Peninsula have shown negligible midday net photo-synthetic rates on warm, sunny, days (canopy air temperature > 20 °C), but nevertheless attained positive photosynthetic rates (Fig. 7.28) on cool days (< 10 °C). It was therefore concluded that although continued warming along the Peninsula will increase the frequency of supra-optimal temperatures, the site averaged increase would be unlikely to exceed the temperature optima for photosynthesis for these species. It is therefore not surprising that the recent warming trend has already resulted in more seeds germinating and an increasing number of seedlings and plants. One report indicates a 25-fold increase in plants together with a southward extension of their range (see also Convey & Smith, 2006).

An additional modern hazard for flowering plants in Antarctica is exposure to high levels of UV-B radiation. Along the west coast of the Antarctic Peninsula springtime ozone depletion events can lead to a two fold increase in biologically effective UV-B radiation. Studies which have examined the influence of solar UV-B on the performance of the Antarctic vascular plants (Colobanthus quitensis and Deschampsia antarctica) have shown that leaf longevity decreased from the first growing season through to the fourth, suggesting that UV-B growth responses tended to be cumulative over successive years.

7.3.2 New Zealand

All scientific examinations require a control. This is just as true for field-based ecological studies as it is for laboratory-based experiments. Before any general conclusions can be made about the nature of the limitations on plant distribution in coastal habitats some attempt should be made to examine shores in a totally different situation. Ideally, an ocean shore on another life-supporting planet would be ideal. However, lacking this possibility at present, the coasts of New Zealand provide an ecologically informative example of a region where both the flora and the fauna have had a very different evolutionary history from that in the northern hemisphere. In New Zealand the native plants evolved without the threat of being grazed by mammals until the Maoris introduced rats. The date of this rodent invasion has been much disputed. However, the application of 14C AMS (accelerator mass spectro-metry) has now allowed this invasion to be dated confidently to the thirteenth century (Wilmshurst & Higham, 2004). This was followed later in the eighteenth and nineteenth centuries by other mammals from Europe and elsewhere. Before the arrival of European

Fig. 7.28 (A) Photosynthetic responses (Pn) to temperature of the whole canopy as shown for Colobanthus quitensis (top) and Deschampsia antarctica (bottom). (B) Dark respiration (Rn) of the species in relation to temperature. The insets show the temperature dependence of Pg calculated as the sum of Pn and Pr assuming that respiration rates are similar in light and dark. (Reproduced with permission from Xiong et al., 1999.)

Fig. 7.28 (A) Photosynthetic responses (Pn) to temperature of the whole canopy as shown for Colobanthus quitensis (top) and Deschampsia antarctica (bottom). (B) Dark respiration (Rn) of the species in relation to temperature. The insets show the temperature dependence of Pg calculated as the sum of Pn and Pr assuming that respiration rates are similar in light and dark. (Reproduced with permission from Xiong et al., 1999.)

settlers the principal graminoid native species that stabilized New Zealand's foredunes were the pingao (Desmoschoenus spiralis), a robust sedge used traditionally by the Maoris for making cloaks and matting, and spinifex grass (Spinifex sericeus) which is common on sand dunes along the coasts of Australia and New Caledonia. Pingao (Desmoschoenus spiralis; Fig. 4.18) has thick rope-like stems forming a vegetative entanglement inside the dune which provides robust anchoring against the strong winds of the South Pacific. Unfortunately, many New Zealand plants lack protection against small grazing mammals as they evolved in an environment devoid of mammals and where herbivory risks were confined to the pecking of shoots by grazing birds, such as the long-extinct giant moa. Consequently, pingao, although physically robust, lacks adequate protection against rabbits.

The introduced marram grass has high silica deposits in its leaves and shoots, like the common reed (Phragmites australis) and the sea club-rush (Bol-boschoenus maritimus), and is more resistant to rabbits; as a result the native pingao has been brought near to extinction (see Section 4.6.1, Fig. 4.18). Fortunately, there is now a renewed interest in preserving the pingao due to the revival of Maori weaving that uses its leaves, and attempts are being made to protect and propagate the species (Wardle, 1991). Spinifex has also been greatly reduced by grazing but with protection is proving capable of recovering. Prostrate woody species with a capacity for layering have always been successful in establishing themselves on foredunes in both the northern and southern hemispheres. Coprosma (Cop-rosma acerosa) is a notable species in New Zealand that can be compared to northern hemisphere creeping willow (Salix repens) which although more commonly a dune-slack species will grow on sand dunes in areas with sufficient rainfall.

The New Zealand foreshore has now lost much of its native vegetation to alien invaders. In addition to Ammophila arenaria from Europe, now Leymus race-mosus from China, Carpobrotus edulis from South Africa, and tree lupins (Lupinus arboreus) from California provide a cosmopolitan grazing-tolerant beach flora. This inter-hemisphere comparison is therefore a pertinent reminder of the powerful effects of herbivory even in a very marginal environment where it might be thought that physical constraints on plant survival were paramount and biotic factors would be minimal.

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