Zonation and succession

There is ample evidence that a progressive change in the intensity of an important environmental factor leads to the formation of zones or belt-like communities in which the plant species reflect a fairly distinct range of tolerance for that factor (Daubenmire 1968). Zonation has been defined as a sequence of vegetation in space and succession as a sequence of vegetation in time (McIntosh 1980). A zone is an area occupied by a plant community that is distinctly different from other zones and can be readily recognized by a change in dominant vegetation. Striking examples of zonation are found in salt marshes, mountain slopes and ponds because of soil salinity in salt marshes, decrease in temperature on mountain slopes and increase in water depth in ponds (Daubenmire 1968; Chapman 1976; Partridge and Wilson 1988). Similarly, it has long been known (Beck 1819) that sand dunes along sea coasts exhibit a zonation pattern extending from the beach to inland dunes. The zones are discrete and occur in parallel series with distinctly different species composition that is related to the ability of plant species to withstand the environmental factors prevailing in that zone (Doing 1985). Many later studies using transects from the shoreline to the inland dunes have confirmed that the taxa are not randomly distributed; they peak at definite distances from the beach (Oosting and Billings 1942; Boyce 1954; Martin 1959; Barbour 1978; Barbour et al. 1985). Succession in coastal dunes is an example of primary succession because the sandy material deposited on the shoreline by waves is inert. The term is generally used to denote a directional change in species composition and physiognomy of vegetation at the same site over time (Drury and Nisbet 1973). However, only the very early stages of dune succession can be observed during the life time of a plant ecologist and the later stages are usually inferred from plant communities represented on older sand dunes. It is hypothesized that the autogenic influence of early colonizers alters environmental conditions in the habitat and facilitates the establishment of new species better adapted to live in the altered habitat.

Animal–plant interactions

Population dynamics of plant species of coastal sand dunes is influenced directly, both above and below the soil surface, by a wide variety of organisms. Plants serve as sources of carbon and pathogens including viruses, insects, bacteria, fungi, birds, and mammals of various kinds. Some enhance plant performance while others have deleterious effects. Positive interactions include pollination of flowers by useful insects in return for nectar and pollen, nutrient acquisition from soil by mycorrhizal fungi in exchange for carbon and acquiring nitrogen (N) from N-fixing bacteria. In the history of co-evolution between plants and organisms over one hundred million years plants have developed many mechanisms to defend themselves from pathogens. Morphology may be altered by producing epicuticular waxes, developing trichomes over leaves, producing tough leaves with deposition of celluloses, lignin, suberin and callose, developing thorns on stems and branches or producing secondary plant metabolites that retard development, intoxicate or kill herbivorous insects. Herbivory may induce a plant to produce chemicals that signal to advertise the presence of insects feeding on them and attract parasites to reduce their numbers. Phenological escape is also employed, such as delay of leaf expansion during periods of insect abundance. Some indirect mechanisms of plant defence involve the use of insects such as ants for protection from other phytophagous insects. However, the predators have also evolved the ability to break down the defence mechanisms of the plant. For example, they may use phytochemicals for their own defence or as olfactory clues for feeding. In this chapter a brief account of organisms of the coastal dune communities, including species of the intertidal zone, scavengers of the sea coast, reptiles, birds, insects, mammals and their possible interactions with terrestrial vegetation is presented. For biological organisms of the seashore the intertidal zone is the most important for food and shelter. The sand-dwelling species of the seashore must be able to contend with four limiting factors: (i) rush of water from the approaching or receding high tide and pounding breakers, (ii) low salinity of the top surface of sand (iii) desiccation of surface by high winds and sunshine and (iv) extreme changes in temperature of topsoil.

Mycorrhizal fungi

Mycorrhizal fungi (mycobionts) form a ubiquitous mutualistic symbiotic association with the roots of higher plants (phytobionts) in coastal sand dunes worldwide. These obligate biotrophs perform vital functions in the survival, establishment and growth of plants by playing an active role in nutrient cycling. As such they serve as a crucial link between plants, fungi and soil at the soil–root interface (Rillig and Allen 1999). Mycorrhizas occur in a wide variety of habitats and ecosystems including aquatic habitats, cold or hot deserts, temperate and tropical coastal dunes, tropical rainforests, saline soils, volcanic tephra soils, prairies and coral substrates (Klironomos and Kendrick 1993). Simon et al. (1993) sequenced ribosomal DNA genes from 12 species of arbuscular mycorrhizal (AM) fungi and confirmed that mycorrhizas (fungal roots) fall into three families. He estimated that they originated about 353–462 million years ago and were instrumental in facilitating the colonization of ancient plants on land. Further evidence was provided by Remy et al. (1994) who discovered arbuscules in an early Devonian land plant, Aglaophyton major, and concluded that mycorrhizal fungi were already established on land > 400 million years ago. Thus the nutrient transfer mechanism of AM fungi was already in existence before the origin of roots. Plant roots probably evolved from rhizomes and AM fungi served as an important evolutionary step in the acquisition of water and mineral nutrients (Brundrett 2002). Over evolutionary time the divergence among these fungi has accompanied the radiation of land plants, and about 200 species of AM fungi have been recognized (Klironomos and Kendrick 1993) that exist in association with about 300 000 plant species in 90% of families (Smith and Read 1997), indicating that AM fungi are capable of colonizing many host species. Approximately 150 of the described mycorrhizal species may occur in sand dunes (Koske et al. 2004). Most host–fungus associations are beneficial to both the plant and the fungus and are thus regarded as mutualistic (++); however, the widespread use of the term mutualism (mutual benefit) for mycorrhizal interactions has been questioned because all associations are not beneficial to both the plant and fungus (Brundrett 2004).

The Ammophila problem

Even a cursory look on foredune plant communities shows vigorous dense stands of dune species in areas with moderate recurrent sand accretion levels specific for each plant species (Disraeli 1984; Maun and Baye 1989; Maun 1998). The phenomenon has been well documented in species of Ammophila arenaria (Carey and Oliver 1918; Tansley 1953), Corynephorus canescens (Marshall 1965), A. breviligulata (Eldred and Maun 1982) and Calamovilfa longifolia (Maun 1985). Burial has a positive influence on growth and flowering of plants and debilitated populations of foredune plant species can be rejuvenated by sand deposition (Maun 1998). Clear evidence of this phenomenon was presented by Maze and Whalley (1992a), who examined population dynamics of Spinifex sericeus in five zones receiving different amounts of sand deposition on a coastal dune system of Australia: the sea side of the first dune ridge, crest of first dune ridge, swale, Acacia thickets and stable hind dunes. In the very dynamic area on the sea side or toe of the first dune ridge (high beach) with regular burial or erosion of up to 1 m or more the plants produced very vigorous stolons with long internodes. On the crest of the dune ridge with sand deposition of about 17.5 cm per year even though plants had fewer stolons, they responded to burial by growing upwards with long internodes. In Acacia thickets in spite of very little sand deposition, plants were vigorous with little or no dead material, produced stolons and grew upwards with some long and some short internodes, probably because of greater nitrogen content in the soil. However, in the swale (slack) with little or no sand deposition, plants showed strong clumping tendency with very short internodes, a large amount of dead material on the surface and very low vigour. Unburied nodes usually died. Similarly, in the stable sand dunes with little or no sand deposition debilitated low-vigour clumps with very few stolons were abundant. Another example of this decline was presented by Martin (1959) on a shoreline along the Atlantic coast of North Carolina. He measured deposition and deflation of sand on two transects and showed that as one moved inland from the shoreline the total deposition of sand decreased.

Burial by sand

In coastal dune systems, plant communities are fundamentally the product of interaction between disturbance of the substrate, impact of high wind velocities, salt spray episodes, sand accretion levels and other factors of the environmental complex. Burial by sand is probably the most important physical stress that alters species diversity by eliminating disturbance-prone species (Maun 1998). There is a close correlation between sand movement and species composition, coverage and density (Moreno-Casasola 1986; Perumal 1994; Martínez et al. 2001). Sand accretion kills intolerant species, reduces the relative abundance of less tolerant species and increases the abundance of tolerant species. It filters out species as the level of burial starts to exceed their levels of tolerance. For example, lichens and mosses are the first to be eliminated, then the annuals and biennials and finally the herbaceous and woody perennials. Again within each life form and genus there are significant differences in survivability. Burial imposes a strong stress on production by altering normal growth conditions and exposing plants to extreme physiological limits of tolerance. Do plant communities occurring in different locations within a dune system correspond to the amount of sand deposition? Several studies (Birse et al. 1957; Moreno- Casasola 1986; Perumal 1994) show that the species composition and their distribution are strongly related to the long-term average sand deposition. The evolution of a plant community in coastal foredunes requires frequent and persistent predictable burial events specific to a particular coast. In a large majority of sea coasts burial occurrences are of relatively low magnitude and species occupying the coasts are well adapted to withstand the stress imposed by burial. This recurring event within the generation times of plant species allows them to acquire genes of resistance over time and evolution of adaptations to live in this habitat. A prerequisite to survive in this habitat happens to be the ability to withstand partial inundation by sand. To survive the dynamic substrate movement a plant species must be a perennial, be able to withstand burial, endure xerophytic environment, spread radially and vertically, and adapt to exposure on deflation and coverage on burial (Cowles 1899).

Seed germination and seedling establishment

For the transformation of a seed to a seedling complex physical and biochemical changes occur within a seed before germination can proceed. Germination is controlled by diverse seed dormancy mechanisms in plant species that delays germination until the conditions are most favourable for seed germination and seedling establishment (Thompson 1970). Baskin and Baskin (1998) identified four benefits for the evolution of seed dormancy in plants: (i) persistence in risky environments as seed banks, (ii) decreased intraspecific competition, (iii) improved chances of seedling establishment and (iv) increased fitness (seed production) of the individual and the species as a whole. They showed that seed dormancy may be caused by any one of physiological, morphological, physical, chemical and mechanical constraints or by a combination of more than one of these factors. For instance, seeds may possess an embryo with a physiological inhibiting mechanism, immature embryo, impermeable seed coat or may contain chemical inhibitors and hard woody fruit walls. In all of these cases seed dormancy is eventually broken by one or more of the following treatments: after ripening, heat treatment, cold temperature stratification, prolonged exposure to high temperatures, exposure to light, softening of seed coat by microbes or physical scarification, leaching of inhibiting chemicals, ageing of seeds and other subtle changes in the habitat. In temperate North America with snow cover during winter months the seeds of a large majority of sand dune species—Cakile edentula, Ammophila breviligulata, Calamovilfa longifolia, Iva imbricata, Croton punctatus, Uniola paniculata—and others require cold stratification at <4°C for 4–6 weeks to break their dormancy requirements. Seeds of some species such as A. breviligulata and U. paniculata that require cold stratification at the northern end of their range lose this requirement in the south (Seneca 1972). At southern locations exposure to high temperatures may be required to fulfil the dormancy requirements. Winter annuals, Vulpia ciliata, Cerastium atrovirens, Mibora minima and Saxifraga tridactylites, that grow and mature their seeds in early summer on sand dunes at Aberffraw, North Wales, require exposure to high soil temperatures to overcome a state of dormancy in a certain proportion of seeds at the time of dispersal (Carey and Watkinson 1993; Pemadasa and Lovell 1975).

Dune systems in relation to rising seas

Beaches and associated dunes are constituted of unconsolidated materials, such as sand, and thus are low-strength land forms less robust than rocky cliffs (van der Meulen et al. 1991). It is estimated that 70% of sand-based coastlines in the world are presently subject to erosion (Bird 1985; Wind and Peerbolte 1993). However, natural dune systems are inclined to adjust after stress without permanent damage (Brown and McLachlin 2002), and when stabilized by plant cover they offer a first line of coastal defence against assault from wave action (Wind and Peerbolte 1993; Broadus 1993; De Ronde 1993). Natural self-sustaining dune systems interact with the sea and closely reflect changes in sea levels. At any given time no single sea level characterizes all oceans, that is, the resting position of the ocean surface, or geoid, is not uniformly elevated over the earth. Eustatic sea levels, free of influence from tides, waves and storms, thus vary from place to place as well as over time. Satellite altimetry, which permits more accurate as well as more numerous observations than older tide-gage methods of measuring sea levels, shows that the ocean is actually a spheroid modified by depressions and elevations. For example, in parts of the Indian Ocean sea levels are as much as 70 m lower than the global mean and in the North Atlantic 80 m higher (Carter 1988). Climate is governed by long-term periodic variations in the earth’s orbit that effect changes in solar radiation and, consequently, also in sea levels (Bartlein and Prentice 1989; Woodroffe 2002). As a result, ice ages repeatedly alternate with periods of interglacial warming in which ice masses contract and sea levels increase. Most of the time that has passed since the Cambrian period—approximately 500 million years—sea levels, although fluctuating on several timescales, have been higher than they are today. Because of the difficulties in documenting conditions so far in the distant past estimates of these sea levels vary considerably, but those shown in Fig. 13.1, based on different kinds of evidence, are representative of attempts at reconstruction (personal communication RA Rohde 2008).

Seed banks

The soil seed bank refers to a reservoir of viable seeds present on the soil surface or buried in the soil. It has the potential to augment or replace adult plants. Such reservoirs have regular inputs and outputs. Outputs are losses of seeds by germination, predation or other causes, while inputs include dispersal of fresh seeds from local sources and immigration from distant sources (Harper 1977). Since sand dunes are dynamic because of erosion, re-arrangement or burial by wind and wave action, efforts to find seed banks have largely been unsuccessful. Following dispersal, seeds accumulate in depressions, in the lee of plants, on sand surfaces, on the base of lee slopes and on the driftline. These seeds are often buried by varying amounts of sand. Buried seeds may subsequently be re-exposed or possibly lost over time. However, the existence of a seed bank can not be denied. Plant species may maintain a transient or a persistent seed bank depending on the longevity of seeds. In species with transient seed banks, all seeds germinate or are lost to other agencies and none is carried over to more than one year. In contrast, in species with a persistent seed bank at least some seeds live for more than one year. The four types of seed banks described by Thompson and Grime (1979) provide useful categories for discussion of coastal seed bank dynamics of different species. Type I species possess a transient seed bank after the maturation and dispersal of their seeds in spring that remain in the seed bank during summer until they germinate in autumn. Type II species possess a transient seed bank during winter but all seeds germinate and colonize vegetation gaps in early spring. Seeds of both types are often but not always dormant and dormancy is usually broken by high temperatures in type I and low temperature in type II. Type III species are annual and perennial herbs in which a certain proportion of seeds enters the persistent seed bank each year, while the remainder germinate soon after dispersal, and Type IV species are annual and perennial herbs and shrubs in which most seeds enter the persistent seed bank and very few germinate after dispersal.

The sand dune environment

The micro-environmental conditions of different soil habitats are influenced by prevailing vegetation, aspect, soil texture, soil colour and other variables that influence the incoming and outgoing solar energy. The variability is especially pronounced in sand dunes because of shifting substrate, burial by sand, bare areas among plants, porous nature of sand and little or no organic matter, especially during the early stages of dune development. Even within a dune system there is disparity in radiative heating of different habitats that is manifested as variation in micro-environmental factors such as relative humidity, temperature, light, moisture content and wind turbulence. The major factor affecting these changes is the establishment of vegetation that stabilizes the surface, adds humus, develops shade, aids in the development of soil structure and reduces the severity of drought on the soil surface. The system changes from an open desert-like sandy substrate on the beach to a mature, well-developed soil system with luxuriant plant communities. The principal topics discussed in this chapter include accounts of micro-environmental factors of coastal sand dunes that influence the growth and reproduction of colonizing species. The water content of the substratum in sandy soils is one of the most important limiting factors in plant growth. Sandy soils have high porosity and after a rain most of the water is drained away from the habitat because of the large interstitial spaces between soil particles and the low capacity of sand to retain water. Evaporation in open dune systems also removes substantial quantities of water. Lichter (1998) showed that evaporation was greater on non-forested dune ridges than on forested areas and the rate of soil drying was influenced by soil depth and dune location. After 3 days of a heavy rainfall there was a drastic decrease in the percentage of moisture (67–80%) at 0–5 cm levels in open habitats compared to only 30–36% in the forested dune ridges. The same measurements at 10–15 cm depths showed much lower reduction in the percentage of moisture. In the swale (slack) even though the evaporative demand was the same, there was actually an increase in moisture because of seepage from the dune ridges.