Useful information is contained in a collection of general articles (Rao et al., 1978) and the synthesis of lignans and neolignans has been reviewed in detail (Ward, 1982). In this review some attention has been paid to old-established methods but the writer has concentrated on developments post 1980.

Owing to the existence of large groups of reference compounds and to the power of modern spectroscopic methods, synthetic techniques are no longer essential to the proof of structure. However, the synthetic approach is vital for the preparation of lignans with useful physiological properties whenever the yields from natural sources are inadequate.

Oxidative coupling

Numerous examples of biomimetic synthesis exist which depend upon this procedure. These were illustrated in the previous chapter (cf. Schemes 7.7, 7.8, 7.9, 7.11) and others which relate to the various classes of lignans are given here.

This method is of interest because of the analogy with biosynthesis. However, as we have seen in oxidative lignin synthesis (p. 272), a number of pathways are usually open and low yields of specific lignans are commonly obtained. An early review (Harkin, 1967) discusses the simulated synthesis of lignin by the oxidative coupling of p-hydroxycinnamyl alcohols. Another contribution (Weinges and Spanig, 1967) in the same source describes the limited examples then available of this route to lignans.

In fundamental studies (Freudenberg, 1959, 1965) of lignin synthesis the free radical (8.1; X = H) derived from eugenol and the coupled quinone methide (8.2; X = H) were identified as intermediates but their half-lives are limited, being only 45 seconds and 1 hour respectively in 1:1 dioxan: water at 20°C.

Lignans (Whiting, 1987; Chatterjeee et al., 1984) occur typically in vascular plants (Hearon and MacGregor, 1955) and are found in roots and rhizomes and the woody parts, stems, leaves, seeds and fruits. In primitive species, such as those of the Podophyllaceae, they are the principal organic inclusions. With a few notable exceptions these sources do not provide commercially useful quantities. However, the wound resins of trees are a valuable major source of lignans, which here occur in simple mixtures with other natural products and are readily separated in substantial quantities (Table 5.1).

This review has been subdivided on the basis of parts of plants as sources because this is still the approach most often taken by chemists. It is unfortunate that relatively few studies have been made on the variations of inclusion compounds between the parts of whole plants.

Plant root sources

The roots and rhizomes of Podophyllum plants yield up to ten individual aryltetralins (Jackson and Dewick, 1985) which may also be found as their glycosidic variants. The flavones quercetin and rhamnetin are the only other identified products (Hartwell and Schrecker, 1958).

The roots and rhizomes of Podophyllum hexandrum afford commercially useful quantities of podophyllotoxin (2.357) with yields in the range of 1.5–4.0% of the dry weight (Hartwell and Schrecker, 1958). This will depend on the age of the plant but those which have come to maturity produce the lignan more economically than any existing laboratory synthesis. The American May apple, Podophyllum peltatum produces commercial quantities of podophyllotoxin and also of α-peltatin (2.354) and β-peltatin (2.355).

The dimeric lignans can be related to lignin, the three-dimensional polymer which is intercalated with cellulose and hemicelluloses so as to rigidify the structure of vascular plants. In the absence of direct evidence of lignan biogenesis it was assumed that dimerisation involved similar precursors and processes to those that had been demonstrated for lignin. Although some direct evidence of the formation of lignans from cinnamyl precursors is now available, much still depends on the lignin analogy. There is also some indication that experiments designed to evaluate pathways in the lignan pool are affected by the co-existence in vivo of a pool of intermediates for lignin synthesis and a brief account of its chemistry is therefore given by way of introduction.

The chemistry of lignin

One-third of the world's land surface is covered by forest containing 3 × 105 million cubic metres of timber and some 2.6 × 109 cubic metres are harvested annually. This amounts to a production of 1.3 × 109 metric tons or twice the world production of steel. Cellulose and hemicelluloses form over half of this but lignin is the remaining bulk constituent and is therefore the second largest natural source of organic material: it is restricted to vascular plants (Fengel and Wegener, 1984).

A diagramatic representation (Pettersen, 1984) of a lignin structure is shown (Scheme 7.1). The structure is constructed by the random linking of the three trans-(E)-monolignols (7.1) through free radicals (7.2A–C) generated by peroxidases bound to the cell wall. (Grisebach, 1981).

This chapter will in the main concentrate attention on spectroscopic methods for establishing lignan structures, but complementary chemical methods will be mentioned as appropriate.

Ultraviolet absorption spectra

All lignans show a basic UV absorption pattern typical of aromatic compounds with three bands in the regions of 210, 230, and 280 nm which correspond to the singlet excited states defined by Platt (1949; see Murrell, 1971) as 1Ba,b; 1La and 1Lb respectively. As the greater majority of lignans are optically active these maxima may usually be correlated with those in circular dichroism (CD) plots (p. 000). Clearly this basic absorption pattern will be modified in compounds with additional conjugation including those lignans which are fully aromatic.

The most intense absorption maximum arising from a component of the 1B excitation in the region of 210 nm has a molecular extinction (ε) of about 5 × 104 but this is rarely recorded. Peaks in the range 220–240 nm (ε = 10 to 20 × 103) and at about 280nm (ε = 2 to 10 × 103) are of diagnostic value since the absorption is of sufficient intensity to allow monitoring at all stages of the isolation procedure (p. 000).

In lignans where there is no conjugation to or between the aryl groups the absorption at the longest wavelength is sensibly the sum of their molar extinction coefficients.

Introduction

The saltmarsh environment is far removed from the optimum for growth of most plant species. Environmental characteristics of saltmarshes which would be inimical to plants include:

Tidal immersion may have a number of effects including:

Interest in the physiological ecology of saltmarsh plants has a long history but most research has been on the effects of salinity and other aspects of the environment have received little attention.

It is the salinity of saltmarshes which distinguishes them from freshwater marshes and fens but the concentration of interest on salinity has possibly led to a narrow view of the important factors operating to differentiate between saltmarsh communities. While it may be appropriate to see the selection of the saltmarsh flora from the broader terrestrial flora as being on the basis of salt tolerance, selection for particular microhabitats within the saltmarsh may have favoured other traits. Within the framework of a salt-tolerant flora selection for occurrence in different communities may have been in terms of tolerance to varying degrees of soil anaerobiosis, ability to thrive under particular soil nitrogen levels, or ability to withstand currents of particular velocity.

Other chapters in this book have been concerned with plant communities on saltmarshes and have stressed the variability in community composition both within and between marshes. While the physiological basis for the survival of saltmarsh plants in a saline, waterlogged environment is now established, the differentiation between species expressed in distribution patterns of species and communities is less clearly understood. Concentration on organisation at the species and community level is appropriate for testing hypotheses of a biogeographic or ecophysiological nature and may provide input towards an eventual synthesis of saltmarsh ecology but any such synthesis will also demand an understanding of processes operating at the ecosystem level. Knowledge of how saltmarshes function as ecosystems will be necessary for long-term management and for full understanding of the linkages between estuaries and saltmarshes. As Mann (1982) has argued ‘we shall never make good predictions about ecosystems unless we learn to observe ecosystems, and make testable hypotheses about them’.

The study of saltmarshes as ecosystems is still in its early stages. Compared to terrestrial systems the relative species paucity of many saltmarshes may simplify ecosystem studies; on the other hand, the intertidal nature of the habitat presents many practical problems.

While there have been many studies of particular processes, there have been few investigations which have adopted an integrative approach to the whole system. Many of the generalisations about saltmarsh ecosystems are based not on complete studies but on assumptions developed to fill the gaps between studies on particular processes.

Despite long-term successional processes, plant communities on most saltmarshes appear to be stable over many years. The little information on the longevity of saltmarsh plants suggests that, at many sites, communities may be stable for periods longer than the lifespan of their individual components. Any understanding of the processes involved, both in the maintenance of communities and the transition between communities, will require knowledge of the regeneration niche (Grubb 1977, 1986) of saltmarsh species. The concept of the regeneration niche encompasses ‘the requirements for effective seed set, characteristics of dispersal in space and time, and requirements for germination, establishment and onward growth that have to do not only with gap shape and size, but also with weather, pests and diseases.’ (Grubb 1986).

At the present time, it would not be possible to provide a fully comprehensive account of the regeneration niche of any saltmarsh species, although data on Salicornia spp. (see pp.330–334) are sufficient to define several components of the niche. The regeneration niche may be unique to genotypes, and so is not necessarily constant across the whole range of a species. Given the great intraspecific genetic diversity within species (pp. 107–131), life history characteristics are likely to be as much subject to variation as other traits. In the case of A. tripolium, within a single marsh there can be variation from long-lived perennials in the low marsh to short-lived perennials (or even annuals) in the upper-marsh (pp. 112–114).

Introduction

Coastal saltmarshes may be defined as areas, vegetated by herbs, grasses or low shrubs, bordering saline water bodies. Although such areas are exposed to the air for the majority of the time, they are subjected to periodic flooding as a result of fluctuations (tidal or non-tidal) in the level of the adjacent water body.

Distinction is drawn between saltmarsh and two other vegetation types found in similar habitats. Seagrass beds are for the most part permanently submerged but in some localities are found on mud or sandflats exposed at low water during spring tide periods. The characteristic floristic composition and structure of these intertidal stands is sufficient to maintain a distinction from any areas conventionally regarded as saltmarsh. Mangroves differ from saltmarsh in being dominated by trees. Where mangroves are well developed, forming forests 30 m high, the distinction between mangrove and saltmarsh is clear. However, there are areas where the differences are less striking. In southern Australia, at the southern limit of mangrove distribution, Avicennia marina forms a low shrubland, which can be shorter in stature than Sclerostegia arbuscula on adjacent saltmarsh. In such cases the distinction is on the basis of floristics and convention. Avicennia, which at lower latitudes is a medium-sized tree, is regarded as always being a mangrove, while Sclerostegia is never accorded mangrove status and is conventionally accounted as a member of the saltmarsh flora.

Many of the world's major cities are sited on estuaries. In consequence, saltmarsh often provides the only extensive areas of apparently natural vegetation close to major conurbations. Saltmarshes are almost always dominated by native species and look very different from the farmland which may fringe the city – however, there are many human impacts on saltmarshes and the species and community composition of many sites may be a consequence of these impacts. If these areas are to be managed, it is important that decision makers are aware of past impacts and the sensitivity of the ecosystem to changes in the pattern of human impacts.

Grazing

Livestock have been grazed on saltmarshes for centuries. At the present time, grazing is an important use of saltmarshes in northern Europe (Dijkema 1984d), eastern Canada (Roberts & Robertson 1986) and Japan (Ishizuka 1974), and occurs widely on marshes from high latitudes to the tropics. In Europe, grazing is most intensive in northern and central regions and is less intensive in the south (Beeftink 1977a). Grazing may, however, be an important ecological factor even on sites where it is carried out on an irregular casual basis.

Introduction

Saltmarshes are found on many of the world's coasts and experience a wide range of environmental conditions.

There is a tendency, however, to regard all saltmarshes as being very similar and to base broad generalisations on the results of studies of particular sites. It is important to be aware of the variability of saltmarshes if generalisations about their ecology are to be sustained.

In this chapter variation in saltmarsh vegetation at various spatial scales is discussed. After a broad-brush account of global variation, the saltmarshes of Britain and temperate Europe are discussed in detail and some factors which might be responsible for variation between sites are explored.

The extent to which other attributes, particularly those related to ecosystem functions, co-vary with the flora and vegetation remains to be documented.

Patterns of variation

Although in certain features there are resemblences between all saltmarshes, there are also major differences. Frey & Basan (1985) suggest that differences between saltmarshes are related to eight factors:

1. (1) Nature of the local marsh flora (and fauna);

2. (2) Effects of climate, hydrology and soil factors on the flora;

3. (3) Availability, composition, mode of deposition and compaction of sediments;

4. (4) Organism–substrate interrelationships;

5. (5) Topography and areal extent of the depositional surface;

6. (6) Tidal range;

7. (7) Wave and current energy; and

8. (8) Tectonic and eustatic stability of the coastal area.

Introduction

On casual inspection, saltmarshes might appear dull, monotonous places. After more detailed investigation, the range of organisms which will have been encountered is very diverse. The great diversity reflects the fact that saltmarsh straddles the boundary between land and sea and provides habitats for both terrestrial and marine organisms.

In this chapter, the biota of saltmarshes is introduced, with special reference to the flora. The relatively brief treatment of the fauna does not imply that animals are unimportant components of saltmarsh ecosystems. However, the biology of the saltmarsh fauna has been reviewed by Daiber (1982), a publication to which readers are referred for a more comprehensive treatment.

The genecology of some saltmarsh vascular plants is discussed. At a time when plant physiologists are increasingly looking to saltmarshes for experimental plants and systems and ecologists are seeking generalisations about species' behaviour to feed into ecosystem models it is appropriate to emphasise that species are inherently variable and that this variability is the basis for ongoing evolution.

The flora

The vascular flora

Saltmarsh floras are small when compared with those from most other habitats but it is nevertheless difficult to compile complete floristic lists for many parts of the world. In part, this reflects lack of scientific recording from many coasts but, more fundamentally, it is due to the lack of agreement regarding the definition of saltmarsh and the constitution of its flora.

Saltmarsh has a very important place in the history of ecology. Some of the earliest field courses involved extensive study of saltmarshes; early volumes of the New Phytologist, Journal of Ecology and Ecology contain papers describing saltmarshes and introductory studies on physiological ecology. The ritual saltmarsh excursion is still an essential part of the curriculum in many courses, at both secondary and tertiary levels. While this educational role partly reflects the importance of tradition, it is also an acknowledgement of the enormous opportunities for demonstrating ecological phenomena provided by the saltmarsh ecosystem. In the 1960s, saltmarshes were the venue for some important studies which developed systems ecology; in the 1970s and 1980s, studies on the physiology for salt tolerance in halophytes have been one of the most active areas in the development of ecophysiology. Research is being carried out today on many aspects of the biology and ecology of saltmarsh in many parts of the world.

Despite the great number of studies on saltmarshes, there is still much we do not know. Some topics, such as the study of nutrient and energy cycles, provide great challenges and will require interdisciplinary collaboration. Others, although equally important to our understanding of the total saltmarsh resource, are more easily studied. For example, in many parts of the world, there is little documentation of the distribution of marshes or of their species composition.