CASE STUDY 17.1  Mangroves and saltmarsh communities

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P. Adam

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Figure 1 Mangrove represents a characteristic life form for many species of perennial plants adapted to sediments that are both saline and waterlogged. Convergent evolution across a number of taxonomically remote families has resulted in a diversity of anatomical structures for root gas exchange. These include pneumatophores in Avicennia marina (a), stilt roots in Rhizophora stylosa (b), cone roots in Sonneratia sp. (c), and stem buttresses in Xylocarpus sp. (d) (Photographs courtesy R.J. King)

 

Mangroves occupy extensive areas of sheltered intertidal soft sediment coast in Australasia. Adaptation to saline and water-logged sediments has resulted in convergent evolution into this particular life form by a number of plant families, and on a number of occasions. ‘Mangrove’ is thus an ecological rather than taxonomic term, and refers to this characteristic life form manifest by any one of a number of species. Their salt tolerance varies, but all are equipped with some form of root aeration device that helps sustain oxidative metabolism which in turn supports energy-dependent exclusion of salt in an anaerobic root zone. These devices include pneumatophores, as produced by Avicennia marina (Figure 1a), stilt roots on Rhizophora stylosa (Figure 1b), cone roots of Sonneratia sp. (Figure 1c) and stem buttresses on Xylocarpus sp. (Figure 1d). Morphology varies widely, but all have evolved with an accompanying aerenchyma to serve a common purpose, namely, root gas exchange.

Australia has a particularly rich mangrove flora (about 40 species) with the greatest diversity in communities of northeast Queensland. Species richness declines with increasing latitude, and mangrove communities at the southern limits of their distribution are represented by only a single species, namely Avicennia marina. This decline in species richness is paralleled by a reduction in stature and stand complexity from forests >30 m tall in northeast Queensland to low shrublands in Victoria.

Where mangrove and saltmarsh co-occur, there is normally a characteristic zonation with mangrove in the lower intertidal zone and saltmarsh at higher levels. This distribution reflects salinity tolerance. Lower intertidal areas are frequently flooded by seawater so that upper-profile soil water salinities are generally similar to those of seawater, and fluctuations limited. By contrast, upper intertidal soil salinities are influenced by evaporation from sediments and plants as well as by rainfall, so that soil water can experience periods of either hypersalinity or hyposalinity. Landward salinity gradients above high water level are increasingly restrictive on growth, and although some mangroves persist they commonly diminish in size and are soon replaced by saltmarsh communities more tolerant of hypersalinity.

Mangrove communities in tropical regions are extensive and taxonomically diverse, but tropical saltmarshes are species poor. Species richness does increase with latitude, and saltmarsh floras in Tasmania are especially rich together with those on southeast coastlines of mainland Australia and New Zealand (where saltmarsh floras are similar to those of southeastern Australia; Adam 1990). In southern Australia, saltmarsh flora and vegetation of high rainfall areas carry an abundance of grasses, sedges and rushes. By contrast, those on winter rainfall and arid coasts support a great diversity of dwarf shrubs (Adam 1990).

A feature of many Australasian plant communities is their susceptibility to weed invasion. Mangroves are almost unique in their absence of weeds. By contrast, saltmarshes in Australasia are not weed free. Saltmarsh communities commonly provide a habitat for numerous introduced species with Spartina anglica being especially troublesome in lower marshland. S. anglica is a natural hybrid between the American S. alterniflora and the European S. maritima, and was first recorded in Southampton Water (southern England) in the late nineteenth century. This hybrid form proved to be a vigorous colonist of low-level mudflats and was widely planted to aid in stabilisation of sediment. Various attempts were made to introduce it to Australasia, but it is now regarded with concern in New Zealand, Tasmania and Victoria. S. anglica can endure prolonged submergence, and has even extended beyond seaward mangroves in Victoria.

S. anglica is a C4 plant, but has not established on tropical coastlines despite obvious physiological advantages over C3 counterparts in terms of seed set, leaf development and photo-synthetic rate in warm environments. An explanation to this paradox relates to temperature effects on seed germination. Marks and Truscott (1985) discovered that germination might be more rapid at 20°C compared with 10°C, but that per-centage germination is enhanced by storage at low temperature (5°C) and diminished by storage at higher temperatures. Establishment and colonisation is thus favoured in temperate rather than tropical environments.

Mangroves and coastal saltmarshes occupy a tidal gradient from sites regularly flooded by high tides (seaward side) to sites reached by high tides on only one or two occasions each year (landward side). Vertical extent of this gradient will be determined by tidal range which varies around the coast of Australia from microtidal (<1 m) to macrotidal (>10 m in river estuaries of northwest regions). Horizontal expression of these gradients (i.e. the width of mangrove and saltmarsh zones) will vary according to tidal range and geomorphology. The seaward extent of mangroves will be dictated by the carbon cost of sustaining a large submerged root system, plus physical limits on ventilating those roots. Both problems intensify with increasing depth.

Intertidal wetlands are important components of bio--diversity and conservation in their own right is thus essential. Maintaining these ecosystems is also of direct benefit to humankind. Mangrove and saltmarshes are major nurseries for fish, including many commercially important species. More-over, mangrove and saltmarsh detritus in estuarine waters is an important input to food chains supporting major invertebrate and vertebrate fisheries.

Unlike the situation in Southeast Asia, where mangroves constitute a forestry resource, direct utilisation of mangroves and saltmarshes in Australasia is limited to specialist wood, casual grazing and production of honey.

One potential threat to survival of mangrove and saltmarsh ecosystems is a rise in sea level that could accompany global change. Coastlines are ephemeral in geological terms, and Australia’s current coastline was established ‘only’ 6000 years ago, and subsequent to a postglacial sea-level rise. Intertidal communities have accommodated past changes, but human development of shore lines may preclude future retreats. Mangrove and saltmarsh communities in northern Australia should adjust to a rise in sea level but opportunities are more limited in southeast Australia where many intertidal wetlands have already been lost. Northern Australia is credited with some of the most extensive and pristine wetlands on earth, and protection of this resource is an issue of international significance.

References

Adam, P. (1990). Saltmarsh Ecology, Cambridge University Press: Cambridge.

Hutchings, P. and Saenger, P. (1987). Ecology of Mangroves, University of Queensland Press: Brisbane.

Marks, T.C. and Truscott, A.J. (1985). ‘Variation in seed production and germination of Spartina anglica within a zoned saltmarsh’, Journal of Ecology, 73, 695–705.

Further reading

Adam, P. (1994). ‘Saltmarsh and mangrove’, in Australian Vegetation, 2nd edn, ed. R.H. Groves, 395–435, Cambridge University Press: Cambridge.

Allaway, W.G. (1987). ‘Exploitation and destruction of mangroves in Australia’, in Mangrove Ecosystems of Asia and the Pacific. Status, Exploitation and Management, eds C.D. Field and A.J. Dartnall, 183–192, Australian Institute of Marine Science: Townsville.

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