Tidal wetlands of mangroves and tidal saltmarsh are universally recognised as remarkable ecosystems with often dramatic zonation and notably distinct patterns in diversity and structure (Duke et al. 1998). The defining zones appear to conform to gradients in underlying physical and chemical characteristics uniquely present at the dynamic land-sea interface. As factors like mean sea level, inundation frequency, salinity, temperature and exposure change with climate change and other direct human influences, the diversity and structure of tidal wetland ecosystems has altered and adapted to survive. Their presence today is testament to their success so far. However, the future survival of these habitats depends on maintaining such adaptive abilities, and keeping up with overall rates of change in their tidal wetland setting. But, the odds are against them where direct human pressures seriously reduce their resilience and adaptive capacity.

Characteristic descriptors of the tidal wetland zone

Tidal wetland plants worldwide are influenced by tidal range to form distinct zones as a direct response to inundation by marine and estuarine waters because each plant has different tolerances to salinity and inundation.  As such, the zones are determined largely, but not entirely, by tidal position, substrate type, slope, estuarine location, and exposure (see, Duke at al., 1998). Tidal range (or elevation profile) of tidal wetlands usually includes only the upper part of the tidal zone above mean sea level (MSL) up to the level of Highest Astronomical Tides (HAT). The tidal elevation zone may or may not be colonised by macrophytes (= the noticeable plants), but when it is, it is usually mangrove or tidal saltmarsh vegetation; collectively known as tidal wetlands, or to be precise; the upper tidal wetlands. Substrate type of tidal wetlands often consists of soft fine sediments, but at times it might also be hard coral, rock, rubble or sand. Slope of the tidal wetland zone is often gentle, but it might also be quite steep with a low density of stems. Estuarine location of tidal wetlands and other ecophysiological constraints based on specific tolerances of each vegetation type are defined by, temperature (dependent on latitude), salinity (dependant on catchment size, tidal range and climate) and the supply of nutrients (also dependant on catchment runoff, and rarely by coastal upwelling). Exposure of tidal wetlands is usually quite restrictive, where they mostly (but not always) occupy sheltered estuarine and coastal embayment locations, consistent with low energy waves, wind and currents.

Components of the tidal wetland niche

Tidal wetlands, above mean sea level, are broadly comprised of a specialised assemblage of macrophytes uniquely adapted to the prevailing biogeographical environment. The niche is unique and variously dominated by one or two chief assemblages, with mangroves predominant in the tropics and sub-tropics, and tidal saltmarsh throughout - extending to temperate shorelines (Tomlinson 1986; Spalding et al. 1997; Saenger 2002). In the tropics, these often co-exist along with the ‘vegetation-free zones’, termed tidal saltpans. As such, tidal wetlands are believed to consist of three dominant vegetation assemblages, such that:
 

Tidal Wetlands = Mangrove + Tidal Saltmarsh + Tidal Saltpan
 

This definition conforms with natural ecological conditions where tidal wetlands appear to function as a more or less unified, specialised niche with each component vegetation assemblage at times occupying different portions of the total area, depending on prevailing biogeochemical conditions. And, these portions might be relatively stable where key influencing factors, like sea level, sedimentation, nutrient levels and rainfall, remain constant and apply broadly. When influencing factors do change however, it seems highly likely that respective proportions of these vegetation units will also change with consequential gains or losses, at the expense or advantage of each other.

Mangroves are typically, tropical and sub-tropical tree and shrub macrophytes, often exceeding 0.5 m in mature height, growing mostly above mean sea level on unconsolidated sediments of the intertidal zone in marine coastal environments, and along estuarine margins. Stands of mangroves occur chiefly between MSL and HAT. Mangroves are an ecological entity, comprised of genetically diverse macrophyte taxa, dominated by angiosperm flowering plants (Duke 1992). 

Tidal Saltmarsh are typically temperate and tropical succulent herbs, sedges and grasses, often less than 0.5 m in height, also growing mostly above mean sea level on unconsolidated sediments of the intertidal zone in marine coastal environments, along estuarine margins, or bordering saline water bodies whose water levels fluctuate tidally or non-tidally (modified from Daiber 1959). Tidal saltmarsh plants are an important part of tidal wetlands worldwide. It is important not to confuse these plants with the more widely defined saltmarsh plants, with a considerable number not found in tidal wetlands (e.g., Sainty et al. 2012).

Tidal Saltpan, including the so-called ‘vegetation-free zone’ (also known as salinas elsewhere) occur mostly in tropical and sub-tropical areas within the tidal wetland zone. These areas lack macrophyte vegetation, and the muddy flats are often covered (more so seasonally) by layers (a carpet) of benthic microalgae. As such, the zone is not ‘vegetation-free’, but rather the ‘macrohyte-free zone’. These saltpan areas, occurring mostly between MSL and HAT, are recognised as key functional components of tidal wetlands.

For more information:

If you want to explore these matters further, then start with the following references. 

Daiber. F.C. 1959. Mangroves - the tidal marshes of the tropics. Estuar. Bull.: 10-15.

Duke. N. C. 1992. Mangrove floristics and biogeography. Pages 63-100 in A. I. Robertson, D. M. Alongi, eds. Tropical Mangrove Ecosystems. American Geophysical Union, Washington, D.C. 63-100.

Duke, N.C. 2006. Australia’s Mangroves. The authoritative guide to Australia’s mangrove plants. University of Queensland and Norman C Duke, Brisbane, 200 pages.

Duke, N.C., M.C. Ball, J.C. Ellison. 1998. Factors influencing biodiversity and distributional gradients in mangroves. Global Ecology and Biogeography Letters, Mangrove Special Issue 7: 27-47.

Fosberg, F. R. 1961. Vegetation-free zone on dry mangrove coastline. U.S. Geol. Soc. Prof. Papers 424, 216-218.

Saenger, P. J. 2002. Mangrove ecology, silviculture and conservation (Kluwer Academic Publishers, Dordrecht.

Sainty, G., J. Hosking, G. Carr and P. Adam (ed.) 2012. Estuary plants and what's happening to them in South-East Australia. Potts Point, NSW, Sainty and Associates Pty Ltd, 654 pp.

Spalding, M.D. F. Blasco, C. D. Field, eds. 1997. World mangrove atlas. International Society for Mangrove Ecosystems, Okinawa, Japan.

Tomlinson. P.B. 1986. The Botany of Mangroves. Cambridge University Press, Cambridge.