Discussion of the adaptations of plants in a salt marsh community.

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Discussion of the adaptations of plants

in a salt marsh community

Salt marshes are intertidal ecosystems, surrounded by land and open to the sea by way of an estuary (Pomeroy & Wiegert 1981). As a result salt marshes are constrained, by silt and saline water, to the type of communities it can sustain (Ranwell 1972). However because of the tidal nature of these ecosystems and the presence of many migratory birds we find some of the the most widley distributed plant species in the world (Ranwell 1972). The habitat is typically high in phosphates but low in nitrogen. Sulphur washed in from the sea collects in the soil and during dry periods lowers the soil pH (Chapman & Reiss 2000).

Essentially the main problem for salt marsh plants is that salt marsh ecosystems are perminantly waterlogged by seawater. Land plants obtain oxygen for their roots by diffusion of oxygen found in air spaces in the soil. When soil is waterlogged, water displaces oxygen from the air spaces and reduces oxygen transport to as much as 1/300000 of its original value ().

Most land plants are used to water that has an osmotis potential close to zero, however salt marsh plant communities have to exist in water conditions of much lower osmotic potential, approximately -2 Mpa. Most land plants subjected to such low osmotic potentials would loose water to its environment and die ().

It is the aim of this paper to discuss some of the common mechanisms which plants colonising salt marsh habitats have evolved to make effiecient use of what oxygen is available and be able to exclude salt and absourb water or excrete any excess salt that is absourbed as a result, with reference to specific salt marsh species.

Salt-marsh plants are halophytes (Gr halos:salt + phyton:plant), meaning they can tolerate excessive salinity levels (e.g. >0.5% NaCl), and have characteristics of both terrestrial and marine environments (Pomeroy & Wiegert 1981).

The salinity may vary and depends on the structure of the marsh, rainfall, and how often it floods (Chapman and Reiss 2000). If rainfall is high the marsh is washed of some of its salinity and will be colonised by different species such as Limonium spp. (sea lavender) and Triglochin spp. (arrow grass) (Chapman and Reiss 2000).

Grasses and rushes, such as Spartina spp. & Juncus spp. retrospectively, dominate salt marsh communities, however in the lower, muddy levels of salt marshes pioneer species such as Salicornia europaea are more common (Rose 1981) (Chapman and Reiss 2000).

Many of the plants are terrestrial species and in the upper parts of the marsh, where salinity concentrations are prodominantly affected by the amount of rainfall and not tidal influences, soils similar too more obvious terrestrial habitats are witnessed (Pomeroy & Wiegert 1981) (Ranwell 1972).

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Plants found in more northenly located salt marshes tend to be more tollerant of higher salinities, e.g. Spartina anglica can tolerate salinities up to twice that of sea water (Ranwell 1972).

Salt marshes have a particularly low osmotic potential due to its high sodium chloride concentration. To prevent excess loss of water and to obtain water from its environment it is vital that plants maintain a lower internal osmotic potential than that of its external environment (Purvis 2001).

This is a problem for non-halophytes at concentrations >0.05 M (1/10 sea water). However halophytes, subject to sea water (0.5 M), can ...

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