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Impacts of additional aerial inputs of nitrogen to salt marsh

In 2012 LAB Coastal carried out a desk-based investigation on the impacts of ...

A Portable Erosion Measuring Device (EMD)

When a river overtops its artificial flood banks, water running down the land...

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LAB Coastal
The Maylands
St Ives, Cambs
PE27 4TQ

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What can mangroves teach us about salt marshes?

In temperate regions along the coast where the soil surface is covered by sea water for a part of the day the result will be salt marshes – areas where the vegetation is composed mainly of annual and herbaceous perennial plants which are tolerant of limited periods of immersion in salt water.  However, in subtropical and tropical areas a very different type of vegetation occurs – these are the mangroves.  They are intertidal woodland or forest communities dominated by tree species, often tall, and shrubs.   Mangrove species are limited to areas where minimum air temperatures never fall below 5oC and where mean annual sea surface temperatures are not less than 24oC.  In addition mangroves benefit from fresh water inputs and are absent from arid coastlines. While mangroves are tropical or subtropical and salt marshes are temperate plant communities there are a few areas, notably in Australia, where mangrove plant species and salt marsh species do occur together.   Further north the full development of mangrove communities excludes the low-growing herbaceous salt marsh species.

The key physiological character affecting both mangrove and salt marsh plants is their salt tolerance, their ability to grow under saline conditions.   Both salt marsh and mangrove plant species show a range of similar adaptations to achieve this.   However, the physiological adaptations to growth in saline environments are only achievable at the expense of significantly increased energy requirements.

There is also a morphological character separating salt marshes and mangroves and this is the occurrence of woody tree and shrub species in the latter.  A few salt marsh plants such as Halimione portulacoides develop a degree of woodiness but it should be noted that this species is often cut back by winter frosts.  It appears that the development of wood tissues is inhibited by the stresses of saline immersion.  There is, however, an analogy for this in that the development of wood tissues is also inhibited by the stresses of high altitude or latitude habitats. In general trees will only survive when the mean annual temperature is greater than 6oC.  There are thus comparisons to be made between the absence of trees at high altitudes and the absence of trees in saline habitats at higher latitudes.

Despite the differences both salt marshes and mangroves have economically and ecologically important roles in protecting the coast from erosion, in providing nurseries for fish and shellfish as well as their contributions to biodiversity.   There are thus very good reasons for considering salt marshes with their tropical analogue as well as considering both salt marshes and mangroves in relation to other plant communities in stressful situations.