Lateral extent • Elevation change • Changes in creek and marsh • Changes in floristic composition • Changes in vegetation vigour

Changes in floristic composition and vegetation characteristics

Vegetation succession is a natural process which occurs during the development of a saltmarsh (Gray, 1992; Adam, 1994; Willis, 2000). Most saltmarsh systems also show marked species zonation, since different species vary in their tolerances to salinity, tidal inundation and energy conditions. There are three main communities of marsh pioneer species which are important in the UK , namely Salicornia spp., Puccinellia spp. and Spartina anglica. Salicornia has traditionally been dominant in the more muddy systems of the east and south coasts, but during this century Spartina has become increasingly important in these areas. Puccinellia has traditionally been the dominant species in the more sandy marshes of western and northern England but, again, Spartina anglica has become increasingly important during the past 100 years.

Each plant species/community has different characteristics in terms of seasonal growth patterns and plant physiology. From a geomorphological process point of view, the density of plant stems, stem height and leaf surface area are properties of particular relevance in terms of the way in which marsh plant interact with tidal currents and waves. Seasonal variations in vegetation cover and plant height have been demonstrated to have a significant effect on wave dissipation over marshes (Moller and Spencer, 2002). Consequently, longer-term changes in plant community composition and growth characteristics may also be expected to have an important effect on marsh sedimentation rates and wave energy dissipation, which in turn is of importance to the flood defence properties of a saltmarsh.

As an example, the timing of the introduction and growth of new Spartina marshes varied from area to area in the UK with different consequences. In many locations on the south coast rapid expansion took place between 1900 and 1930, and was followed by a more variable pattern of localised die-back (which began in the early 1940’s) and recolonisation. In Southwest England and South Wales, Spartina and other marshes experienced a marked phase of expansion between about 1920 and 1940, continuing until the 1960’s, while in West Wales and Northwest England the major phase of expansion did not take place in most areas until the 1960’s and 1970’s, even though a vigorous Spartina hybrid was introduced to these areas in the 1930’s and 1940’s. During the 1980’s and 1990’s, further spread of Spartina marshes has been very localised. The older Spartina marshes have stabilised, been invaded by higher marsh communities or have experienced erosion, particularly in more exposed locations.

In Southeast England, extensive colonisation by Spartina, and to a lesser extent other pioneer species, has occurred only in a relatively few areas, for example Shellness at the eastern end of the Swale, and scattered localities in the Medway, the Thames and in the Essex and Suffolk estuaries. Growth of these new marshes has not been sufficient to offset erosional losses of the older marshes elsewhere in the south east, although recent survey data from the Medway Estuary indicates that Spartina growth is outpacing erosional loss of non-Spartina communities (North Kent CHaMP, 2002).