A striking feature of saltmarsh vegetation is the zonation of different communities with increasing elevation. This zonation is generally displayed as bands of characteristic species assemblages that run more or less parallel to the shoreline, although in many sites this is more complex due to the presence of creeks which have a subset of zonation patterns along creek sides. The distinctions between zones are often blurred since the pattern is generated by different, but usually overlapping, vertical distributions of individual species. A typical example of the different vertical range of plants and how it produces zonation is shown in the figure below.

Elevation ranges of some widespread saltmarsh species around Morecambe Bay; the thicker parts of the lines represent the range where a species is most frequent (after Gray & Scott, 1987)

Although there is variation around the UK in which species are likely to occur in each zone, the species which typify each zone are remarkably constant from marsh to marsh. It has, therefore, been possible to describe distinct 'communities' or assemblages of characteristic species.

The causes of zonation are connected with the gradient of changing conditions related to decreasing tidal submergence as marsh elevation increases, as well as differences in the efficacy of the tolerance mechanisms of the different species. The interplay of these factors determines the vertical distribution, and hence zonation, of individual species. To understand zonation, however, a third element is required, the biotic interactions between individuals. The two most important of these are competition and facilitation.

Various studies have demonstrated the importance of elevation and of biotic interactions in determining both broad zonation and local distribution patterns among saltmarsh plants. The manipulation of these variables provides a useful potential means of managing saltmarsh vegetation. For example, repeated clipping or mowing of Spartina marsh will, at appropriate elevations, convert that marsh into one dominated by common saltmarsh grasses. This occurred in only three years in experimental plots on the Ribble estuary near Southport, when Spartina grass was harvested by cutting it to ground level each September.

There is also, clearly, the potential to change the species composition of saltmarshes by altering marsh topography or drainage. Similarly, successional changes may be initiated by sowing a facilitating pioneer species into saltmarsh sites. However, it is important to note that if conditions are not right (e.g. elevation and gradient) planting may not be a viable option or success will be short-lived. Seed sowing and planting experiments were tried at Tollesbury, Essex with mixed results. In the short term, the use of turfs of vegetation proved to be more effective than planting with small plants or sowing seed, which had relatively poor success in all but the highest areas (Boorman, 2003). In contrast, no planting was undertaken in the Trimley Marshes managed realignment site, on the Orwell Estuary, Suffolk , however, within 6 months significant areas of the site had been colonised by Salicornia spp. (Posford Haskoning, 2003).

On the whole, saltmarshes do not tend to conform to the general picture of a successional and increasingly complex community developing under the driving forces of sediment accretion, increasing elevation and drainage and decreasing tidal submergence. On the contrary, the vast majority of saltmarshes south of a line from (approximately) Suffolk to South Wales, although often displaying successional changes in the upper marshes, appear to be in decline. Far from being replaced by low marsh species, the pioneer zones of these marshes are often either eroding or absent. Reasons for this include the extensive die-back of Spartina marsh and the combination of rising relative sea levels causing submergence and coastal squeeze.