|This article does not cite any references or sources. (December 2009)|
The earliest plant colonizers are algae and eel grass which can tolerate submergence by the tide for most of the 12-hour cycle and which trap mud, causing it to accumulate. Two other colonisers are salicornia and spartina which are halophytes -i.e. plants that can tolerate saline conditions. They grow on the inter-tidal mudflats with a maximum of 4 hours' and exposure to air every 12 hours.
Spartina has long roots enabling it to trap more mud than the initial conlonizing plants and salicornia, and so on. In most places this becomes dominant vegetation. The initial tidal flats receive new sediments daily, are waterlogged to the exclusion of oxygen, and have a high pH value.
The sward zone, in contrast, is inhabited by plants that can only tolerate a maximum of four hours submergence every day (24 hours). The dominant species here are sea lavender and other numerous types of grasses.
However, although the vegetation here tends to form a thick mat, it is not continuous. Hollow may remain where the seawater becomes trapped elaving, after evaporation, saltpans in which the salinity is too great for plants. As the ride ebbs, water draining off the land may be concentrated into creeks
http://home.earthlink.net/~yvonr/trees/science/success.html (Seral succession)