Spartina Marshes in Southern England: VI. Pattern of Invasion in Poole Harbour

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Abstract
Population changes resulting in a pattern of invasion by Apartina townsendii (s.l.) in Poole Harbour have been described from a study of historical and photographic records. The origin and relationships of the British species of Spartina are discussed. The basic pattern of Spartina marsh distribution was determined by the distribution of mudflats above O.D. (Newlyn) prior to the advent of Spartina. Two distinctive patterns of spread have been recognized. The first results from initial extensive seedling establishment and subsequent rapid expansion of tussocks by vegetative growth in optimum conditions on level mud and is characteristic of the early phase of development. The second results from much more limited seedling establishment on sloping mud surfaces adjoining accreting swards. It is estimated that the area occupied by Spartina reached a maximum of 2124 acres (867 ha) about 30 years after its initial establishment at the end of the 19th century. During the last 40 years the area of Spartina marsh has been reduced, by various factors, by about 20% of the maximum and there is evidence that the decline is still continuing. The importance of Poole Harbour as the main center for the distribution of Spartina planting material is exemplified by the fact that over 175,000 plant fragments and many samples of seed have been dispatched from this area to at least 130 sites around the world.