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Not banking on getting into the zone
I live near the shore of a large estuary in southwest England. My concept of a salt marsh is moulded by my knowledge of the local salt marshes and in particular that the tide flows in and out twice per day. However, there are examples of inland salt marshes where the tide is not a complicating factor in ecological research. Such a situation is described by Christy Carter and Irwin Ungar (Athens, Ohio, USA, pp. 119-125). The marsh is at the site of an abandoned salt mine. There is a gradient of salinity within the site but actually, the differences in salt concentration along the gradient are rather less than the variation over a year. Autumn and winter precipitation lead to flooding and hence dilution of the salt; the salt concentration starts to rise again in the spring. The vegetation in the marsh shows clear zonation with different species dominating different zones. The authors have selected Spergularia marina in order to investigate whether its zonation results from its germination behaviour. S. marina forms large seed banks with differing depths of dormancy. In general, seeds are dormant when shed from the plant and dormancy is broken by exposure to low winter temperatures. In the field, germination occurs in spring when both temperature and soil salinity are still relatively low. Germination does not occur in the heat of summer, even though the seeds are no longer dormant. This germination pattern clearly allows establishment of the plants in time to reach maturity and set seed in the summer. However, it does not explain zonation because firstly, this species banks seeds in all the zones and secondly because it can actually germinate in all zones (although germination may be inhibited by shading in the Phragmites zone). As the authors rightly conclude, they now need to examine later stages of growth and development.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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