Vegetation and ecological gradients in abandoned salt pans in western France


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Abstract. Salt marshes along the Atlantic coast of France have been converted into solar salt pans since the 7th century. Salt production declined strongly from the 18th century onward, leading to the abandonment of many of these salt pans. High soil salinity is a residue from the original salt production and varies among salt pans according to time since abandonment, the current flooding regime and the effect of drainage measures. The relationships between the plant communities and seven environmental variables were investigated by Detrended Correspondence Analyses (DCA). Duration of flooding, electric conductivity and sodium saturation were most strongly related to variation in vegetation. The Heleo-chareto-Hippuridetum vulgaris and the Caricetum ripariae occur in lagoons with slightly saline soil that are flooded for the longest time period; the Scirpetum maritimi compacti occurs in salt pans with saline soil flooded for a long period; the Alopecuro-Juncetum gerardii occurs on saline soil where flooding is of short duration whereas the Carici-Lolietum perennis is never flooded and occurs on only slightly saline soil.

Soil salinity and duration of flooding provide a satisfactory explanation of the variation in species composition in abandoned salt pans but land-use practices, especially grazing, have to be taken into account to fully understand their floristic composition.