THE VEGETATION OF A CAMARGUE PASTURE

SUMMARY (1) A vegetation and soil survey of a 400-ha pasture in the Camargue (southern France) was carried out as part of a programme on the ecology of grazing animals and their impact on the vegetation. (2) A classification of the vegetation by indicator-species analysis gave eight groups. One of these was a closed grassland dominated by Brachypodium phoenicoides. Two others typically contained rich swards with abundant Limonium vulgare, Halimione portulacoides and annual grasses. The fourth group was very species-poor, containing sparse clumps of Arthrocnemum glaucum separated by large expanses of bare ground, while the fifth was similar but richer in species. Two groups were characteristic of marshes; one group, dominated by Scirpus maritimus or Phragmites communis, was typical of the wetter parts, while the other, with abundant Scirpus maritimus and Aeluropus littoralis, was found at the edges. Finally, there was a group restricted to disused rice-fields, rich in ruderal species. (3) An ordination of the stands by correspondence analysis (reciprocal averaging) separated the main marsh sites from the remainder on the first axis. Stands in the old rice-fields and in small temporary marshes were separated out on the second axis, while the remainder of the stands (the majority) were spread fairly evenly along the third axis, showing a gradation from completely glycophytic sites to extremely halophytic ones. (4) Of the eleven soil factors measured in each stand, four (pH, depth of the water table, percentage of organic matter, and amount of extractable potassium) were significantly correlated with the first axis of the ordination; these were shown to be related to soil hydrology. Four more (extractable sodium and magnesium, the total amount of extractable bases, and the depth of the mottled horizon) were significantly correlated with both the second and the third axes; these factors were connected with soil salinity. The remaining three (extractable calcium, and the percentages of phosphorus and nitrogen) were significantly correlated with all three axes. A cluster analysis of these factors showed a similar grouping. It was concluded that the vegetation and soil show parallel responses to variations in the hydrology and salinity of the area. (5) A number of normally glycophytic species were found in saline soils; some hypotheses are put forward to explain this occurrence. (6) Comparisons with previous phytosociological work in southern France suggests that the vegetation in the pasture shows affinities with four Zurich-Montpellier orders: Thero-Brachypodietalia, Salicornietalia, Juncetalia maritimi and Phragmitetalia. These groups are important components of Mediterranean halophytic vegetation.