Plant community patterns in old embanked grasslands and the survival of halophytic flora | Patrons d'organisation des communautés végétales dans un ancien polder et maintient d'une flore halophytique.

[Departement_IRSTEA]GT [TR1_IRSTEA]SET / DYMON

International audience

anglais. The presence of halophytic and sub-halophytic flora in grasslands reclaimed from the sea several centuries ago on the west Atlantic coast in France appears very unusual and the reasons for its survival are still uncertain. our objectives is to document its distribution pattern and to investigate whether its persistence is related to abiotic conditions and/or management practices. Vegetation survey were carried out in seven different sites with 26 relevés on grazed sites and 11 on mown sites. This flora shows some similarity with the vegetation of recent polders and can be classified into groups of species differing by their degree of salt tolerance but also their response to management mode. It was found that halophytes are only present on very saline soils, in a situation where the level of competition with more competitive species is reduced. The sub-halophytes also required low competition on lower soil salinity. In old polders, these two groups only occured on grazed slopes and declined or even disappeared in the absence of grazing. In exclosures, the community became strongly dominated by competitive species and the soil salinity decreased on the slopes. The maintenance of sub-halophytes on old polders seems to be mainly conditioned by competition limitation whereas halophytes showed also a strong dependency to soil salt level, two requirements, which are met on continuously grazed slopes. Grazing maintains inter-specific competition at a low level while trampling increases soil compaction. Soil compaction modifies water and salt circulation, which could explain the high level of soil conductivity observed on grazed slopes.