Effets des contraintes (vents et embruns) sur la composition et la structure de la végétation des pentes drainées de l'île de la Possession (archipel Crozet, subantarctique)

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Abstract
A study of the vegetation communities along two transects on the well drained slope of the Possession Island (subantarctic island, Indian Ocean) was carried out. The aim of this study was to investigate the structure of the vegetation under permanent pressure from gales and salt sprays and where there was no major perturbation. In the areas protected from gales, a Blechnum pennamarina community dominates. This fern hinders the vegetation dynamics in two ways : (i) by forming a very dense canopy that prevents sunlight from reaching shorter plants (passive competition) and (ii) by the accumulation of thick litter resulting from dead fronds. This community thus acts as the "inhibition" model of Connell and Slatyer. In areas exposed to gales and salt spray, B. pennamarina partly or completley disappears. The soil surface is consequently covered with several bryophytes and Azorella selago that form a mosaic on the soil surface. The vegetation dynamics end at this stage, despite the ability of B. pennamarina to colonize moss litter. The bryophyte community probably represent a stational climax stage acting as a "facilitation" model to B. pennamarina in an area protected from gales. It has therefore been concluded that the fernbrake community constitutes the optimal vegetation development on the well drained slopes of the Possession Island. Key words: climatic pressure, community ecology, bryophytes, Blechnum pennamarina, transect.

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