Les bryocénoses du chaos gréseux du Moulin-des-Roches (commune de Senlisse, Yvelines, Île-de-France)

Abstract The populations of bryophytes on a sandy boulder field on the slopes of Ru-des-Vaux, Le Moulin-des-Roches, in the forest of Rambouillet (Ile-de-France) were analyzed. This site is already recognized as one of the most interesting for its bryoflora. The cartography and precise topology of more than six hundred stones throughout the entire boulder field were determined during summer. Then, two kinds of studies were undertaken. The first consisted of selecting 108 stones and analyzing the populations of bryophytes in terms of communities present and the position of species on the stones in relation to topology. All species were recorded, and the principle populations were mapped. Transects were used to describe the position of each species in relation to the others. The second kind of study involved enumerating all species present at the level of the sandy boulder field. The results show that two major topological situations occur : the colonization of saxicolous species on strong slopes lacking an organic deposit ; and a dominance of humicolous an lignicolous species on weak slopes lacking an accumulation. The mapped distribution of the 15 species makes it possible to determine areas of the high species richness and areas of conservation interest. Microtopography plays a major role in determining the distribution of species. The autecology of species and the various communities to which they belong are determined and linked to a sketch of bryosociologic dynamics. Recommendations for conservation management are proposed. It is important to maintain the light-reducing canopy of Quercus, Carpinus and Fagus because the bryophyte communities depend this for survival. Simple means are recommended to limit excess frequenting of the most remarkable sectors.