THE INFLUENCE OF SOIL COVER ORGANIZATION ON THE FLORISTIC AND STRUCTURAL HETEROGENEITY OF A GUIANAN RAIN-FOREST

The impact of soil cover organization on the forest community has been studied in a 19-ha tract at Piste de St Elie station in French Guiana . 195 species each represented by at least 10 individuals were chosen from records of the position, diameter at breast height (dbh) and prec ise identification by botanical sampling of 12 104 ligneous plants (db h greater than or equal to 10 cm). Spatial variations in the soil were mapped using the method proposed by Boulet et al. (1982). The soil ma pping units correspond to the successive stages of evolution of a curr ently unbalanced ferralitic cover. These stages describe firstly the t hinning by erosion of the microaggregated upper horizon and secondly t he mineralogical changes under more or less extended hydromorphic cond itions. The degree of evolution of ferralitic cover is also related to the hydrodynamic functioning and chemical properties of the soil. Geo logical substrate, topography and slope have also been taken into acco unt. Analysis of the influence of environmental variables on plant cov er has been performed using the Ecological Profiles method and Corresp ondence Analysis (CA) of the table of ecological profiles. The forest community seems to be dependent on the soil and the topographical feat ures that govern it. There are significant, exclusive soil-species lin ks for each soil functioning mapping unit. However, the highest propor tion of significant positive links is connected with a thick microaggr egated horizon (25%). Several species are of real value as indicators and more particularly enable differentiation between the forest stands of typical ferralitic soil and the ones of thinned out, transformed a nd hydromorphic soils. The CA of the species by environmental variable s matrices reveals two significant factorial axes. The first can be as sociated with the drainage mainly related to the thinning of the soil and the second with the hydromorphic conditions related to the topogra phy. The vegetation ordination of the stands (congruent to 0.25 ha) de limited in the various soil domains clearly shows that changes in ferr alitic cover and in particular the transition from soil with deep vert ical drainage to soil with superficial lateral drainage is accompanied by substantial changes in the forest community.