Qualitative and quantitative studies were carried out at three localities at different times of the year, from 1970 to 1974.Multivariate analysis, using inertia techniques, elucidated the major features of the principal sources of variation in time and space. The first main variation, in spite of the similarity of edaphic factors, was related to the spatial distribution of the samples among the stations.Replicate sampling procedures facilitated identification of the heterogeneous components of the environment. To estimate the importance of the time fluctuations at each of the three stations, an attempt was made to locate them along a climatic gradient.Time evolution is governed by two phenomena: annual cycle and climactic trend. Species diversity analysis gives an interpretation of this evolution. [NOT CONTROLLED OCR]