Since a long time throughout the World, presence of a particular flora has been observed on archaeological occupations. Archaeologists are interested in this relationship for surveying purposes. These long term man impacts are more and more intergrated in present ecological studies. Study of four archaeological sites in semi-natural areas allows to evaluate the link between sites and vegetation in North-Western France. Impact of structures on flora is obvious on each site. Soils of three of them are analysed : their physical and chemical properties are modified by ancient occupations. Ecological analysis of two sites flora provides answers. Plant species observed on specific sites were not introduced by man. They reflect this sustainable change of the soils. There is an early closure of the environment related to a better availability of mineral and water resources on old occupations. Bioindication software is developed to apply our approach to archaeological surveys. This discipline called “phytoarchaeology” is redefined.