Résumé
Pollen and plant macrofossil analysis were undertaken on two small peatlands, situated on the facing slopes of the Guisane Valley (internal Southern French Alps). Their comparative study allowed for the determination of the role played by exposition and human activity in the Holocene forest dynamic of the valley. The vegetation distribution between 9000 and 4000 cal. BP was influenced primarily by the microclimatic context related to exposition. During the period around 8500 cal. BP, the northern flank was already covered by Pinus forests, while the non-forested southern side was colonised by Pinus and Betula woodlands. After 6500 cal. BP, Abies woodlands and Pinus woodlands became dominant on the northern and southern sides, respectively. The onset of human perturbation around 4000 cal. BP caused a retreat of the Abies forests on the northern side and an opening of the plant cover on the southern side. The latter is associated to an important increase in pollen abundance, which appears, in this study, to be a good indicator of forested and non-forested phases in the surrounding vegetation. Human activities, which radically modified the vegetation of the valley for 4000 years, masked the influence of the exposition; Larix forests expanded on the northern side, while the southern side was invaded by grasslands and Pinus forests. Finally, the reconstruction of local vegetation history stresses the major influence of the surrounding environment on sites development.