Résumé
In the mountains of the Mediterranean Basin, the modifications of the land use drive a strong dynamics of the landscape. The woody species colonize the open area, but the maintenance of grazing and its variability in time and space prevent the existence of any equilibrium. The dynamics of the tree species accounts for a large part of this transformation. In this thesis, we studied the demographic dynamics of Fagus sylvatica and Que cus pubescens in representative habitats of the landscapes of the Causse du Larzac. Light-growth and light-mortality models show that the differences of shade tolerance explain the higher regeneration of the beech in pine understory, in comparison to the oak. We show then that the dynamics of establishment of these species in open habitat is limited by herbaceous competition especially for beech. The shrubs can indirectly facilitate the establishment of trees by reducing the biomass of highly competitive herbs. The advantage of the oak compared to the beech in open environment doesn't result therefore from a better growth in full light but of a better tolerance to herbs competition. The strategies of regeneration of the trees in open habitat with large herbivores, hardly covered in the literature, constitute yet an important part of their regeneration niche. We analyzed the dispersal by animals. The oak is dispersed to longer distance than the beech in open habitat, and the jay, an important dispersal agent, drives to a directed dispersal of the two species toward pine woods in a heterogeneous landscape. This dispersal can thus drive to an important dynamics of beech in the pine groves, whereas for oak the dynamics is limited by its low survival.