Résumé
There is a current trend towards managing forests with multiple objectives, in particular to preserve or increase biodiversity and sustainability. There is renewed interest in understorey vegetation as a way both to increase the number of species and, indirectly, to favour fauna, including game, or improve soil quality. However, this stratum of herbaceous or shrubby species can also compete with young tree seedling and jeopardise tree regeneration. Hence a compromise has to be found among the different management objectives for the forest and in particular the understorey. Light is one of the main environmental factors controlling ecological and biological processes in forests. For example, light quantity and light quality control the success of seed germination and the establishment and growth of tree seedlings in the understorey. Light also promotes the development of the floor vegetation, the composition of which varies with site conditions in addition to light. Modifying light availability in the understorey, for example with thinning operations, thus interferes with tree regeneration and flora cover and composition. The understorey plants can intercept a significant fraction of the light and so the resulting amount of light available for tree seedlings varies greatly depending on species and composition of the floor vegetation. Light is successively intercepted and transmitted by the different strata composing the forest stand, i.e. overstorey trees, midstorey trees, shrubs and herbaceous species. The different strata interact, the development of each one being controlled by the others, often through modification of light availability. Foresters have to control this chain of light sharing to steer forest stands towards different objectives. We report results concerning the light sharing chain and discuss their implications for management.