Forest regeneration can be inhibited by competition for environmental resources (water, nutrients, light) between tree seedlings and some competitive species that are generally light-demanding species developing in gaps. The study's aim was to quantify the development of two competitive species (Rubus idaeus and Cytisus scoparius) present in the chaine des Puys, France, relatively to light in gaps inside Picea abies stands. On 29 transects linking the stand to the gap centre (223 points), light intensity was measured (0-80% of relative light) and floristic measurements (cover and height of the different species) were done. Development of both competitive species is positively connected to fight, with a bell-shaped curve with a maximum of 40-50% for R. idaeus and a quite constant increase to 80% (maximum of light recorded in the experimentation) for C scoparius. These results are discussed relatively to understorey vegetation management in order to favour forest regeneration.