For most European temperate forests, recurrent droughts during seasonal growth are now recorded. To cope with these ongoing climate changes one attempt considered by forest managers is to reduce wood standing volume to globally decrease water consumption. Low wood standing volumes, and in particular a reduced tree leaf area index (LAI), globally lead to an increase of resources, light and water, in the understorey, as well as a modification of the microclimate. Previous studies reported that these modifications could benefit to the understorey flora, and in particular to some monopolistic genus such as Molinia, Rubus, Pteridium, Calluna,… What are the consequences of such modifications in resources, microclimate and flora composition induced by stand density management on biodiversity is the main question the project IMPREBIO attempted to resolve. The diversity in plants, including bryophytes, insects, soil macro- and meso-fauna, and gastropods, in interaction with game herbivory, was recorded in the field. It was then related to resources and microclimate generated by contrasted thinning regimes leading to different stand densities in pure even-aged oak (Quercus petraea and Q. robur) stands in Northern France. Two networks were used; the national cooperative for data on forest tree and stand growth modelling, and the LERFOB network on oak growth. In both networks contrasted thinning regimes were applied to oak stands with target levels of relative density index (RDI, following Reinecke) ranging from 0, totally open woodland, to 1, no thinning at all. A total of 100 plots distributed mostly in the northern mid-part of France were sampled, aged from 20 to 170 year-old. Light, air and soil temperatures, soil water content, and soil fertility were measured. A Structural Equation Modelling (SEM) approach was used to analyse the functional links between the oak stand main characteristics (RDI, stand basal area, age) and the species richness (alpha) of the different considered guilds. Overall no unequivocal trend was recorded between biodiversity considered as a whole and the contrasted stand densities. Conversely results depended strongly on the considered guilds and also on site features, highlighting the importance of the local soil and climate characteristics as primary environmental filters acting on understorey biodiversity. Stand age also influenced significantly the results, leading to complex interactions between the different ecosystem’s components. Many relationships between species richness and stand density were not linear, whereas a humped-model was often recorded, with the highest effects not recorded at both ends of the RDI gradient. The over-development of monopolistic plants in some sites at low RDI (low tree LAI) had a negative domino effect on other guilds such as other plants and insects that needs to be considered and further analysed. The understorey woody species such as Carpinus betulus also have a fundamental role on the microclimate and thus, in many relationships with species richness. As a consequence, low stand densities are not always beneficial to the diversity of every guild, whereas the different functional types must be considered in a given guild.