Mountain forests are multifunctional forests that play a key role both on biodiversity conservation and rockfall protection. Most of these forests are under a process of secondary succession, where we can distinguish five major stages with well differentiated diametric structures. Using a network of permanent plots in the french Alps, we selected 22 plots that best fit the five successional stages. For each plot, we first evaluated the function of biodiversity conservation using indirect indicators based on diametric structure and deadwood. Then, we used the rockfall simulation model Rockyfor3D to evaluate the function of rockfall protection. The advanced stages of succession, representing mature forest, are the most important stages for conservation. The first one, so-called ageing stage and still dominated by the initial cohort, contains numerous very large dominant trees. The two following ones, so-called renewal and irregular stages, are very heterogeneous in terms of tree dimensions and contain a large amount of deadwood. The most effective stages to protect from rockfalls are the densest stages, i.e. the first stage of the succession, so-called initial stage, and the irregular stage. In certain mountain forest contexts, the irregular stage which optimizes the two functions is difficult to perennialize. Consequently, when the function of protection is a priority, it is recommended to anticipate the renewal of the stands by creating openings large enough to promote a new succession. Forest management organizes the spatial and temporal distribution of these openings on a forested slope and aims at creating a perennial mosaic of small groups of trees at initial, intermediate (self-thinning) and ageing stages. The latest stages, the most interesting in terms of biodiversity, are often under-represented within this forest mosaic. The installation of small islands of sensible located old growth stands makes it possible to compensate for this under-representativeness.