Knowledge on the natural potential of oaks (Quercus spp.) in Switzerland is important for both nature conservation and adaptation to climate change. To assess this natural potential, we analysed tree ring and inventory data collected in Swiss natural forest reserves. Influences of monthly climate variables on oak growth were determined by response functions, whereas generalized linear mixed-effects models were used to identify the influence of climate and site variables on oak mortality. Oak growth was most strongly limited by summer precipitation. Oak mortality reached values between 0% and 25% per year for the individual inventory period; it decreased with tree diameter and increased with stand basal area and mean precipitation during the inventory period. Because stand basal area has continuously increased since the first inventory, the oak proportion decreased in most reserves. In increasingly dense stands, oaks were outcompeted by more shade-tolerant species such as beech. For the investigated sites, a gain of relative competitiveness of oaks in the course of ongoing climate change was not evident. Regarding the preservation of oak proportions in managed forests, protection measures should therefore still be combined with targeted interventions for reducing the competitive pressure.