This study identifies the important role of climate change and photosynthetic photon flux density (PPFD) in the regenerative competence of ash and beech seedlings in 12 inter- and intra-specific competition designs in simulated mixed ash-beech forest gaps under conditions of non-limiting soil volume, water and nutrient supply. The growth conditions simulated natural forest conditions as closely as possible. Simulations were performed by growing interacting seedling canopies for one season in temperature-regulated closed-top chambers (CTCs). Eight CTCs were used in a factorial design with replicate treatments of [CO2] × temperature × PPFD × competition design. [CO2] tracked ambient levels or was 360 µmol mol-1 higher. Temperature tracked ambient levels or was 2.8 °C higher. PPFD on two plant tables inside each CTC was 16 % and 5 % of open-field levels, respectively, representative of typical light flux levels in a natural forest gap. In several of the competition designs, climate change made the ash seedlings grow taller than the beech seedlings and, at the same time, attain a larger leaf area and a larger total biomass. Advantages of this type for ash were found particularly at lower PPFD. There was a positive synergistic interaction of elevated temperature × [CO2] for both species, but more so for ash. There are many uncertainties when a study of chambered seedlings is to be projected to real changes in natural forests. Nevertheless, this study supports a possible future shift towards ash in north European, unmanaged, mixed ash-beech forests in response to the predicted climate change.