Climate change constitutes a major threat to global biodiversity and to the success of natural and assisted tree regeneration. Oaks are among the most emblematic tree species in the Northern Hemisphere, so it is crucial to understand the impact of changing climate on seedling recruitment and early development. In this study, we investigated the effect of air warming on the early development of one deciduous species - Quercus faginea - and one evergreen species - Quercus ilex subsp. ballota. Acorns of both species were seeded in an alluvial valley in southern Spain and subjected to an air warming treatment with Open-Top Chambers (OTC), which increased air temperature by 2 degrees C. We monitored seedling emergence, growth, chlorophyll concentration, and mortality in the first growing season. The simulated climate change treatment accelerated plant emergence in early spring, reduced spring shoot growth, and increased mortality from - 23 % in control plots to - 40 % inside OTCs. Although Q. ilex and Q. faginea are sympatric species, Q. faginea showed lower performance under simulated climate change in terms of growth. In addition, acorn fresh weight was positively related with the probability and speed of emergence (only for Q. faginea), seedling size, and relative chlorophyll content, and plants that emerged earlier had a greater likelihood of surviving. In short, larger acorns partly counter-balanced the negative impact of temperature increase on plants. This study highlights the importance of understanding plant response to climate change both to forecast potential changes in species composition and to choose adequate species and traits such as acorn size in restoration projects.