The size of seeds and the microsite of seed dispersal may affect the early establishment of seedlings through different physiological processes. Here, we examined the effects of seed size and light availability on seedling growth and survival, and whether such effects were mediated by water use efficiency. Acorns of Quercus petraea and the more drought-tolerant Quercus pyrenaica were sowed within and around a tree canopy gap in a subMediterranean forest stand. We monitored seedling emergence and measured predawn leaf water potential (^Ppd), leaf nitrogen per unit area (N a ), leaf mass per area, leaf carbon isotope composition (5 C) and plant growth at the end of the first summer. Survival was measured on the next year. Path analysis revealed a consistent pattern in both species of higher <5 13 C as W pd decreased and higher <5 13 C as seedlings emerged later in the season, indicating an increase in 13 C as the growing season is shorter and drier. There was a direct positive effect of seed size on S C in Q. petraea that was absent in Q. pyrenaica. Leaf 5 C had no effect on growth but the probability of surviving until the second year was higher for those seedlings of Q. pyrenaica that had lower <5 13 C on the first year. In conclusion, leaf <5 13 C is affected by seed size, seedling emergence time and the availability of light and water, however, leaf <5 13 C is irrelevant for first year growth, which is directly dependent on the amount of seed reserves.