We investigated the influence of mature of oak trees of various dimensions on soil properties, acorn and oak seedling characteristics in semi-arid forests in western Iran. A total of 24 oak trees were selected in comparable site conditions according to three size categories: small trees (DBH< 20 cm), medium trees (DBH: 20-50 cm) and large trees (DBH> 50 cm). Soil properties, light availability below canopy, acorn dimensions and weight, various below-and above-ground seedling morphological traits were measured. Besides, a seedling quality index (SQI) was also produced as an integrative measure of the seedling response. We found an increasing light availability from small trees to large trees (1512-103 μmol m − 2 s − 1) and soil fertility was largely improved from small trees to large trees: soil organic carbon (1.33-2.2%), available phosphorus (12.9-18.1 ppm) and potassium (301.2-470.4 ppm). However, soil properties did not significantly differ between medium and large trees. In contrast, acorn weight and dimensions as well as many seedling traits, including the aerial and belowground biomass and the SQI, were the highest in the medium tree category. To fully explore the relationships among our large set of variables, we produced a partial least square path model which explained 72% of the variation of SQI across the three tree classes. To conclude, we identified a clear effect by mature trees which provided favourable conditions for seedling establishment, but that effect was mediated by tree size and optimal conditions were found below the canopy of medium trees.