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17 pages, 3 figures, 5 tables, 63 references. We thank the Consejería de Medio Ambiente (Andalusian Government) and the directors of the Sierra Nevada National Park and Alcornocales Natural Park for facilities and support to carry out the field work. We also thank Rafael Villar for his advice in the experimental design of seedling growth measurements and for providing Q. pyrenaica acorns; and Luis V. García, Eduardo Gutiérrez, Ana Polo, Rocío Sánchez, Maite Domínguez, Carmen Navarro, Sergio de Haro, Asier Herrero, Ángel Navarro and Joaquín Sánchez for invaluable lab and field assistance. Many thanks to Lora Murphy and Charlie Canham for developing and providing the likelihood package for statistical analyses. Eugene W. Schupp provided helpful comments on an earlier version of the manuscript., Understanding seedling performance across resource gradients is crucial for defining the regeneration niche of plant species under current environmental conditions and for predicting potential changes under a global change scenario. A 2-year field experiment was conducted to determine how seedling survival and growth of two evergreen and two deciduous Quercus species vary along gradients of light and soil properties in two Mediterranean forests with contrasting soils and climatic conditions. Half the seedlings were subjected to an irrigation treatment during the first year to quantify the effects on performance of an alteration in the summer drought intensity. Linear and non-linear models were parameterized and compared to identify major resources controlling seedling performance. We found both site-specific and general patterns of regeneration. Strong site-specificity was found in the identity of the best predictors of seedling survival: survival decreased linearly with increasing light (i.e. increasing desiccation risk) in the drier site, whereas it decreased logistically with increasing spring soil water content (i.e. increasing waterlogging risk) in the wetter site. We found strong empirical support for multiple resource limitation at the drier site, the response to light being modulated by the availability of soil resources (water and P). Evidence for regeneration niche partitioning among Quercus species was only found at the wetter site. However, at both sites Quercus species shared the same response to summer drought alleviation through water addition: increased first-year survival but not final survival (i.e. after two years). This suggests that extremely dry summers (i.e. the second summer in the experiment) can cancel out the positive effects of previous wetter summers. Therefore, an increase in the intensity and frequency of summer drought with climate change might cause a double negative impact on Quercus regeneration, due to a general reduction in survival probability and the annulment of the positive effects of (infrequent) ‘wet’ years. Overall, results presented in this study are a major step towards the development of a mechanistic model of Mediterranean forest dynamics that incorporates the idiosyncrasies and generalities of tree regeneration in these systems, and that allow simulation and prediction of the ecological consequences of resource level alterations due to global change., This study was supported by a Juan de la Cierva contract to LGA, grants FPU-MEC to IMPR and IM, grants FPIMEC to J.L.Q. and L.M. and by the coordinated Spanish MEC projects HETEROMED (REN2002-04041-C02) and DINAMED (CGL2005-05830-C03). This research is part of REDBOME (www.ugr.es/ redbome/ ) and GLOBIMED (www.globimed.net) networks on forest ecology.