Abstract: Plant species can change biotic or abiotic soil properties with implications for the growth and survival of the own plant community. These plant-soil feedback processes could play an important role in determining the abundance and coexistence of species in plant communities. We evaluated the existence of plant-soil feedback processes in two declining Quercus suber forests at SW Spain: an open woodland with high abundance of the soil-borne pathogen Phytophthora cinnamomi, and a closed forest with low abundance of the same pathogen. In particular, a greenhouse experiment was conducted in order to evaluate the effects of Q. suber decline on conspecific seedling growth via soil changes, and to determine the nature (biotic vs. abiotic) of these soils changes. Q. suber acorns were sown in pots filled with soil cores collected in the two study sites under Q. suber adult trees of three health categories: healthy, defoliated, and dead. Half of each soil type was sterilized to separate biotic vs abiotic effects on seedling performance. After 4 months, shoot and root dry biomass of the seedlings were measured. Seedling growth did not vary among soil types collected in the open woodland. However, it was positively affected by soil sterilization, suggesting the existence of a net negative effect of the soil microorganism community. In the closed forest, seedling growth in soils collected under dead trees was much lower than in soils collected under healthy trees. In this case, soil sterilization did not affect seedling growth, suggesting a mainly effect of adverse abiotic factors. Overall, the results revealed that the decline and mortality of Q. suber trees induce biotic and abiotic soil changes that decrease the growth of conspecific seedlings. These changes could imply a competitive disadvantage for this species, with negative consequences for the maintenance of its abundance.