Soil-borne pathogens limit Quercus suber regeneration in Mediterranean forests

Abstract: In the Iberian Peninsula, evergreen Quercus species are suffering a severe adult tree mortality caused by the exotic soil-borne pathogen Phytophthora cinnamomi. Nevertheless, the effect of this pathogen on oak regeneration has been poorly investigated under field conditions. In this work, we experimentally assess the role of P. cinnamomi as a limiting factor of Q. suber seedling recruitment under field and controlled conditions. A seed sowing experiment was conducted in declining Q. suber forests at Los Alcornocales Natural Park (SW Spain). Q. suber acorns were sown with and without fungicide in 3 different sites (49 sowing points per site), and their emergence and survival were monitored monthly during 18 months. We also conducted a greenhouse experiment where Q. suber seedlings were sowed in soils collected under 4 types of adult trees: healthy, defoliated and dead Q. suber and under Olea europaea (the coexisting tree species). Half of the seedlings were treated with a fungicide monthly during 4 months. At the end of the experiment, aboveground biomass was weighed in all the seedlings. In the field experiment, the application of fungicide increased Q. suber emergence and survival. In the greenhouse experiment, the fungicide increased seedling aboveground biomass in all type of soils. Overall, our results experimentally demonstrate that P. cinnamomi has negative effects on the performance of Q. suber seedlings in declining Mediterranean forests. Soil-borne pathogens should therefore be added to the list of factors (e.g. seed predation, summer drought) that limit the natural regeneration of Q. suber.