Plant–soil feedbacks in mixed pine-oak Mediterranean forests under drought conditions

Abstract Background and aims Pine-oak mixed forests are characteristic of the Mediterranean landscape. Understanding which types of plant–soil feedbacks (PSFs) operate in these communities and how such PSFs could be altered by severe summer droughts is needed in the face of current climate change. Methods We conducted a fully reciprocal PSF experiment using eight Mediterranean pine and oak species. Seedlings were grown under controlled conditions in substrate containing sterilised soil and a small amount of soil collected under congeneric or heterogeneric mature trees. Half of the seedlings were subjected to a severe drought during the second growing season. Results Differences in plant biomass and responses to drought were linked to fungal communities in the rhizosphere. Negative PSFs were detected amongst pines and oaks. Pine and oak seedlings grew better in heterogeneric soil than in congeneric soil. Fitness differences correlated with a higher relative abundance of growth-promoting ectomycorrhizal fungi (EMF) in heterogeneric soils. Under severe drought, no differences in seedling biomass, seedling survival or stomatal conductance were observed between seedlings growing in heterogeneric or congeneric soil. Conclusion Seedlings grew better in heterogeneric soils which suggests that Mediterranean pine-oak mixtures could be maintained by negative PSFs. Severe drought could decrease the strength of the negative PSFs, implying that tree diversity in Mediterranean forests could decline in a drier climate.