Disentangling the interactive effects of climate change and Phytophthora cinnamomi on coexisting Mediterranean tree species

Abstract

Abstract The structure and function of Mediterranean mixed oak forests are seriously threatened by climate change and the exotic pathogen Phytophthora cinnamomi. Both factors have been individually studied in some detail, but we have a poor understanding of their interactive effects on plant performance and species coexistence. We conducted a greenhouse experiment where seedlings of three coexisting species with contrasting susceptibility to the pathogen (Quercus suber, Quercus canariensis and Olea europaea var. sylvestris) were planted in pairs under different scenarios of drought (control vs. 30% rainfall reduction), warming (control vs. 3°C increase) and pathogen presence (control vs. soil infested with P. cinnamomi). At the end of the experiment, we assessed the severity of root symptoms and plant morphological traits (shoot and root biomass, root tissue density and specific root length). Results showed that seedlings of the three species, including O. europaea and Q. canariensis (species considered as resistant and tolerant to the pathogen, respectively), showed in general significantly higher root symptoms and lower shoot and root biomass than non-inoculated seedlings, independently of the drought, warming and plant treatments. We found, however, some interactive effects of the climatic treatments and P. cinnamomi on root traits of the two Quercus species. Variations in the specific root length (SRL) and root tissue mass density (RTD) of Quercus spp. suggest a compensation of the negative effects of both stressors on seedling performance. Overall, our results suggest that the warming and drought predicted for Mediterranean forests during spring might not have a relevant impact on the infectivity ability of P. cinnamomi, supporting its great adaptability to these changing climatic conditions. Therefore, in Mediterranean forests infected by P. cinnamomi, it becomes crucial to avoid the spread and multiplication of the pathogen during spring, since its destructive ability at high inoculum levels is largely independent of environmental conditions.