Impact of a drier climate on the exotic pathogen Phytophthora cinnamomi in Mediterranean forests differing in soil properties and species composition

Abstract

As climate becomes drier, plant disease dynamics will change. However, there is a lack of experimental data exploring how climate change scenarios will modify the abundance of aggressive pathogens threating forest ecosystems. In this study, we aimed to fill this gap by analysing the effect of a drier climate on the population dynamics of Phytophthora cinnamomi, one of the 100 worst invasive alien species on earth. The study was conducted over 5 years (2016–2020) in two Mediterranean mixed forests of Southern Spain invaded by the pathogen. The two forests varied in soil properties (texture) and species composition (closed forest of Quercus suber and Quercus canariensis vs. open woodland of Quercus suber and Olea europaea). The abundance of P. cinnamomi resistance spores in the forest soil was analysed every spring taking advantage of rainfall exclusion infrastructures that removed 30 % of the rainfall, simulating predicted climate change scenarios for the Mediterranean basin. Results showed that P. cinnamomi abundance in the forest soil was influenced by both soil texture and tree species composition, being lower in sandy soils and under Olea europaea compared to Quercus species. More importantly, we found a general negative effect of the rainfall exclusion treatment on pathogen abundance across soils and species. The longitudinal assessment of P. cinnamomi abundance in the soil also revealed that the pathogen had the capacity to survive at low density during dry years and increase its population in response to subsequent wet years. Overall, our findings suggest that the aridification of the Mediterranean climate might imply a reduction in average pathogen abundance, but that it would not be enough to preclude peaks of high pathogen abundance in response to the extreme heavy rains and floods also predicted by climate change models.