Climate change impacts on Phytophthora cinnamomi population in a Mediterranean mixed oak forest

Abstract: Mediterranean mixed oak forests are seriously threatened by climate change and exotic pathogens, specifically Phytophthora cinnamomi. This study examines the effects of experimental climate change on the soil population dynamics of P. cinnamomi in a mixed oak forest. For that purpose, in spring 2016 a rainfall exclusion experiment (control vs. 30% rainfall reduction, 3 plots per treatment) was set up in a Quercus suber – Quercus canariensis forest in southern Spain. One year later, open top chambers (OTC) were also set up in the experiment to simulate the increase in temperature (control vs. 3 °C increase) predicted by climate change models. Six OTCs were established per plot, together with six sampling points outside the OTCs to serve as controls (n = 72 sampling points). Changes in soil inoculum density of P. cinnamomi under each climate treatment were determined in spring from 2016 (just before the beginning of the rainfall exclusion treatment) to 2019. For each year, no significant differences in the density of viable inoculum of P. cinnamomi in the soil were found among the climatic treatments. However, in spring 2018, when precipitation was particularly high, the viable inoculum density in soil under natural conditions was significantly higher than in other years. A similar trend was obtained in soil under warming, although this increase was not significant. Meanwhile, rainfall reduction treatments showed a uniform density of P. cinnamomi in soil, even during 2018. These findings indicate the high adaptability of P. cinnamomi to new climatic conditions and consequently the great threat to Mediterranean forest conservation that both factors together represent.