1. Understanding the interactive effects of global change drivers on tree demography is fundamental for realistic predictions of future forest dynamics. Multiple studies have shown increasing drought and exotic pathogens to severely threaten forest persistence by increasing mortality and decreasing growth of adult trees. However, much less is known about their effects on regeneration, and how they might affect seedling performance in additive and non-additive (synergistic or antagonistic) ways.
2. Here we aimed to fill this gap by experimentally exploring the effects of increasing drought and soil-borne pathogens on tree regeneration in two types of mixed oak forests (Quercus suber-Q. canariensis and Q. suber-Olea europaea) invaded by the exotic soil-borne oomycete Phytophthora cinnamomi, one of the most aggressive plant pathogens on earth. We conducted a seed-sowing experiment with oomycete-specific fungicide taking advantage of rainfall exclusion infrastructures that excluded 30% of the annual rainfall, simulating predictions of climate change models for Mediterranean systems. Seedling emergence, survival and growth of the three tree species were followed over 3 years.
3. We found that neutral or positive drought effects on regeneration dominated over negative effects in the tree community. Moreover, most positive drought effects on the dominant species (Q. suber) were not direct, but rather indirectly mediated by soil-borne pathogens. This was shown by the fact that positive drought effects disappeared with fungicide application. 
4. Synthesis: Overall, our results suggest that rainfall reductions predicted by climate change models for the Mediterranean region might have minor direct negative effects on early regeneration of tree species, but that could play a major indirect role by limiting the negative effects of exotic pathogens on highly susceptible tree species. These findings highlight that antagonisms among global change drivers should be recognized as important forces that might slow down the current loss of tree health.

We conducted a sowing experiment using seeds of the two dominant species at each forest type: Q. suber and Q. canariensis in the closed forest, and Q. suber and O. europaea in the open woodland. In December 2016, two 25 × 25 cm sowing quadrats (“Control” and “Fungicide”) were established under each of the 86 study trees. At each quadrat we established 16 sowing points. In the closed forest, one acorn of Q. canariensis or Q. suber was sown per point, making a total of 688 seeds per species (8 seeds × 2 treatments × 43 trees). In the open woodland, one acorn of Q. suber was also sown per point (n = 688 acorns). However, we sowed a higher number of O. europaea seeds (17 seeds per point, 11696 seeds in total) to compensate for the low germination rate of the species. Seeds in the fungicide quadrats were treated monthly with an oomycete-specific fungicide (Armetil 25 WP, 25% metalaxyl), whereas Control seedlings were treated with an identical volume of water. Seedling emergence, survival, and growth were followed monthly for 34 months. At each census, the state of each seed (emerged/not emerged) or seedling (alive/dead) was registered. In October 2019, all the living seedlings were harvested. Once in the lab, seedlings were divided into stem, leaves and roots. All plant material was dried at 70 ºC for a minimum of 48 h to estimate shoot and root biomass.