Summary• Soil-borne pathogens are a key component of the belowground community because of the significance of their ecological and socio-economic impacts. However, very little is known about the complexity of their distribution patterns in natural systems. Here, we explored the patterns, causes and ecological consequences of spatial variability in pathogen abundance in Mediterranean forests affected by oak decline.• We used spatially explicit neighborhood models to predict the abundance of soil-borne pathogen species (Phytophthora cinnamomi, Pythium spiculum and Pythium spp.) as a function of local abiotic conditions (soil texture) and the characteristics of the tree and shrub neighborhoods (species composition, size and health status). The implications of pathogen abundance for tree seedling performance were explored by conducting a sowing experiment in the same locations in which pathogen abundance was quantified.• Pathogen abundance in the forest soil was not randomly distributed, but exhibited spatially predictable patterns influenced by both abiotic and, particularly, biotic factors (tree and shrub species). Pathogen abundance reduced seedling emergence and survival, but not in all sites or tree species.• Our findings suggest that heterogeneous spatial patterns of pathogen abundance at fine spatial scale can be important for the dynamics and restoration of declining Mediterranean forests.

Plant-soil feedbacks (PSFs) play a relevant role as drivers of species abundance, coexistence, and succession in plant communities. However, the potential contribution of PSFs to community dynamics in changing forest ecosystems affected by global change drivers is still largely unexplored. We measured the direction, strength and nature (biological vs. chemical) of PSFs experienced by coexisting tree species in two types of declining Quercus suber forests of southwestern Spain (open woodland vs. closed forest) invaded by the exotic soil pathogen Phytophthora cinnamomi. To test PSFs in a realistic community context, we focused not only on individual PSFs (i.e., comparing the growth of a tree species on conspecific vs. heterospecific soil) but also calculated net-pairwise PSFs by comparing performance of coexisting tree species on their own and each other's soils. We hypothesized that the decline and death of Q. suber would alter the direction and strength of individual and net-pairwise PSFs due to the associated changes in soil nutrients and microbial communities, with implications for recruitment dynamics and species coexistence. In support of our hypothesis, we found that the decline of Q. suber translated into substantial alterations of individual and net-pairwise PSFs, which shifted from mostly neutral to significantly positive or negative, depending on the forest type. In both cases however the identified PSFs benefited other species more than Q. suber (i.e., heterospecific positive PSF in the open woodland, conspecific negative PSF in the closed forest). Our results supported PSFs driven by changes in chemical soil properties (mainly phosphorus) and arbuscular mycorrhizal fungi, but not in pathogen abundance. Overall, our study suggests that PSFs might reinforce the loss of dominance of Q. suber in declining forests invaded by P. cinnamomi by promoting the relative performance of non-declining coexisting species. More generally, our results indicate an increase in the strength of net PSFs as natural forests become disturbed by global change drivers (e.g., invasive species), suggesting an increasingly important role of PSFs in forest community dynamics in the near future.

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15In the last decades widespread tree decline and mortality has been documented in forests 16 worldwide. These mortality events usually show certain level of host-specificity, translating 17 into rapid changes in the relative abundance of the adult community. Despite these short-term 18 changes, it is poorly understood whether the decline and mortality of certain tree species are 19 likely to result in long-term vegetation shifts. Trajectories of forest recovery and the 20 probability of occurrence of permanent vegetation shifts are to a large extent determined by 21 post-mortality regeneration dynamics. Using a spatially-explicit neighborhood approach we 22 evaluated the spatial patterns of natural regeneration of the woody plant community in mixed 23 Mediterranean forests affected by the decline of their dominant tree species, Quercus suber. 24 We predicted the abundance, survival and richness of the seedling and sapling bank as a 25 function of the distribution and health status of the tree and shrub community. Results 26 indicated that Q. suber decline had detectable effects on seedlings and saplings of coexistent 27 woody species from very different functional groups (trees, shrubs and lianas). The sign and 28 magnitude of these effects varied substantially among coexistent species, which could imply 29 shifts in the species ranking of seedling and sapling abundance, affecting successional 30 trajectories and potentially leading to vegetation shifts. Because most of these changes 31 pointed towards a loss of dominance of Q. suber, management strategies are urgently needed 32 in order to attenuate adult mortality or promote its regeneration, counteracting the negative 33 effects of global change drivers (exotic pathogens, climate change) on these valuable forests.34 35

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