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.

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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In forests, the vulnerable seedling stage is largely influenced by the canopy, which modifies the surrounding environment. Consequently, any alteration in the characteristics of the canopy, such as those promoted by forest dieback, might impact regeneration dynamics. Our work analyzes the interaction between canopy neighbors and seedlings in Mediterranean forests affected by the decline of their dominant species (Quercus suber). Our objective was to understand how the impacts of neighbor trees and shrubs on recruitment could affect future dynamics of these declining forests. Seeds of the three dominant tree species (Quercus suber, Olea europaea and Quercus canariensis) were sown in six sites during two consecutive years. Using a spatially-explicit, neighborhood approach we developed models that explained the observed spatial variation in seedling emergence, survival, growth and photochemical efficiency as a function of the size, identity, health, abundance and distribution of adult trees and shrubs in the neighborhood. We found strong neighborhood effects for all the performance estimators, particularly seedling emergence and survival. Tree neighbors positively affected emergence, independently of species identity or health. Alternatively, seedling survival was much lower in neighborhoods dominated by defoliated and dead Q. suber trees than in neighborhoods dominated by healthy trees. For the two oak species, these negative effects were consistent over the three years of the experimental seedlings. These results indicate that ongoing changes in species’ relative abundance and canopy trees’ health might alter the successional trajectories of Mediterranean oak-forests through neighbor-specific impacts on seedlings. The recruitment failure of dominant late-successional oaks in the gaps opened after Q. suber death would indirectly favor the establishment of other coexisting woody species, such as drought-tolerant shrubs. This could lead current forests to shift into open systems with lower tree cover. Adult canopy decline would therefore represent an additional factor threatening the recruitment of Quercus forests worldwide.

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1. An increase in tree mortality rates has been recently detected in forests world-wide. However, few works have focused on the potential consequences of forest dieback for ecosystem functioning. 2. Here we assessed the effect of Quercus suber dieback on carbon, nitrogen and phosphorus cycles in two types of Mediterranean forests (woodlands and closed forests) affected by the aggressive pathogen Phytophthora cinnamomi. We used a spatially explicit neighbourhood approach to analyse the direct effects of Q. suber dieback on soil variables, comparing the impact of Q. suber trees with different health status, as well as its potential long-term indirect effects, comparing the impact of non-declining coexistent species. 3. Quercus suber dieback translated into lower soil respiration rates and phosphorus availability, whereas its effects on nitrogen varied depending on forest type. Coexistent species differed strongly from Q. suber in their effects on nutrient availability, but not on soil respiration rates. Our models showed low interannual but high intra-annual variation in the ecosystem impacts of tree dieback. 4. Synthesis. Our results support that tree dieback might have important short-and long-term impacts on ecosystem processes in Mediterranean forests. With this work, we provide valuable insights to fill the existent gap in knowledge on the ecosystem-level impacts of forest dieback in general and P. cinnamomi-driven mortality in particular. Because the activity and range of this pathogen is predicted to increase due to climate warming, these impacts could also increase in the near future altering ecosystem functioning world-wide.

Aim For tree species, adult survival and seedling and sapling recruitment dynamics are the main processes that determine forest structure and composition. Thus, studying how these two life stages may be affected by climate change in the context of other abiotic and biotic variables is critical to understand future population trends. The aim of this study was to assess the sustainability of cork oak (Quercus suber) forests at the core of its distributional range under future climatic conditions. Location Southern Spain. Methods Using forest inventory data collected at two periods 10 years apart, we performed a comprehensive analysis to evaluate the role of different abiotic and biotic factors on adult survival and recruitment patterns. Results We found that both life stages were influenced by climatic conditions, but in different ways. Adult tree survival was negatively impacted by warmer spring temperatures, while recruitment was positively affected by warmer winter temperatures. Our results also revealed the importance of soil texture as a modulator of winter precipitation effects on adult survival. With higher winter precipitation, adult survival increased in sandy soils and decreased in clayish soils. Therefore, under predicted future climate scenarios of wetter winters and warmer temperatures, the presence of cork oaks is more likely to occur in sandy soils vs. clayish soils. Biotic conditions also affected these life stages. We found a negative effect of heterospecific but not conspecific trees on both adult survival and seedling recruitment. Main conclusions Overall, the sustainability of the studied forests will be highly dependent not only on future climatic trends, but also on their interaction with other key factors – soil properties in particular – that modulate the effects of climate on demographic rates.