Forested landscapes are rapidly being transformed by increasing rates of tree mortality caused by global-change factors such as exotic pathogens or extreme droughts. Disturbance of tree communities can have cascading impacts on biodiversity at all organization levels. However, how soil microorganisms respond to tree dieback and the implications for ecosystem recovery is largely unknown.
Here we combined spatially-explicit neighborhood models, DNA metabarcoding, and a field experiment to analyse how the decline of the Mediterranean tree Quercus suber altered the diversity, composition and network structure of belowground fungal communities in forests invaded by the aggressive exotic pathogen Phytophthora cinnamomi. Moreover, we explored whether such changes had cascading impacts on Q. suber recruitment.
Results/Conclusions
We found that Q. suber dieback caused a reduction of the taxonomic diversity of soil fungal communities but increased their phylogenetic diversity, likely due to the colonization of unhealthy soils by rare taxa not present in healthy soils. Soil fungal communities in neighborhoods dominated by defoliated or dead oaks were characterized by a higher relative abundance of pathogenic and saprophytic fungi, lower abundance of ectomycorrhizal fungi, and a very distinct network structure (lower modularity but higher connectivity) than fungal communities in healthy neighborhoods. Such microbial changes had cascading effects on Q. suber regeneration, since seedling establishment was negatively related to fungal phylogenetic diversity but positively, strongly influenced by ectomycorrhizal abundance.
Overall, our results showed that Q. suber dieback exerted a strong influence on the diversity and structure of soil fungal networks promoting less stable assemblages impoverished in mutualistic taxa, with negative feedbacks on conspecific regeneration. This implies that oak dieback has far-reaching multi-trophic effects that may affect post-mortality successional dynamics and condition a slow ecosystem recovery.