Tree cavities as key microhabitats: ecosystem function and conservation in Iberian Quercus forests

Abstract

Mediterranean Quercus woodlands are socio-ecological systems where biodiversity associated with tree-related microhabitats remains poorly known despite its potential importance for ecosystem functioning and conservation. Tree cavities, in particular, provide buffered microclimates and key nesting and shelter resources for numerous taxa, yet the diversity of organisms associated with these microhabitats remains incompletely documented across multiple trophic groups in Mediterranean woodlands. Here we compile an integrative inventory of cavity-dwelling biota and their ecological roles by combining high-throughput sequencing of fungal communities in holm oak ( Quercus ilex ) cavities with long-term data on saproxylic beetles and vertebrates across Mediterranean Quercus forests in Spain. Fungal communities are taxonomically diverse (> 2,000 OTUs) and functionally heterogeneous, dominated by saprotrophic Ascomycota but including plant pathogens, endophytes, and ligninolytic Basidiomycota, indicating multi-stage decay processes. Cavities also hosted a diverse saproxylic beetle communities (> 200 species, > 40 families), rich in wood-dependent, mycophagous, and predatory taxa, including numerous EU- and regionally threatened hollow specialists. Vertebrate inventories documented 67 cavity-using species (birds, mammals, reptiles, amphibians), largely secondary cavity users spanning broad dietary and foraging guilds, with woodpeckers representing a small excavator guild and several threatened mammals and reptiles depending on cavity-bearing trees. Taken together, these inventories document substantial taxonomic and functional diversity associated with tree cavities across fungi, saproxylic beetles, and vertebrates, highlighting their relevance as microhabitats for a wide range of organisms. Conserving cavity-bearing trees and the processes that generate them can therefore contribute to maintaining habitat heterogeneity and biodiversity associated with mature trees under ongoing global change in the Mediterranean region.

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