Tree cavity formation in Mediterranean oak woodlands: Integrating tree traits, biotic interactions, and management for biodiversity conservation

Tree cavities are keystone structural resources that sustain forest biodiversity, yet their formation remains poorly understood in Mediterranean woodlands. Here, we synthesize evidence on cavity formation in Iberian oak woodlands by integrating tree traits, biotic interactions, management history, and climate-driven constraints. To place Iberian oaks in a broader functional context, we combined a structured narrative review with phylogenetic comparative analyses across 18 European tree species, evaluating how wood properties filter cavity excavation and how these constraints scale to regional cavity supply. Across tree species, wood hardness, approximated by basic specific gravity, emerged as a major constraint on excavation, with higher wood density strongly associated with a lower proportion of excavated cavities. Within this comparative framework, Iberian oaks occupy the upper end of the gradient, and their low excavation rates are consistent with their intrinsically resistant wood. Therefore, cavity formation in Iberian oak woodlands is largely decoupled from vertebrate excavation and instead largely depends on decay processes. Traditional practices such as pruning, cork extraction, and selective cutting can promote cavity initiation by creating entry points for fungi and other decay agents, whereas intensive management, including clear-cutting and snag removal, suppresses cavity availability. Fungal communities, saproxylic insects, and moisture dynamics then act as amplifiers of decay, shaping cavity persistence and development. Climate change, especially rising maximum temperatures and increasing drought stress, may weaken these facilitative pathways by reducing moisture availability and altering the activity of cavity-forming organisms. We conclude that cavity dynamics in Mediterranean oak woodlands arise from the interplay of intrinsic wood resistance, anthropogenic disturbance, and biotic facilitation. Management that retains veteran trees and maintains moderate disturbance is essential to conserve cavity networks and the biodiversity they support under ongoing climate change.