The whole greater than the part: Assessing the relationships between Black Woodpecker cavity trees and diversity of tree-related microhabitats

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Abstract
Tree-related microhabitats (TreMs) are widely used as indicators of structural complexity and biodiversity in temperate forests, yet the contribution of individual habitat trees to plot-level TreM diversity has not been quantified. Particularly, the role of Black Woodpecker (Dryocopus martius L.) cavity trees, which are considered key elements in close-to-nature forest practices, has rarely been examined. We combined an observational paired-plot design with a counterfactual approach to assess the magnitude and mechanisms by which Black Woodpecker cavity trees contribute to TreM diversity across a wide range of temperate forests in France, spanning from lowland and mountain sites. To isolate the specific contribution of the cavity tree, we constructed four counterfactual scenarios in which the Black Woodpecker cavity tree was either removed or replaced by alternative TreM-bearing trees. Observational comparisons showed that the plots containing a Black woodpecker cavity tree supported higher TreM alpha and gamma richness compared to control plots. Across all counterfactual scenarios, TreM alpha, beta and gamma diversities declined, meaning that no replacement strategy restored observed richness or composition of the Black Woodpecker cavity tree plot. Multivariate analysis revealed that the importance of the cavity tree is driven by saproxylic TreMs, reflecting the decay processes, excavation scars and secondary microhabitats. Our findings strengthen the importance of Black Woodpecker cavity trees as keystone habitat structures in managed forests and suggest that TreM-based biodiversity indicators and retention forestry guidelines should explicitly incorporate their presence.

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