Multitaxon assessment reveals inconsistent biodiversity responses to forest structural complexity in Flemish forests

Increasing forest structural complexity is a key objective of future-proof forest management, with benefits for biodiversity. However, empirical evidence for generalizable biodiversity-structure relationships across taxa is still limited. We investigated whether structurally more complex forests support greater species richness within individual taxa and higher multidiversity (i.e. a composite diversity metric) across taxonomic and functional groups in 19 mature forest plots in Flanders, Belgium. As one of the most densely populated regions in Europe, Flanders provides a realistic and policy-relevant context to test structure-biodiversity relationships, despite not capturing old-growth forest conditions. Its forests, often dominated by a few tree species and subject to long-term anthropogenic pressures and management, represent a realistic gradient of structural complexity. Structural complexity was quantified using a Structural Complexity Index (SCI), and biodiversity was assessed using a multidiversity index integrating scaled species richness across five taxonomic and seven arthropod functional groups, as well as the species richness of individual groups. Using mixed-effects models and multivariate Bayesian analyses, we tested both direct effects of SCI on biodiversity and the covariation in species richness among groups. Contrary to expectations, SCI was not a consistent predictor of multidiversity, and most groups showed weak or inconsistent responses. These findings might suggest that, within the range of structural complexity currently present in managed forests, structural complexity alone may be insufficient to enhance biodiversity in simplified forests.