Structural Attributes Outweigh Topographical Effects in Determining Vascular Plant Diversity Within Southern Apennine Old-Growth Forests.

Old-growth forests (OGFs) provide key benchmarks for biodiversity and ecosystem functioning, yet relationships among stand structural heterogeneity, topographic gradients, and multilayered understorey composition remain insufficiently quantified in Mediterranean mountain systems. This study aimed to (i) quantify forest structural heterogeneity in Southern Apennine OGFs, (ii) examine relationships among site conditions, vascular plant diversity and forest structure and (iii) identify the structural attributes most strongly associated with vascular plant species composition while disentangling their contribution from that of topography. Location: Pollino National Park, southern Italy. Structural attributes, topographic variables and vascular plant species composition were recorded across nine candidate OGF sites. After exclusion of one site, the analytical dataset comprised 142 subplots from eight sites. Structural complexity was quantified using a Structural Heterogeneity Index (SHI). Linear mixed-effects models accounted for subplot nesting within site, while canonical correspondence analysis (CCA), principal coordinate analysis (PCoA) and variation partitioning (VP) assessed structural-topographic-floristic relationships. Floristic sampling completeness and sensitivity to the taxon-frequency threshold were also evaluated. SHI values varied markedly among forest types, with beech and mixed forests exhibiting the highest structural heterogeneity. Across the full multivariate gradient, topographic variables explained 18.8% of variation in structural attributes. In site-adjusted mixed models, aspect had a significant joint association with SHI, whereas elevation and slope were not significant. Floristic ordination was highly stable across taxon-frequency thresholds, and pooled sample coverage was 98.65%. VP analysis revealed that the forest-structure predictor set had a larger unique contribution to floristic variation (23.5%) than topography alone (12.8%), with a substantial shared fraction; large-tree abundance, canopy cover and decay-class diversity were among the structural variables significantly associated with vascular plant composition. The ecological complexity of Southern Apennine OGFs reflects an interplay of structural, topographic, historical and floristic dimensions. Integrating stand structure, topographic context and vascular plant composition provides a more robust framework for interpreting and conserving old-growth forests than any single structural metric.