Forest structure and understory functional diversity at multiple scales: The importance of median tree height

The understorey holds most of the vascular plant diversity in temperate forests. Functional diversity is a key aspect, commonly linked to forest age and management practices, but its relationship to specific structural features such as tree height and canopy cover is understudied. We studied 28 beech forest plots in an Italian National Park encompassing coppice, high forest and unmanaged old-growth stands. Using a novel multiscale approach (increasing grain size by combining adjacent sampling units), we assessed functional diversity at plot scale (γ), in small-scale subunits (α), between subunits (β), further decomposing β into functional redundancy, uniqueness and clustering. Forest structural features and canopy cover were quantified via terrestrial laser scanning and digital cover photography and related to understorey functional diversity across scales. We found that higher median tree height at the plot scale—though not canopy cover—is associated with increased functional diversity at scales up to ∼10 m. This is accompanied by greater redundancy and functional clustering. However, no changes in functional diversity or redundancy were observed at the plot scale, meaning that small scales host a greater portion of trait diversity. Our findings highlight the ecological relevance of median tree height, often overlooked in favour of canopy cover as a structural indicator of understorey functional diversity, for monitoring and management. Furthermore, our sampling design enabled the detection of scale-specific relationships that may have been missed by conventional plot-level vegetation surveys, i.e., an increase in diversity at the α level that does not translate to the plot level.