Multiscale assessment of understory functional diversity components and their relationships with forest structure and canopy cover

The understory accounts for the majority of the diversity in temperate forests (Gilliam, 2014). Functional diversity is an especially relevant indicator, and previous studies have shown how it relates to forest age and management types (Chelli et al., 2024; Chianucci et al., 2024). We studied 28 beech forest sites in a National Park in Italy (Foreste Casentinesi, Monte Falterona e Campigna) that features coppice, high forest and unmanaged old-growth stands. We apply a novel multiscale approach based on Rao’s Quadratic Entropy. We considered functional diversity at stand scale (γ), in small-scale subunits (α), between subunits (β), as well as decomposing the latter in terms of redundancy, uniqueness and clustering. Forest structural features and canopy cover have been quantified in each stand with terrestrial laser scanning and digital cover photography and related to the understory functional diversity at each scale through the use of functional linear models. We show that stand-scale higher median tree height, but not canopy cover, is related to an increase in functional diversity at small scales, together with an increase in redundancy and functional clustering, while at stand scale there is no evidence of a change in functional diversity and redundancy. Our findings have implications for forest management and monitoring. First, we highlight the importance of median tree height, which is often overlooked in favour of canopy closure when studying relationships with understory functional diversity. Second, the employed sampling design enabled the identification of relationships at particular spatial and ecological scales that might have been overlooked with the classic standlevel vegetation survey.