Contrasting responses of understory plant functional groups to canopy cover in Italian forests

Despite their relatively low biomass, forest understories play a crucial role in forest plant biodiversity and ecosystem functioning (1). Understory composition and diversity are strongly influenced by canopy cover through its effects on light availability, microclimate, and environmental heterogeneity, yet responses may differ substantially among plant functional groups (PFGs). Although PFG-specific responses of understory vegetation to canopy structure are increasingly recognized, most studies focus on limited taxonomic or functional contrasts (such as forbs vs. graminoids, pteridophytes vs. seed plants, woody plants vs. herbaceous plants), leaving broader comparative patterns among major understory PFGs poorly resolved. As a substantial innovation compared to previous inventories, the new Italian NFI (National Forest Inventory - IFNI 2025) introduces the measurement of understory biodiversity metrics (2), providing a large-scale dataset representative of Italian forests and wooded areas that may prove useful to fill this literature gap. In particular, in over 100 locations (10% of the 2025 IFNI second sample points) in four 10mx10m plots in each location (for a total of 438 sampling units), all understory vascular plant species were identified and their percent cover quantified by botanical experts selected by the SBI. Following the EU Directive 2016/2284 (NEC - National Emission reduction Commitments Directive), the species were subsequently assigned to five PFGs: forbs, graminoids, legumes (Fabaceae), woody plants, and pteridophytes. For each PFG, we calculated total cover, relative cover, and species richness, and examined the relationship of these metrics with each other and with canopy cover. The shape and strength of richness–abundance relationships varied among PFGs: forb richness, though remaining consistently the highest among all PFGs, exhibited a clear unimodal “hump-shaped” relationship with both absolute and relative cover, with maximum richness occurring at intermediate abundance level; legumes also presented a unimodal relationship between richness and abundance, whereas woody plant and graminoid richness remained mostly stable or even increased as their cover increased. Graminoids dominated understory cover under low canopy closure, whereas communities under denser canopies exhibited more balanced relative abundances among graminoids, forbs, and woody plants. By contrast, forbs consistently supported the highest species richness across the canopy gradient, followed by woody plants and graminoids, indicating a marked decoupling between abundance and diversity among PFGs. Overall understory cover declined with increasing canopy cover, largely driven by a strong reduction in graminoid abundance, whereas forb, pteridophyte, and woody plant cover remained stable or increased under closed canopies. These results suggest that canopy closure differentially affects understory PFGs, and that intermediate canopy conditions may maximize understory diversity by maintaining heterogeneous environmental conditions. These preliminary results highlight the importance of considering understory vegetation as a functionally heterogeneous assemblage rather than a single ecological layer: plant functional groups differed in their richness–abundance relationships and their responses to canopy closure, indicating that common community-level metrics may obscure contrasting ecological dynamics among understory components. Incorporating plant functional groups into understory analyses may therefore improve our understanding of forest biodiversity dynamics and enhance the ecological sensitivity of forest management and conservation assessments.