Forest structural conditions and taxonomic and functional diversity of breeding birds in a temperate forest

Forest vertical structure influences habitat complexity and resource availability, thereby affecting biodiversity. However, most studies have focused primarily on taxonomic diversity, while the effects of vegetation structure on functional diversity remain less understood. This study investigated the relationships between forest vertical structure and both taxonomic and functional diversity of breeding bird communities in the Baekdudaegan Protected Area, South Korea. Taxonomic diversity was quantified using the Shannon–Weaver index (H′), and functional diversity was estimated using Rao’s quadratic entropy (Rao’s Q) based on five ecological traits: body mass, diet, foraging guild, nesting type, and migratory status. Vegetation structure was characterized by the cover of four vertical strata (ground, shrub, subtree, and tree layers). Generalized linear models and an information-theoretic approach based on Akaike’s Information Criterion were used to evaluate the relationships between vegetation structure and bird diversity. The best-supported model for taxonomic diversity included tree, subtree, and ground cover, whereas functional diversity was best explained by subtree and ground cover. Taxonomic diversity was positively associated with tree cover (β = 0.004) and subtree cover (β = 0.007), whereas ground cover showed a negative association (β = −0.002). Functional diversity was positively associated with subtree and ground cover. Although regression coefficients were not statistically significant, variable importance analysis consistently identified subtree cover as the most influential predictor for both diversity indices. These findings suggest that upper vegetation layers contribute to maintaining species diversity, whereas midstory and ground vegetation enhance structural heterogeneity and support functional diversity. Conserving vertically complex forests may therefore be important for maintaining multiple dimensions of biodiversity in temperate forest ecosystems.