Elevation differentially shapes functional diversity patterns in understorey forest communities when considering intraspecific and interspecific trait variability

Questions: What is the relative importance of interspecific and intraspecific trait variation and their covariation in the herb layer of European temperate beech forests, and how do they vary with elevation? Is there evidence of interspecific trait convergence at higher elevations, as postulated by the habitat-filtering hypothesis, and is this convergence enhanced or counteracted by intraspecific variation?. Location: National Park “Foreste Casentinesi, Monte Falterona and Campigna”, Italy. Methods: We measured four functional traits – plant height, specific leaf area (SLA), leaf dry matter content (LDMC) and leaf area – across 775 individuals from 60 herb-layer species in 28 forest plots (10 m × 10 m) spanning an 800 m elevation gradient. For each trait in each plot, we computed community-weighted means (CWMs) and the standardized effect size of functional diversity (SES-FD). We decomposed total trait variation into its interspecific and intraspecific components, and their covariation. We run linear regression models to assess the impact of elevation on these three components of functional variation. Lastly, we investigated whether higher elevation communities exhibited lower SES-FD, indicating functional convergence that could hint to a stronger habitat filtering. Results: Interspecific trait variation was more important than the intraspecific counterpart both for CWMs and SES-FD. Only CWMs calculated for plant height and LDMC showed a significant relationship with elevation. Low-elevation communities featured taller, more-conservative species, whereas shorter, faster-growing species were more common at higher elevations. SES-FD remained consistently negative for species turnover and total variation, suggesting stable functional convergence across the gradient. Conclusions: Our findings suggest that interspecific and intraspecific trait variability can be decoupled along an elevation gradient, stressing the importance of individually considering each component of trait variation when studying community composition. Elevation significantly influenced various components of plant community trait variation, with habitat filtering playing a substantial role in selecting plants with specific traits across elevations.