Studies addressing multiple aspects of biodiversity simultaneously (i.e., multifaceted approaches) can quantify plant diversity-environment links comprehensively, however they are scant in forests. This is because of the multidimensional nature of plant diversity. We studied taxonomic, functional and phylogenetic diversity patterns in 19 beech forest understory plots in two areas belonging to a biodiversity monitoring plan in Tuscany, Italy. We performed linear-mixed-effect models to quantify the influence of elevation (proxy for macroclimate), aspect (affecting microclimate), and basal area (related to microclimate and stand maturity) on diversity facets of vascular plants. Elevation played a major role in shaping diversity: high-elevation plots were less rich in species and had a reduced functional diversity of storage organs that may promote cold-tolerance. Conversely, the diversity of flowering phenology increased with elevation, thus low-elevation vegetation converged functionally towards a common, short blooming period. This strategy may be advantageous for understory plants in the deciduous beech forests experiencing longer growing seasons, hence more extended canopy closure at lower elevations. Basal area negatively affected foliar and multiple-trait functional diversity which may be associated with highly selective and competitive environment for light capture in closed canopy, mature stands. Slope aspect did not exert any significant effect on diversity facets, neither did interactions among predictors. Overall, these results confirm the usefulness of implementing multifaceted approaches to i) better understand the influence of environmental drivers on different aspects of plant diversity, and ii) inform the biodiversity monitoring plan that is in place in the study forests by systematically including functional diversity instead of taxonomic metrics only.
