Topography and Canopy Structure Interactively Shape Understory Functional Diversity

Aims Topography and canopy structure are key environmental filters for the trait-mediated assembly of understories, especially in mountain areas. Although changes in functional diversity (FD) along elevational gradients have been described in the past, the importance of slope aspect or the coincidental change of the canopy structure is generally unaccounted for. In this study, we aim to assess (i) whether the relationship between understory FD and elevation varies between north- and south-facing slopes and (ii) whether the canopy structure acts as a mediator between local topography and FD. Location Central Apennines, Italy. Methods We conducted a transect-based field sampling in forests on opposing slopes in the central Apennines, stretching from the valley bottom to the climatic treeline at ~2000 m. By linking the species distribution and abundance data with plant functional traits from publicly available databases, we modeled the change of single- and multi-trait FD along the elevational gradient. To test for the importance of slope aspect and the mediatory effect of canopy structure on understory FD, we used regression analysis and comparisons between two competing multiple linear models (topography vs. canopy structure). Results We found that FD generally decreased along south-facing slopes, with no clear pattern on north-facing slopes, which indicates that different filtering regimes act on the understory in temperate forests depending on the slope aspect. Secondly, we found that for multi-trait FD, elevation and aspect influence the understory both directly and indirectly through differences in the canopy structure, which changes from mixed broadleaf forest to monodominant stands of Fagus sylvatica. For the diversity of single-trait FD, topography itself explained a higher amount of variation. Conclusion Our results underline the need to account for slope aspect in the study of understory FD and the role that the canopy layer plays in mediating between elevation, aspect, and the understory.