Altitude and aspect filter the herb layer functional structure of sub- Mediterranean forests

Question: Do topographic factors that determine the distribution pattern of sub-Mediterranean coppiced woods influence the functional structure of their herb layer? Study area: Umbria-Marche Apennines (Italy). Methods: In 156 plots we collected topographic data and species cover. We identified the main forest communities using a Multivariate Regression Tree (MRT) analysis on the relevés by species matrix constrained by altitude, aspect and slope angle. For the herb layer of each plot, we calculated the community-weighted mean trait values (CWMs) and functional diversity (FD) indices. We compared CWM and FD values between MRT groups at each partition of the tree graph and checked linear trends of CWMs and FD indices along gradients illustrated by topographic variables and mean Ellenberg’s Indicator Values. Results: Altitude and aspect segregated the main forest types and filtered the herb layer functional structure, depending on the considered traits. We observed a shift from strategies related to cold stress and the shortness and unpredictability of growing season at the highest altitudes and on north-facing slopes, to those related to dryness at lower elevations and on south-facing slopes. Environmental filtering changed the relative abundance of traits between mountain and hilly woods, as well as functional richness of life form, vegetative propagation, pollen/spore vector, and functional evenness, divergence, and Rao’s quadratic entropy of storage organs types. Within hilly forests, functional divergence increased toward north-facing slopes, while evenness increased toward south-facing slopes. Conclusions: Altitude and aspect, besides shaping the distribution patterns of sub-Mediterranean coppiced woods, influence the functional structure of their herb layer in a different manner. In mountain woods, higher altitude fosters convergence of the storage organs more suitable to tolerate cold stress. In hilly woods, drought and the heterogeneous distribution of resources on south-facing slopes increase functional evenness, whereas the higher availability of resources on north-facing slopes promotes divergence.