Large standard trees and deadwood promote functional divergence in the understory of beech coppice forests

How species assemble in a community is still an unresolved question in ecology, especially in forest ecosystems. In temperate forests, the understory layer includes most of the plant diversity and significantly contributes to ecosystem functions. Understory communities are susceptible to changes in environmental conditions linked to forest structural features. Understanding how understory assemblages respond to these features can provide useful suggestions for sustainable forest management. We selected 68 abandoned coppice-with-standards beech forest stands in central Italy. We recorded plant species presence and abundance and several structural variables, including total stem density, height and basal area of standard trees, the abundance of lying deadwood, and shrub layer cover. Different plant traits informative on key ecological functions were attributed to understory plants to calculate the single- and multi-trait functional diversity (FD) expressed as Rao's quadratic entropy. Linear-mixed models were used to assess the relationship between structural parameters and single- and multi-trait FD. We found that the size of standard trees and lying deadwood were the main structural drivers of trait-based understory assemblages. Larger standards and a higher amount of lying deadwood contributed to reduce multi-trait convergent patterns or to shift patterns from convergence to divergence (for reproductive height, seed mass, specific leaf area, and leaf area), probably due to modulation of resource amount and heterogeneity (i.e., light and nutrients). Our results feed the debate on the sustainable management of coppice forests. We suggest that the understory functional diversity of montane beech forests could be enhanced if forest management practices allow the release of larger standards (height > 19 m; basal area > 35 m2/ha) and the accumulation of deadwood (cover > 7%).