a higher number of tree species in the overstorey is accompanied by a different composition of the understorey and positively influences understorey diversity

Trees have a species-specific impact on local environmental: the mean amount, variability and nature of the light reaching the forest floor (Messier et al. 1998), soil water content via differences in throughfall, transpiration and water uptake by roots (e.g. Barbier et al. 2008, 2009; Geiβler et al. 2012), soil nutrient availability   and acidity via differences in litter quality and quantity, nitrogen fixation, nutrient uptake and atmospheric deposition on leaves and needles (e.g. Augusto et al. 2003; Hagen-Thorn et al. 2004), phytotoxic compounds, thickness litter layer (Rodríguez-Calcerrada et al. 2011).
The local-scale tree species diversity influences the heterogeneity of resource availability and soil conditions at the forest floor. Namely, comparatively homogeneous environmental conditions in monocultures contrast with the heterogeneous pattern of patches with distinct resource availability and soil conditions within mixed stands (Morin et al., 2011; Yankelevich et al., 2006).

Hence, the composition of the tree layer may substantially affect herb layer composition and productivity as the germination and performance of herb layer species depends on the environmental conditions The relatively higher environmental heterogeneity within a mixed stand might be reflected in an elevated compositional dissimilarity between patches (Golodets et al., 2011). Moreover, resource heterogeneity increases niche differentiation, resource partitioning and complementarity, which are known to be among the major drivers of species diversity in natural ecosystems. In a heterogeneous environment interspecific competition is reduced because different species can occupy different microhabitats (niches) and use different resources. This may all lead to a higher stand-level understorey diversity (Reich et al., 2012), compared to monocultures. Silvicultural management of most forests usually aims at favouring one or a few highly productive tree species, therefore reducing both the species and structural diversity of the community. Understanding how this affects the diversity of other biotic components, such as the understorey, is not only crucial for biodiversity conservation but also for a sustainable and multifunctional management of the forests.

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