Recent Hypothesis

The composition of the understorey shows a clear turnover over time due to tree canopy development.

During the first years after afforestation, the different tree species influence the environmental conditions and the understorey in a similar way due to the small size and hence, limited impact of the trees. In a later phase, the planted trees induce changes in environmental conditions:

The species-specific impact of trees on environmental conditions induces differences in understorey species richness and productivity

Trees have a species-specific impact on local environmental conditions. Messier et al. (1998) found differences among tree species in the

the understorey compositional dissimilarity between patches within a stand increases with increasing number of tree species in the overstorey, while the compositional dissimilarity between different stands decreases with higher tree species richness

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). Each understorey species has its own optimal requirements concerning resources and soil conditions. The relatively higher environmental heterogeneity within a mixed stand might thus be reflected in an elevated compositional dissimilarity between patches (Golodets et al., 2011).

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.

Soil biological activity is influenced by tree species composition

Earthworms can cause different effects on the soil by their activity: increased nutrient availability, better drainage or a more stable soil structure. Factors that influence the spatial variability of earthworm populations in the forest soil remain largely unclear. Variability can be explained by biotic interactions within populations, abiotic soil heterogeneity or the aboveground tree species composition.

Water balance and water use efficiency on different scales

Competition and complementarity related to biodiversity affect the physiological parameters controlling the water balance. Spatial differentiation in the rooting zone and hydraulic redistribution increase stand water availability and thus drought resistance of diverse stands. Therefore it can be hypothesized that resistance to soil drought and water use efficiency are greater in diverse than in species poor communities.

Resistance to Fungal Pathogens

The species number of foliar pathogenic fungi is positively, and the pathogen load per individual is negatively related to the number of host tree species, in particular for specialist species; therefore tree
diversity reduces the disease risk and pathogen load by transmission reduction.

Diversity and abundance of the understorey vegetation increases with increasing overstorey diversity in the canopy

It is assumed that the diversity of the understorey vegetation increases with increasing overstorey diversity in the canopy. The overstorey diversity effects on herb layer diversity and abundance are caused by a direct effect of canopy complexity and an indirect effect of the overstorey diversity on the humus layer dynamics; in turn, amore diverse and abundant understorey has a positive feedback on litter decomposition and nutrient cycling.

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