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.
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.
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.
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.
