Vertical distribution and radial growth of coarse roots in pure and mixed stands of Fagus sylvatica and Picea abies

The vertical distribution of roots greater than 2 mm diameter was determined from digital images covering 116-m2 profile walls in soil pits in pure stands of European beech (Fagus sylvatica L.) and of Norway spruce (Picea abies (L.) Karst.) and in a mixed stand of both species. Radial root growth was assessed for roots greater than 5 mm diameter by growth ring analysis. Beech roots showed maximum density at the 10- to 20-cm depth, whereas the density of spruce roots decreased exponentially with increasing soil depth. Roots of both species reached the maximum excavation depth (1 m) in their monospecific stands. However, the root system of spruce was shallower in the mixture with beech, where large roots (diameter (d) >20 mm) were limited to the upper 10 cm. Beech roots reached the same rooting depth as in the pure stand but showed higher root densities in deeper soil layers. Neither root diameter nor root growth of any species was correlated with soil depth. Radial root growth of beech exceeded that of spruce significantly in both pure and mixed stands. Radial growth rate of beech roots further increased when mixed with spruce. The enhancement of beech root growth in the mixed stand suggests a higher belowground competitive ability of beech compared with spruce; as a result the spruce root system developed even shallower in the mixed versus in the pure stand.