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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.

Susceptibility of crops to drought may change under atmospheric CO 2 enrichment. We tested the effects of CO 2 enrichment and drought on the older malting barley cultivar Golden Promise (GP) and the recent variety Bambina (BA). Hypothesizing that CO 2 enrichment mitigates the adverse effects of drought and that GP shows a stronger response to CO 2 enrichment than BA, plants of both cultivars were grown in climate chambers. Optimal and reduced watering levels and two CO 2 concentrations (380 and 550 ppm) were used to investigate photosynthetic parameters, growth and yield. In contrast to expectations, CO 2 increased total plant biomass by 34 % in the modern cultivar while the growth stimulation was not significant in GP. As a reaction to drought, BA showed reduced biomass under elevated CO 2 , which was not seen in GP. Grain yield and harvest index (HI) were negatively influenced by drought and increased by CO 2 enrichment. BA formed higher grain yield and had higher water-use efficiency of grain yield and HI compared to GP. CO 2 fertilization compensated for the negative effect of drought on grain yield and HI, especially in GP. Stomatal conductance proved to be the gas exchange parameter most sensitive to drought. Photosynthetic rate of BA showed more pronounced reaction to drought compared to GP. Overall, BA turned out to respond more intense to changes in water supply and CO 2 enrichment than the older GP.