Experimentelle Untersuchung der Wasseraufnahme und der hydraulischen Eigenschaften des Wurzelsystems von sechs heimischen Baumarten

The aim of this study was to investigate the water uptake of the tree species sycamore (Acer pseudoplatanus), ash (Fraxinus excelsior), small-leaved lime (Tilia cordata), hornbeam (Carpinus betulus), pedunculate oak (Quercus robur) and beech (Fagus sylvatica). Miniature sap flow gauges were installed on 20 roots (3-4 mm in diameter) per species, recording the water uptake. After completion of the sap flow measurements the roots were excavated for the determination of the root surface area. The study took place in a mixed forest, composed of eleven tree species, and in an adjacent beech forest in the national park Hainich (Thuringia, Germany). Sycamore, ash and lime trees had the lowest sap flow densities. The flow densities of roots of hornbeam, pedunculate oak and beech were 1.5 to twofold higher. The surface area of the examined roots declined in the following order: hornbeam > pedunculate oak > beech > sycamore > ash > lime. The surface areas of roots of ash and lime trees were sixfold smaller than those of hornbeam. There was a linear relationship between root surface area and sap flow density. Thus the highest surface-specific water uptake rates were found for roots of ash trees. Roots of sycamore trees had similar surface-specific uptake rates. The lowest uptake rates were found in roots of beech and pedunculate oak which took up just a third of the amount of water per cm² root surface area compared to ash trees. The formation of a periderm as a secondary boundary layer was detected in fine and finest roots of all examined tree species. Already at the distance of 0.5 cm from the root tip a periderm build of up to four cell layers could be seen. The hydraulic conductivity of the roots of the lime trees was threefold higher than of the other five tree species. Beech and oak roots had the lowest hydraulic conducticities. There was no linear relationship with the sap flow density. During two desiccation periods with drying of the upper soil in the years 2000 and 2001 a different reaction of the six tree species could be observed. Ash and pedunculate oak retained high water uptake rates while sycamore, lime and beech roots showed a reduction.In a mixed forest of beech (Fagus sylvatica) and sessile oak (Quercus petraea) in the Lüneburger Heide (Lower Saxony, Germany) the water uptake rates of beech and oak in sandy soil was studied comparatively to the loamy soil in the Hainich. In the Lüneburger Heide a thick and intensely rooted organic layer could be found. The sap flow density and the surface area of the roots in this layer was twofold higher than in the mineral soil. The organic layer in the Hainich was a thin OL-layer and very sparsely rooted. Roots of beech and oak trees in the Hainich could just be found in the mineral soil and showed similar sap flow densities to roots of the organic layer in the Lüneburger Heide. In the nutrient rich and fine porous soil in the Hainich the root-strands had smaller surface areas than the roots in the Lüneburger Heide and thus up to sevenfold higher surface area specific water uptake rates.