Wasseraufnahme und artspezifische hydraulische Eigenschaften der Feinwurzeln von Buche, Eiche und Fichte: In situ-Messungen an Altbäumen

Abstract

1. In this study, the application of miniature sap flow gauges to 3-4 mm roots and subsequent determination of absorbing root area allowed measurement of surface-specific water uptake rates of tree roots under in situ conditions. With precise calibration and a modified calculation method even the small sap flow rates in fine roots of central European tree species could be determined. 2. Root and stem sap flow rates are tightly coupled in their daily time course. Thus, water uptake and transpiration are strongly related in the SPAC. Simultaneous measurement on branches, stem and roots of a well watered beech resulted in a time lag of only 5 min between each compartment. 3. Water uptake via different roots of one tree varied widely (variation coefficient: 50-100%). Daily maximum of root sap flow was about 2-10 g h-1, leading to daily totals of 500-1000 g m-2d-1 in surface-specific water uptake. 4. A comparison of root water uptake in three beech stands with different water regime revealed slightly higher surface-specific uptake rates for the driest stand. Here, water was taken from deeper soil horizons. Two beech roots at this site were extremely short and showed water uptake rates up to 7000 g m-2d-1. The relevance of such "high capacity roots" for the water balance of the stand could not be finally clarified due to the lack of additional examples of such roots. 5. Anatomical analysis showed that fine roots of beech, oak, and spruce from the Lüneburger Heide site have a completely differentiated periderm just behind the mycorrhizal root tip. The primary state of root development could not be found in any of the the investigated roots. Direct comparison of periderm thickness and suberin content yielded significant differences among the species in the order oak > beech > spruce. 6. In contrast to great anatomical differences, in situ water uptake rates were in the same range for all 3 tree species at the Lüneburger Heide site. Radial hydraulic conductivities calculated from surface-specific flow rates and root-soil potential differences were remarkably similar to results from laboratory experiments with the root pressure probe, although the results are based on different methods and environmental conditions. 7. Surface-specific water uptake rates in the organic soil layer were distinctly higher than in the mineral soil for all three species. In particular during periods with a rapid decrease in water potential in the coarse sandy soil of the Lüneburger Heide site, the organic layer plays an important role in the water balance of the trees. 8. In field experiments, root radial hydraulic resistance was eliminated by decapitation. This procedure always resulted in a sharp increase of flow rate, leading to the conclusion that root radial resistance is much higher than axial hydraulic resistance inside the xylem. 9. Irrigation experiments in a dry period at the Lüneburger Heide site always resulted in a sharp increase of root sap flow. Thus, irreversible damage of fine roots due to drought is unlikely. Obviously, embolism in the root xylem, if it occurs, can be repaired very quickly. 10. As a conclusion, the combination of sap flow measurement on tree fine roots and subsequent determination of absorbing root area or biomass, respectively, offers a great potential for further investigation of water flow in tree root systems under in situ conditions.