Hydraulic redistribution (HR) is the passive flux of water through plants along a water-potential (ψ) gradient in the soil. Under predicted more frequent dry periods in the current climate scenarios, some temperate species may improve their water balance via HR, however its extent, potentially depending on soil processes and specific plant physiological strategies, remains unclear. This contribution combines field observations and growth chamber experiments to trace, quantify and identify driving factors of HR among multiple temperate tree species under drought. In a split-root growth chamber study, driving factors of HR by diffuse porous (Acer pseudoplatanus, Fagus sylvatica), ring porous (Castanea sativa, Quercus robur) and conifer species (Picea abies, Pseudotsuga menziesii) were identified. Trees were planted with one individual (split-root plant) having its roots split between two pots with additional trees each. A range of ψ gradients was established between the pots and HR was observed by stable isotope labeling. Over one night, species redistributed 0.39 ± 0.14 ml water (equaling 8 ± 1% of total root water). Higher ψ gradients and larger conduit diameters significantly increased HR. Especially trees with high xylem hydraulic conductivity had high HR capacitance, predestinating them as valuable ‘silvicultural tools’ to improve plant water-status under drought. After one week one quarter of the water in the roots, stems and transpiration of the plants in the dry pots originated from the split-root plant, revealing HR as an essential water source for drought stressed trees. In a mixed F. sylvatica – P. abies stand in southern Germany (Kranzberg forest, kroof.wzw.tum.de) HR by 70-year old F. sylvatica trees was studied on six throughfall exclusion plots. Via plastic tubes labeled water was applied to deep (30-50 cm) soil layers and HR was traced by sampling of soil and plant tissue. In the dry topsoil, 4 ± 2% and 5 ± 3 % of the water in fine root branches and in their rhizosphere was HR water, respectively. The HR water was potentially available to neighboring P. abies trees. Concomitant sap flow measurements in coarse roots confirmed HR by F. sylvatica trees. With anticipated precipitation shifts in the future, HR could be an important strategy in the water dynamics of forest ecosystems facing greater ψ gradients because of increasing periods of water limitation.
