Water potential gradient, root conduit size and root xylem hydraulic conductivity determine the extent of hydraulic redistribution in temperate trees

Hydraulic redistribution (HR) of soil water through plant roots is widely described; however its extent, especially in temperate trees, remains unclear. Here, we quantified HR of five temperate tree species. We hypothesized that both, HR within a plant and into the soil increase with higher water-potential gradients, larger root conduit diameters and root-xylem hydraulic conductivities as HR driving factors. Saplings of conifer (Picea abies, Pseudotsuga menziesii), diffuse-porous (Acer pseudoplatanus) and ring-porous species (Castanea sativa, Quercus robur) were planted in split-root systems, where one plant had its roots split between two pots with different water-potential gradients (0.23-4.20 MPa). We quantified HR via deuterium labelling. Species redistributed 0.39 +/- 0.14 ml of water overnight (0.08 +/- 0.01 ml/g root mass). Higher pre-dawn water-potential gradients, hydraulic conductivities and larger conduits significantly increased HR quantity. Hydraulic conductivity was the most important driving factor on HR amounts, within the plants (0.03 +/- 0.01 ml/g) and into the soil (0.06 +/- 0.01 ml/g). Additional factors as soil-root contact should be considered, especially when calculating water transfer into the soil. Nevertheless, trees maintaining high-xylem hydraulic conductivity showed higher HR amounts, potentially making them valuable 'silvicultural tools' to improve plant water status. A free Plain Language Summary can be found within the Supporting Information of this article.