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

1. Hydraulic redistribution (HR) of soil water through plant roots is
widely described, however its extent, especially in temperate trees,
remains unclear. Here, we quantified redistributed water of five temperate
tree species. We hypothesized that both, HR within a plant and into the
soil increases with higher water-potential gradients, larger root conduit
diameters and root-xylem hydraulic conductivities. 2. 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 to 4.20 MPa). Hydraulic
redistribution was quantified via deuterium labeling. 3. On average,
species redistributed 0.39 ± 0.14 ml water overnight (0.08 ± 0.01 ml g-1
root mass). Higher pre-dawn water-potential gradients, xylem hydraulic
conductivities and larger conduit diameters significantly increased HR.
Hydraulic conductivity had the greatest influence on HR, within the plants
(0.03 ± 0.01 ml g-1) and into the soil (0.06 ± 0.01 ml g-1). 4. 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.