High variation in hydraulic efficiency but not xylem safety between roots and branches in four temperate broad-leaved tree species

Xylem hydraulic safety and efficiency are key traits determining tree
fitness in a warmer and drier world. While numerous plant hydraulic
studies have focused on branches, our understanding of root hydraulic
functioning remains limited, although roots control water uptake,
influence stomatal regulation and have commonly been considered as the
most vulnerable organ along the hydraulic pathway. We investigated 11
traits related to xylem safety and efficiency along the hydraulic pathway
in four temperate broad-leaved tree species. Continuous vessel tapering
from coarse roots to stems and branches caused considerable reduction in
hydraulic efficiency. Wood density was always lowest in roots, but did not
decline linearly along the flow path. In contrast, xylem embolism
resistance (P50) did not differ significantly between roots and branches,
except for one species. The limited variation in xylem safety between
organs did not adequately reflect the corresponding reductions in vessel
diameter (by ~70%) and hydraulic efficiency (by ~85%). Although we did not
observe any trade-off between xylem safety and specific conductivity,
vessel diameter, vessel lumen fraction and wood density were related to
embolism resistance, both across and partly within organs. We conclude
that coarse roots are not highly vulnerable to xylem embolism as commonly
believed, indicating that hydraulic failure during soil drying might be
restricted to fine roots.