Abstract
Abstract 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. A free Plain Language Summary can be found within the Supporting Information of this article.
Highlights
Gravitation and frictional forces increasingly constrain water flow and the maintenance of intact water columns in the xylem of tall trees with increasing height, which causes a decline in xylem water potential from roots to leaves (Fulton et al, 2014; Woodruff & Meinzer, 2011)
The continuous vessel tapering paradigm has not been confirmed for species from tropical environments (Kotowska et al, 2015; Schuldt et al, 2013), measurements conducted in temperate trees and in woody plants from arid environments indicate that the largest xylem vessels are located in roots (Domec et al, 2009; Lintunen & Kalliokoski, 2010; Martínez-Vilalta et al, 2002)
The way this trait varies among trees and among organs within a single tree is Highlight vessel tapering resulted in a significant reduction in xylem hydraulic efficiency along the flow path in four temperate deciduous tree species, the resistance to xylem embolism did not vary
Summary
Gravitation and frictional forces increasingly constrain water flow and the maintenance of intact water columns in the xylem of tall trees with increasing height, which causes a decline in xylem water potential from roots to leaves (Fulton et al, 2014; Woodruff & Meinzer, 2011). Thirty years ago, Sperry and Tyree (1988) argued that the mechanism of embolism formation might not be directly related to conduit size but to pit properties In this regard, pit membrane thickness was recently identified as the key determinant of embolism resistance across a broad range of woody angiosperm species (Kaack et al, 2021; Li et al, 2016). Recent observations indicate that pit membrane thickness is comparable between large roots and branches in two Acer species, or even thicker in roots in the case of Corylus avellana (Kotowska et al, 2020; Wu et al, 2020) Overall, it remains unclear whether roots are less resistant to embolism than stems or branches, and how this matches the variability in wood anatomical features along the hydraulic pathway. We hypothesized that (a) vessel diameter and xylem hydraulic efficiency decreases continuously along the hydraulic pathway from roots to branches to compensate for gravitational and frictional constraints with height, but (b) roots are not necessarily more vulnerable to embolism than branches due to a weak or no relationship between xylem efficiency and safety
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