Trees never rest: the multiple facets of hydraulic redistribution

Abstract

The upward movement of water due to transpiration stops when soil water potential ( s ) drops below leaf water potential ( L ). Under these circumstances, water can move in any direction in the plant-soil continuum through the passive conduits of roots and stems towards the lowest  s . This is generally termed as hydraulic redistribution (HR), but the positioning and orientation of the driving water potential gradient may vary. Any experimental method that can measure bi-directional and low flows in the sapwood of roots and stems will be suitable to detect HR. Using one approach for measuring sap flow (the heat field deformation technique, HFD) in several forest species and sites across Europe, we were able to provide evidence on different types of HR: vertical hydraulic redistribution (VHR), horizontal hydraulic redistribution (HHR), foliar uptake (FU) and tissue dehydration (TD). VHR is the vertical water movement through roots in response to water potential differences between deep and topsoil, either hydraulic lift or hydraulic descent. HHR is the lateral water movement through roots in response to horizontal water potential gradients, namely under localised irrigation. FU is the water movement from crown to soil through stems when the crown is wetted by foggy weather. TD is the downward movement of water in stems or roots from above-ground tree tissues to soil under prolonged drought or frost. Results from direct sap flow measurements indicated the vectoral and widespread nature of HR, a phenomenon of paramount importance for overall physiology and ecohydrology.

Rainfall interception and spatial variability of throughfall in spruce stand

“…Recent research utilizing data from Liz experimental catchment included studies on evapotranspiration (Pražák et al, 1996), cloud and fog water deposition (Eliáš et al, 1995), water regime affected by vegetation cover changes (Buchtele et al, 2006), hydraulic redistribution of water by roots (Nadezhdina et al, 2010), development and testing of macroscopic root water uptake model (Vogel et al, 2013), and predictions of heat fluxes in structured soil profile (Votrubová et al, 2012).…”

Global synthesis of forest cover effects on long-term water balance partitioning in large basins

“…We propose the hypothesis that the presence of forests in large basins is determinant for the occurrence of a given partitioning pattern, mainly because forests have a strong potential to modify the 10 components of the surface water balance. There is a variety of mechanisms through which forests can exert strong effects on the components of long-term water balance in river basins These factors include, but are not limited to: accumulation and redistribution of soil moisture by root systems (Nadezhdina et al, 2010;Nepstad et al, 1994;Lee et al, 2005;Bond et al, 2002), strong capacity for stomatal regulation related to the large cumulative surface area of leaves (Berry et al, 2010;Costa and Foley, 1997;Katul et al, 2012), physiological adaptations for water and light use efficiency (Nadezhdina et al, 2010), 15 landscape-scale energy balance effects and overall dynamics of E (Villegas et al, 2014), land-atmosphere interactions that enhance the capacity of river basins to store water as a natural "reservoir" (Salazar et al, 2017), activation of shallow convection through transpiration (Wright et al, 2017), below-canopy stability that restricts direct soil evaporation (Henao et al, Submitted), and soil moisture control via canopy effects on hydrological partitioning (Fleischbein et al, 2005). Collectively, these mechanisms define biophysical relations between the presence of forests and the dynamics of P , E and R, and imply a 20 strong potential of forests for modifying the long-term patterns of water balance partitioning.…”

Differences in transpiration of Norway spruce drought stressed trees and trees well supplied with water

“…In general, the relationship between SF and transpiration is linear, and can be used for the calculation of tree transpiration (Clausnitzer et al 2011). Sap flow and transpiration are closely correlated not only to meteorological factors, such as vapour pressure deficit and potential evapotranspiration (Dragoni et al 2009;Köhler et al 2010), but also to soil water content and physiological parameters (Nadezhdina at al. 2010;Dalsgaard et al 2011;Klein at al.…”