Losing half the conductive area hardly impacts the water status of mature trees

Lars Dietrich,Günter Hoch,Ansgar Kahmen,Christian Körner

doi:10.1038/s41598-018-33465-0

Abstract

The water status of transpiring tree crowns depends on a hydraulic continuum from the soil matrix around roots to the sub-stomatal cavity of leaves, with a multitude of hydraulic resistances along this path. Although the stem xylem path may not be the most critical of these resistances, it had been suggested that a >50% interruption of that path by drought-stress-induced embolization (air filling) of conduits is critical for tree survival. Here we show that cutting the sapwood of mature, 35 m tall trees in half hardly affects crown water status and transpiration. Counter expectation, this first adult tree sapwood interception experiment revealed that shoot water potential in the canopy (assessed by using a 45 m canopy crane) either remained unaffected (spruce) or became less negative (beech), associated with small reductions in leaf diffusive conductance for water vapour. We conclude that the stem xylem of these trees has a large overcapacity and the tree hydraulics debate requires a critical re-visitation.

Highlights

Water transport in trees is ensured by the passive movement of water through capillary pipes in the xylem called conduits
Since water always flows from a point of higher water potential to a point of lower water potential it moves through capillary conduits up the trunk across different resistances
The inter-fibril cohesion of water in transpiring cell walls in the leaf mesophyll is so strong that the cell wall cannot be dehydrated hydraulically, ensuring the capillary water continuum from root tips to the mesophyll surface of transpiring leaves

Summary

Introduction

Water transport in trees is ensured by the passive movement of water through capillary pipes in the xylem called conduits. Vessels are larger diameter conduits composed of a whole series of cells that lost intermittent cell walls, with individual vessels connected by both, pits and so-called perforation plates, that allow water to pass from one vessel to the [1] Depending on their anatomy, tracheids and vessels reveal clearly different hydraulic properties[2]. (2) Under drought, the capillary continuum from the soil to the root surface becomes interrupted and water potentials in the canopy can become very negative (more negative than −2 MPa, down to −6 MPa), despite minute flux Under such tension the conduits can fill with water vapour and air, causing hydraulic failure, which in turn is often assumed to cause tree mortality, a causality still lacking unambiguous evidential data in mature trees. From this point of view, it is rather unlikely that a 50% loss in stem conductivity can cause a critical branch water shortage when only around 1% of the conductive capacity would be required

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Conclusion