A new mechanism for tree mortality due to drought and heatwaves

Hervé Cochard

doi:10.24072/pcjournal.45

Abstract

Abstract Plants tend to die earlier in hot and drought conditions, but the underlying mechanisms are not yet understood. I propose here a new mechanism by which excessive residual water losses caused by high cuticular permeabilities and a high leaf-to-air vapor pressure deficits would trigger uncontrolled and sudden cavitation events. The combination of heat and drought stresses may therefore lead to an unsuspected risk of hydraulic failure. I explored this hypothesis with a new mechanistic model. The simulations support this hypothesis and highlight the critical role played by the cuticle phase transition temperature. Experiments are now awaited to confirm these predictions.

Highlights

In recent years, cases of widespread forest mortality have been recorded worldwide (Allen et al, 2010)
Drought-induced tree mortality is tightly associated with the risk of hydraulic failure, whereby excessive tensions in the xylem tissue provoke cavitation events that block water transport from the roots to the leaves
By integrating a new model of leaf cuticular conductance incorporating its bi-phasic temperature dependence, I propose here a new hypothesis to explain the striking increase in tree mortality under both hot and dry conditions

Summary

Introduction

Cases of widespread forest mortality have been recorded worldwide (Allen et al, 2010). These die-offs seem mostly triggered by extreme drought and heat stresses (Anderreg et al, 2016, Adams et al, 2017a, Williams et al, 2013). Current models predict an increased risk of mortality due to climate warming (McDowell & Allen 2015). The exact physiological mechanisms responsible for tree mortality under these extreme events remain largely unknown, which hinders our ability to model and predict the risk of forest dieback and changes in species distribution ranges. Drought-induced tree mortality is tightly associated with the risk of hydraulic failure, whereby excessive tensions in the xylem tissue provoke cavitation events that block water transport from the roots to the leaves. Most species live on the verge of hydraulic failure (Choat et al, 2012), trees seem remarkably protected against xylem dysfunctions during drought events (Cochard & Delzon 2013) suggesting that drought-induced hydraulic failure may occur only under extreme and peculiar climatic conditions

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