Understanding the combined effects of drought and heat stresses on tree mortality – HydrauLeaks

Droughts cause more dieback in forests when they are associated with strong heat waves, as during the summer of 2003 in France. However, the physiological mechanisms of the exacerbating effect of water stress by temperature on tree mortality remain to be elucidated. The main objective of this project is to explore a new mortality hypothesis, whereby xylem hydraulic failure is caused by an abrupt and uncontrolled increase in residual cuticular water loss (g_min) beyond a critical temperature Tp. Tp values from literature and preliminary results from a new mechanistic model (SurEau) justify this hypothesis.
Here, we will measure g_min and its temperature dependence for selected forest trees species spanning a large range of drought tolerance (including deciduous, evergreen, temperate and Mediterranean species). For two key forest species threatened by climate changes (Fagus sylvatica and Abies alba), we will explore the variability of g_min and Tp across provenances selected from contrasted origins in their natural distribution range by working both on provenance trails as well as on natural populations. g_min will also be compared for selected poplar genotypes differing for their drought tolerance. The phenotypic plasticity of this trait will be evaluated in a poplar genotype grown under contrasted controlled environments for watering, shading or temperature. To phenotype g_min under controlled temperature, air humidity and light conditions, we will develop and distribute across all the partners of the project a new tool (Drought-Box).
We hypothesize (H1) that species, genotypes and phenotypes more resistant to hot droughts display lower and more thermo-stable g_min and higher Tp values. The underlying physical mechanisms will be investigated on real and biomimetic cuticles. The chemical composition of leaf cuticles will be measured on relevant plant material from the above experiments to identify the key structural components associated with the variation of g_min across and within species. The contribution of these components in g_min and its thermo-stability will be evaluated in Arabidopsis mutants modified for their biosynthesis. We hypothesize (H2) that the thermostability of g_min is caused by the presence or proportion of specific molecules in the cuticle. Test experiments will be conducted ex-situ under controlled conditions to assess the impact of a hot drought on hydraulic failure and mortality to validate the predictions of the SurEau model. This will require the parameterization of an explicit 3D model of leaf temperature combining both leaf cooling by latent heat and leaf warming due to radiative heating. Finally, we will implement the SurEau model into ecosystem models of forest fluxes to predict the risk of tree mortality on French ICOS-Ecosystem sites and project the future risk of desiccation caused by hot droughts at stand scale under several climatic scenarios. A consortium of researchers associating ecophysiologists, biomolecularists, chemists, physicists and modelers will ensure the success of the project.
The main contribution of our project will consist in advancing our understanding of plant resistance to hot-drought episodes. We will propose new key physiological traits, new phenotyping tools and putative genes that will help breeder in their screening of genotypes better adapted to future climatic conditions. Other final products of the project will consist of a list of species and beech and fir provenances potentially better performing under hot-drought conditions. Finally, we will also improve ecosystem models used to assess climate change impacts on forest by including desiccation processes related to hot drought.