Doctoral candidate : Pierre-Antoine Chuste
University : Université de Lorraine, France
Contract duration : 2014-2017

Research topic — Study of the sensitivity of Lorraine beech populations to drought and defoliation – What roles of carbon and nitrogen metabolisms in the death of trees?

Research team and supervising scientists —
Equipe Arbeco, UMR INRA- UL (1137) Ecologie et Ecophysiologie forestières (EEF).
PhD supervisor: Pascale Maillard / PhD co-supervisor: Catherine Massonnet

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Context and state of the art —Due to climate changes, forests will increasingly be subjected to serious adverse events such as extreme droughts which represent a major threat to the survival of trees (Allen et al, 2010, Breda et al, 2006). However, physiological mechanisms causing the death of trees due to drought are to date still poorly understood. This question is particularly relevant today and two opposite theories are currently being debated in the scientific community: do trees die due to thirst or hunger?  (McDowell et al, 2008, 2011; Sala et al, 2010). A better understanding of the physiological mechanisms involved in the death of the trees would certainly improve current climate scenario models for species predictions under future climate conditions (Piedallu et al., 2009).

Objectives and specific questions to be adressed — Our project seeks to fuel the debate about the C theory and to provide new information about the physiological mechanisms implied in the cascade of events leading to the death of trees. Specifically, our goal is to analyze the morphological and physiological changes induced by a drastic shortage of carbon (C) and nitrogen (N) and to assess the consequences in terms of survival of trees. These C and N gaps will be created artificially by making several successive experimental defoliations or generating a water deficit (limiting photosynthesis) by the mean of a rain exclusion system. How patterns of allocation of C and N will be modified by water and nutritional constraints and will create high mortality of beech planted in nursery will be investigated (INRA, Champenoux, F-54280).

The second goal of our project is to identify markers of mortality by comparing live and dead trees of different populations of beech and to quantify levels of C or N resource levels below which tree death will be observed. The corollary to this objective will be the possible identification of morphological or physiological traits that confer increased resistance to trees and / or tree populations.

This work will contribute to the collective effort realized in the context of the labex ARBRE Mepib-Death project by the EEF team.

Hypotheses to be tested:

1. 1. Population effect: Beech populations from Lorraine region show differences in season vegetation variables and these characteristics modify their way to withstand drought or to defoliations. Strong constraints will accelerate  ageing of some trees (possibly premature development of reproduction).
2. 2. Mortality and its causes: Beech can die due to a strong disturbance of its C or N metabolisms.

• Dead trees are those who have exhausted their C and or N reserves.

• Water and carbon constraints will result in changes in the allocation of C and N between organs and tissues eg modify tree allometric relationships (sapwood / leaf area).

• Dying trees have a growth strategy that predisposes more to dieback than trees full of vitality.

Science and innovation issues — This study focuses on beech (Fagus sylvatica L.), a major deciduous tree species in Europe that occupies an important rank in the timber production of France with 23% of timber hardwood and 10% softwood total work. Lorraine is the first region of harvest of beech in France. Planted on  176,000 ha and representing 20% of Lorraine woodlands, beech plays an important socio-economic role in the region. Furthermore, in the context of Pacte Lorraine (2014-2016), the region participates to the optimization of beech wood resources and to the development of a label “Hêtre Lorrain”.A better understanding of the risk of loss of productivity and mortality of beech species under future climate thus presents an issue of importance to the region and more broadly for the wood industry in France.

On an ecological and economical point of view, a better understanding of differences in vulnerability of beech populations from Lorraine to extreme climatic constraints would help to predict better the sensitivity of this species to future climate and therefore better anticipate  geographic distribution of risks for hardwood industry.

Methodological approaches and expected results — The vulnerability of beech trees will be assessed by analyzing and comparing the variability of physiological survival thresholds between 14 beech populations from forests distributed along a latitudinal gradient in Lorraine and planted in a common garden. The experimental device used for this study has been set up in the nursery of INRA Champenoux in 2007, and is constituted of more than 1,000 trees issued from seed. The proposed project aims at inducing mortality by manipulating C and water status of trees. To achieve, all the trees of the device were placed under exclusion of rain in Spring 2014. 2/3 of these trees will be irrigated and half of them will be subjected to a severe defoliation in 2014 and 2015 suceptible to disrupt C and N metabolisms. The goal will be to identify what are the physiological properties that allow survival of some trees and to estimate if these properties are population-dependent.

Hypothesis 1 will be tested by observations of phenology, growth measures and architectural properties of trees carried out in different populations and in different treatments. Hypothesis 2 will be assessed by quantifying, through elemental and biochemical analyzes, the distribution of C and N between the different compartments of trees sampled at the end of the growing seasons. Allometric relationships will also be quantified. A comparison of the radial growth of alive and dead trees will validate or not the hypothesis of growth differences. The candidate will have to assess whether beeches sampled at the end of the growing seasons have survival rates varying with the type of population and to estimate significance of effects of inter-population and intra-population on beech survival.