Renaud Decarsin's thesis defense

Summary 

The diversification of forests is a management strategy gaining popularity for maintaining the functionality of forest ecosystems in the context of global changes. Drought is a major disturbance linked to climate change, for which the effects of diversification remain controversial. My thesis aimed to quantify the effects of tree species mixtures on their physiological functioning under drought conditions at different scales (stand, individual trees). To address this objective, I relied on (1) the TreeDivNet network, which offers experimental plantations where tree diversity is manipulated, (2) various metrics of tree hydraulic functioning (e.g., water potential, vulnerability of the hydraulic system to cavitation, transpiration dynamics), and (3) a characterization of the local context in which the trees grew (e.g., leaf area indices, competition indices, soil properties). In the first chapter, I focused on quantifying an indicator of hydraulic stress related to the risk of tree mortality due to hydraulic failure during an extreme drought, at the scale of 20 tree species spread across 5 European sites in TreeDivNet. This hydraulic mortality risk was approximated by combining a measure of hydraulic vulnerability (referred to as cavitation vulnerability) and a measure of exposure to hydraulic stress (referred to as water potential) taken at the drought peak. The analysis of these data shows that species identity is the main determinant of mortality risk. Furthermore, while species richness has little effect on risk, species composition explains a significant part of it. Thus, certain complementary species assemblages have the potential to significantly reduce the hydraulic stress of at least one species in the mixture. In the second chapter, I focused on a single experimental site (BTree, Austria) to evaluate the effect of species mixtures on the dynamics of sap flux density responses during a drought. The results of this chapter provide evidence that the local environment, shaped by the effects of mixtures on tree growth, partly explains tree physiological responses to drought. The observation of converging sap flux density responses to soil water availability among species in two-species mixtures suggests an effect of mixing through neighborhood structure, water use strategies of neighboring trees, and sometimes soil water retention properties. In the third chapter, we reanalyzed the data from the first chapter (at the scale of 5 sites and 20 species) at the individual tree level to quantify the effects of species and neighborhood (approached through competition indices). Our results show that functional identity (approached through hydraulic traits) explains a large part of the hydraulic stress, but also that tree size and the functional identity of neighbors modulate hydraulic stress. Specifically, it seems that a neighborhood characterized by conservative strategies reduces hydraulic stress. In the general discussion, we address the limitations of these results and perspectives for further research, necessary for the development of tools to support forest management practices.

Members of jury :

Virginie Baldy             Professor University, IMBE Marseille    Reporter
Clément Stahl            Researcher, INRAe Nancy                         Reporter
Isabelle Maréchaux  Researcher, INRAe Montpellier               Reviewer
Xavier Morin              Director de research, CEFE Montpellier Reviewer
Marion Jourdan         Researcher, INRAe Nancy                         Reviewer
Guerric Le Maire       Researcher, Cirad Montpellier                 Thesis supervisor
Joannès Guillemot    Researcher, Cirad Montpellier                 Guest
Nicolas Martin           Researcher, INRAe Avignon                     Guest