Bertille LOISEAU thesis defense

Abstract

Forests cover almost one-third of the Earth's land surface and play a central role in the carbon, water and energy cycles. However, their future is uncertain with climate change, which is exacerbating drought problems by increasing their frequency and intensity in many regions of the world, particularly in the Mediterranean region. Soil water availability and its accessibility to trees are key factors regulating transpiration, biomass production and plant species distribution in ecosystems. Characterizing soil water resources and the dynamics of water uptake by trees is essential to improve our understanding of forest ecosystem functioning. The general aim of this thesis is to combine geophysics with ecophysiological methods in order to better quantify the dynamics of water uptake in forests.
The first chapter of this thesis aims to provide an overview on the use of geophysics in forest contexts and its contributions. The second chapter consists of the development of a methodology for processing electrical resistivity tomography (ERT) data to better quantify soil water fluxes. The developed method enables to convert ERT data into soil water content using a merging approach. The third chapter consists of studying the soil water dynamics in relation to vegetation. To achieve this, ERT measurements were carried out to monitor soil water dynamics combined with sap flow monitoring to observe tree transpiration dynamics. This work was carried out on two Mediterranean forest sites in France: the Larzac observatory and the Font-Blanche forest. An additional non-forestry site was used to validate the proposed method in an agricultural context: the “lysimeter” plot at INRAE in Avignon.
This thesis highlights the potential of geophysical methods for enhance forest ecology studies and their complementarity with traditional approaches. The results underline the ERT effectiveness for monitoring soil water dynamics in forest subsoils, which are often heterogeneous. Thanks to the developed EA-ERT method, quantitative water content were obtained with relatively low errors compared to data measured with moisture probes. Comparison of water stock variations derived from ERT with measured transpiration or modeled evapotranspiration through a water balance approach, demonstrated ERT's ability to capture the temporal and spatial heterogeneity of evapotranspired fluxes. The multidisciplinary approach of this thesis, which combines hydrogeophysics and ecohydrology, opens the way to better quantify water flows and, ultimately, to develop tools for their spatialization. This work will contribute to better understand forest functioning and evolution, and in particular the impact of droughts on these ecosystems.

Jury members :

• Konstantinos Chalikakis – PR, Avignon Université, UMR EMMAH – Rapporteur
• Lionel Jarlan – DR, IRD, UMR CESBIO – Rapporteur
• Nolwenn Lesparre – CR, CNRS, UMR ITES – Examinatrice
• Isabelle Maréchaux – CR, INRAE, UMR AMAP – Examinatrice
• Agnès Ducharne – PR, Sorbonne Université – Examinatrice
• Vazken Andréassian – IG, INRAE, UMR HYCAR – Examinateur
• Simon Carrière – MC, Sorbonne Université, UMR METIS – Directeur de thèse
• Damien Jougnot – DR, CNRS, UMR METIS – Co-directeur de thèse
• Nicolas Martin – CR, INRAE, URFM – Co-encadrant – Invité