Description du sujet de thèse

1. Context
Climate change has already dramatically altered the composition and structure of terrestrial ecosystems, accelerating biodiversity loss and degradation of ecosystem functioning in many habitats worldwide (Peñuelas et al. 2013; Brose and Hillebrand 2016). These changes will continue to affect biodiversity, functioning and ecosystem services such as the capacity of forest ecosystems to store carbon (regulating services) or provide biomass and fuel (provisioning services) (Sala et al. 2000; Doblas-Miranda et al. 2015). While the impact of changes in precipitation amounts and patterns (e.g. increased summer drought) on forest ecosystems has focused intense research effort, multiple climate components could change simultaneously (IPCC 2021), impacting diversity and associated ecosystem processes in complex ways (Peñuelas et al. 2013).
More accurate prediction of the impact of climate change on biodiversity and forest ecosystem functioning requires consideration of the different components of climate change, which can act simultaneously (i.e. synergistically) or in opposition (i.e. antagonistically) (Beaumont et al. 2011; Liu et al. 2017). As water availability is a limiting factor in the functioning of forest ecosystems, the impact of decreasing precipitation has often been tested in isolation without taking into account the simultaneous effect of increasing temperatures. Some studies have reported that increasing temperature and decreasing precipitation could impose similar and additive selection pressures as both components of climate change decrease soil moisture (Williams et al. 2013). However, the combined effects of temperature and precipitation are highly dependent on the climatic context. For example, rising temperatures can increase the negative effect of reduced precipitation in an already dry habitat (i.e. during the summer period). However, when water availability is not limiting, rising temperatures can have positive effects on ecosystem functioning by stimulating biological activity (i.e. during winter and spring). In this context, it seems necessary to distinguish between the influences of the two components of climate change (decreasing precipitation and rising temperatures) and their combined effects on the functioning of forest ecosystems.
Research on climate change has long focused on the most visible above-ground processes of ecosystems, such as their primary production (Hooper et al. 2005). More recently, the response of underground ecosystem processes has also emerged as a major theme in ecology (Wardle et al. 2004; Bardgett and Wardle 2010). Indeed, given that over 50% of the carbon assimilated by primary producers re-enters soils via litter decomposition (Wardle et al. 2004), this process and soil biodiversity have naturally attracted the attention of the scientific community. Decomposers play a key role in regulating the carbon and nutrient cycles (Vitousek 1984; Gessner et al. 2010), and decomposition represents one of the main sources of carbon release into the atmosphere (Davidson and Janssens 2006). Heterotrophic soil respiration associated with organic matter decomposition is a major determinant of forest carbon balance (Valentini et al. 2000), and is mainly controlled by temperature and precipitation (Davidson et al. 2006). As current climate change in Europe is simultaneously warming and drying the soil (IPCC 2021), these two variations in climatic components are expected to have antagonistic effects on organic matter decomposition. Consequently, it remains to be predicted which of these two effects will have the greatest impact.
Consequently, it remains to be predicted which of these two effects has the predominant impact on soil organic carbon dynamics, and also whether warming and drying have additive or interactive effects (Sierra et al. 2015).

2. Experimental sites
This PhD project will focus on the Mediterranean region, where the impact of ongoing climate change is already observable on the functioning of forest ecosystems (Allen et al. 2010; Sardans and Peñuelas 2013). The project will focus on two forest ecosystems of major importance in the North of the Mediterranean basin, namely pubescent oak (Quercus pubescens) and holm oak (Quercus ilex) forests, which represent more than 4 million hectares across the Mediterranean basin, including 750,000 ha in France.
The PhD project will be based on two instrumented and experimental in situ platforms of the AnaEE-France national network, aimed at studying the dynamics, functioning and biodiversity of two Mediterranean forest ecosystems subjected to climate change: the Puéchabon experimental site (Quercus ilex forest) and the O3HP experimental site (Quercus pubescens forest). These experimental research facilities have already shed light on numerous changes in the functioning of forest ecosystems subjected to aggravated water stress on tree function (e.g. evapotranspiration, biomass production, CO2 fixation, Limousin et al. 2009; Gavinet et al. 2019; Laoué et al. 2024) or the litter decomposition process (e.g. Santonja et al. 2015, 2017). The simultaneous use of these two platforms will enable 1) the study and comparison of evergreen and deciduous oak forests, and 2) access to knowledge on the multi-year impact of recurrent increases in the duration and intensity of summer drought episodes on the soil compartment.
These two platforms were supplemented in April 2025 by new experimental setups to simulate soil warming and reduced rainfall, conceptually developed as part of the MEDSOCLIM project (AnaEE-France, 2021-2024) and installed on site as part of the Drought ForC project (PEPR FairCarbon, 2023-2028). The experimental set-up comprises 4 experimental treatments (control, rain exclusion, soil warming, rain exclusion + soil warming) replicated three times (i.e. 3 replication blocks) for a total of 12 plots on each of the two experimental platforms. It is designed to be installed in forest undergrowth and to manipulate soil temperature over the top 10 cm, as well as rainfall. This device will enable us to study the combined and isolated effects of increasing soil temperatures and decreasing rainfall on soil functioning in two typical Mediterranean forests.

3. Objectives of the PhD project
Using an integrative, mechanistic approach combining experiments in natura and under controlled conditions, this PhD project aims to assess the impacts of climate change in the Mediterranean region on soil biodiversity and associated C dynamics in Mediterranean forest soils. More specifically, the main objectives will be to study 1) litter decomposition, CO2 and BVOC emissions linked to biological activity, and 2) soil carbon sequestration in response to the isolated and interactive effects of drought and warming in new experimental set-ups.


The PhD project will be organized around 3 complementary axes:


▪ Axis 1: In situ decoupling of the effects of the two climate change components on the litter decomposition process.

A litter decomposition experiment will be carried out using the litterbag method (Swift 1979) with the aim of studying the decomposition of organic matter and the release of nutrients (N, P, K, Ca, Mg, Na, S) in response to changes in climatic conditions. Biogeochemical models focus primarily on the role of microorganisms, ignoring the key importance of soil fauna (Abramoff et al. 2017). As microbial populations and activities are largely dependent on soil fauna, any change in the structure of the soil food web will have significant consequences for the efficiency of the decomposition process and for carbon and nutrient dynamics (Milton and Kaspari 2007). We are therefore interested in the relative contribution of soil microorganisms and soil fauna to the litter decomposition process. Using litter traps, the leaves of two oak species undergoing abscission will be collected at peak annual litterfall. Leaf litter decomposition will be studied over 2 years (two samplings per year, i.e. before and after the summer drought period) with 10 grams of litter initially placed in each litterbag (20 cm x 20 cm). Different mesh sizes will be used to study the impact of experimental treatments on the various soil organisms contributing to the decomposition process.

▪ Axis 2: In situ decoupling of the effects of the two components of climate change on CO2 and BVOC emissions, and on carbon sequestration.
Soil respiration will be measured using a combination of manual and automatic soil chambers to ensure good spatial and temporal coverage of measurements at both study sites. Manual measurements will be carried out using an EGM-5 portable CO2 gas analyzer. Automatic measurements using Licor-8100 systems will be carried out at a high frequency (several times a day) to monitor dynamic changes in soil respiration in response to diurnal temperature fluctuations. Simultaneous measurements of soil respiration using manual and automatic chambers will enable inter-calibration between systems and sites. Respired CO2 fluxes will then be analyzed as a function of soil temperature and water content. A soil respiration model (Misson et al. 2010) will then be fitted to the experimental data to distinguish the relative influences of soil moisture and temperature between seasons and experimental treatments.
Soil emissions of BVOCs will be collected in the various experimental plots at the two forestry sites during 2 field campaigns in the summer of 2026 (1 campaign per site), in order to quantify their contribution to the ecosystem's C balance and assess the impact of drought and warming on their emission rates. BVOC measurements will be carried out in real time using ground-based chambers connected to a PTR-Tof-MS. These measurements will be carried out in collaboration with the Laboratoire Chimie Environnement (LCE), using the MASSALYA mobile platform dedicated to the continuous analysis of VOCs. Quantification of the C content of the soil's organic, organo-mineral and mineral horizons in order to estimate soil C stocks was carried out on the various experimental plots in April 2025. As the C content of the soil's organic horizon can react more rapidly to altered climatic conditions (Santonja et al. 2022), three samplings of this horizon will be carried out during the course of the PhD.

▪ Axis 3: In vitro decoupling of the effects of the two components of climate change on the soil food web and cascading effects on carbon dynamics.

An experiment under controlled conditions in a climate chamber will be developed to test the effects of 4 climatic treatments (optimal temperature and humidity vs. thermal and water stress, vs. optimal temperature and water stress, vs. thermal stress and optimal humidity) on the biomass and activity of species in a model community composed of 4 trophic levels (primary producer = leaf litter, consumer = microorganisms, secondary consumer = fungivorous springtail, predator = gamasid mite, super predator = myriapod). This experiment will provide a more mechanistic approach to interactions within the soil food web and the impact of climate change stresses on the food web's contribution to soil carbon fluxes

Contexte de travail

The PhD will be funded by the national program PEPR FairCarboN Drought ForC and will be integrated to the Mediterranean Institute of Biodiversity and Ecology (IMBE) of the Aix-Marseille University, France, and will be supervised by Virginie Baldy Virginie Baldy (Pr, Aix-Marseille Université), Elena Ormeño (Research director, CNRS) et Mathieu Santonja (Assistant Pr, Aix-Marseille Université).
The PhD student will carry out laboratory and field experiments on various French forest sites. He/she will benefit from the scientific and technical environment of the PEPR Drought ForC project consortium to set up his/her experiments (e.g. collaborations, technical assistance, experimental infrastructures). The PhD student will also have the opportunity to supervise the research work of at least one Master 2 trainee.

Contraintes et risques

MSc degree in ecology, biology or environmental sciences
- Strong interests in functional ecology, soil ecology and in situ experimental approach
- Motivation vis à vis des approches expérimentales in situ
- Skills in R software
- Driving licence