Josephine Huet

UMR IMBE

PhD student / Oct 2023 to Oct 2028

Experimental disentangling of drought vs. warming effects on microbial activity and carbon dynamics in Mediterranean forest soils.

This doctoral project adopts an integrative, mechanistic framework combining in situ manipulations with controlled laboratory experiments to evaluate how climate change affects soil biodiversity and carbon dynamics in Mediterranean forest ecosystems. The research will focus on (1) litter‑decomposition processes, CO₂ and BVOC emissions driven by biological activity, and (2) soil‑carbon sequestration under the individual and combined influences of drought and warming.

Jeanne Poughon

UMR CEFE

PhD student / Oct 2024 to Oct 2027

Effects of drought on carbon assimilation and allocation in the living biomass of forest trees

This doctoral project aims to quantify how drought‑induced reductions in carbon assimilation alter the allocation of carbon among tree organs — including stems, roots, foliage and reproductive tissues. The research will leverage five long‑term precipitation‑manipulation experiments across contrasting forest ecosystems (three Mediterranean forests, one temperate forest and one eucalyptus plantation), complemented by eddy‑covariance carbon‑flux data from several ICOS sites to capture both experimental and interannual variability in water availability.

The work will combine the synthesis of existing datasets with the acquisition of new measurements using standardised and innovative protocols, with a strong focus on below‑ground processes, which remain insufficiently documented in forest ecology. The thesis will revolve around three core research themes: (i) linking photosynthetic carbon uptake to stem growth; (ii) assessing drought effects on tree architecture and above‑ground carbon allocation; and (iii) quantifying below‑ground carbon allocation and its sensitivity to water stress.

My Mission in 180s / Drought effects on carbon assimilation and allocation in living tree biomass in forests  

Philippine Dubertrand

UMR ESE

PhD student / Oct 2024 to Oct 2027

Quantifying the Influence of the Phosphorus Cycle on Growth and Resource Use in Tropical Eucalyptus Plantations: A Process‑Based Modelling Approach

The biogeochemical cycles of carbon (C), nitrogen (N), phosphorus (P) and potassium (K) are tightly interconnected in terrestrial ecosystems, with complex interactions and feedbacks constrained by plant and soil stoichiometry and influenced by climate. Net primary production is often limited or co‑limited by nutrients such as N, P or K, or by other macro‑ and micronutrients. To date, most Terrestrial Ecosystem Models (TEMs) only partially simulate the influence of nutrient limitation on forest productivity. Only a handful include representations of the nitrogen, phosphorus and/or potassium cycles, and are therefore potentially able to assess their effects on the forest carbon balance.

There is an urgent need to improve TEMs so that they integrate the coupled cycles of carbon, water, nitrogen, phosphorus and potassium, along with their interactions. This is a major challenge, as nutrient deficiencies affect numerous processes occurring in plants and soils across different spatial and temporal scales, and modify the cycles of carbon, water and other nutrients.

As part of this PhD project, we will develop a novel process‑based model coupling the carbon, water and mineral (N, P and K) cycles in forest stands: the CASTANEA‑MAESPA‑CNPK model. The model will include mechanistic representations of physiological processes in plants and biogeochemical processes in soils to maximise its generality and (i) the confidence in its application across climate and soil gradients, and (ii) its transferability for future adaptations to different forest types. Fine‑scale mechanistic representations can provide deeper fundamental understanding of how nutrient deficiencies affect forest functioning and the carbon cycle.

The model will be parameterised and validated using a network of Eucalyptus trials across Brazil, covering a wide range of N, P and/or K (co‑)limitations. We will use the model to evaluate the role of mineral nutrition (N, P and K) in determining productivity and resource‑use efficiency in tropical Eucalyptus plantations in Brazil. More specifically, we aim to identify the ecosystem processes involved in these nutrient cycles that limit biomass production in forests.

Model development will build on extensive datasets and previous modelling studies conducted on tropical Eucalyptus plantations, which serve here as a case study: these are simplified ecosystems (a single plant species growing on homogeneous soils) with substantial nutrient removal through harvesting, where the effects of ecosystem manipulations (e.g. rainfall exclusion, fertilisation) on biogeochemical cycles can be studied over a full rotation.