Practical informations :
The PhD thesis is funded for a period of 3 years (DroughtForC project, PEPR FairCarbon).
The PhD can start from October 2023. The position will remain open until filled.
PhD supervisors :
Nicolas Delpierre (PR UPSaclay, laboratoire ESE), nicolas.delpierre@universite-paris-saclay.fr
Guerric Le Maire (DR CIRAD, laboratoire Eco&Sols), guerric.le_maire@cirad.fr
Hosting labs :
Ecologie, Systématique, Evolution (UMR 8079)
Bâtiment 680, IDEEV
12, route 128
Université Paris-Saclay
91190 Gif-sur-Yvette – France
CIRAD, UMR Eco&Sols
(CIRAD, IRD, INRAE, Institut Agro)
Institut Agro – Bât 12
2 place Viala
34060 Montpellier Cedex 2
Summary
The biogeochemical cycles of carbon (C), nitrogen, (N), phosphorus (P) and potassium (K) are strongly
linked in terrestrial ecosystems, with complex interactions and feedbacks, constrained by
stoichiometry in the plant and in the soil, and influenced by the climate. Net primary production is
frequently limited or co-limited by nutrients such as N, P or K, or by other macro- and micro-nutrients.
The widespread nutrient limitation of forest productivity calls for the incorporation of the cycles of
nutrients that limit the sequestration of carbon in plant biomass and soil in process-based Terrestrial
Ecosystem Models (TEMs). To date, most TEMs do not simulate the influence of nutrient limitation on
forest productivity. Only a handful incorporate coarse representations of the N, P and/or K cycles and
are thus potentially able to evaluate their feedback on the forests C balance. There is an urgent need
to improve the TEMs for integrating the cycles of C, water, N, P, and K and their interactions. This is a
challenge, as deficiencies of these nutrients impact numerous processes occurring in the plant and in
the soil at different time and space scales, and modify the carbon, water, and other nutrient cycles.
In this PhD, we will develop an unprecedented process-based model, coupling representations of the
carbon, water and mineral (N, P, and K) cycles in forest stands (the CASTANEA-MAESPA-CNPK model).
The model will encapsulate mechanistic representations of the physiological processes in the plant and
biogeochemical processes in the soil to maximize the genericity of the model and (i) the confidence in
its application under climate and soil gradients and (ii) its transferability for future adaptations to
different forest types. Fine-scaled mechanistic representations can give access to a better fundamental
understanding of the impacts nutrient deficiencies have on forest functioning and carbon cycling. The
model will be parameterized and validated on an ensemble of Eucalypt trials located across Brazil, with
a large range of N, P and/or K (co)-limitations.
We will use the model to evaluate the role of mineral nutrition (considering N, P and K) in determining
the production and resource-use efficiencies of Brazilian tropical Eucalypt plantations. More
specifically, we aim to identify which of the processes involved in the cycles of these nutrients within
the ecosystem limit the production of biomass in forests. The model development, will rely on large
datasets and past modelling studies conducted on tropical Eucalypt forest plantations, which are
considered here as a “forest model” study case : they are simplified ecosystems (only one plant species
growing on homogeneous soils) with high nutrient removals through harvests, where the effects of
ecosystem manipulations (e.g. rainfall exclusion and fertilizations) on the biogeochemical cycles can
be studied over an entire rotation.
Introduction
The sustainability of the forest net carbon sink is a hot topic. On the one hand, increased temperatures
and a more frequent occurrence of extreme climatic events (heatwaves and drought; IPCC, 2021) are
likely to reduce forest productivity and threaten the viability of vulnerable forest stands (Hartmann et
al. 2022). On the other hand, the enhancement of productivity under elevated CO2 may be
compromised by progressive nutrient limitation, as demonstrated in free-air CO2 enrichment
experiments in temperate forests where nitrogen (N) limitation has been evidenced (Norby et al.,
2010). In response to unbalanced N and phosphorus (P) deposition and given the crucial role of P in
plant development, northern ecosystems (with young soils characterized by a relatively high
availability of P and other rock-derived nutrients such as potassium (K); Vitousek et al., 2010) are
gradually moving from N to P limitation (Peñuelas et al., 2012; Jonard et al., 2015) and K limitation
(Manning, 2010; Sardans & Peñuelas, 2015). A situation that is already widespread in tropical
ecosystems with ancient, highly-weathered and P- and K-poor soils (Vitousek et al., 2010; Manning,
2010; Cornut et al. 2021). Other nutrients such as magnesium (Mg), and calcium (Ca) could also
become limiting in the future or are already limiting now in nutrient-poor ecosystems (Laclau et al.,
2010). Recent studies show that multi-element deficiencies are probably much more frequent in forest
ecosystems than commonly admitted (Townsend et al. 2011). In order to avoid overestimations of the
C sequestration capacity of forests, it is therefore crucial that terrestrial ecosystem models (TEMs)
account for the effects of nutrient limitation on the forest response to global changes (Norby et al.,
2010; Peñuelas et al., 2013; Sardans & Peñuelas, 2015).
To date, most TEMs still do not simulate the influence of nutrient limitation on forest productivity, and
only a handful incorporate (coarse) representations of the N and P cycles (Wang et al. 2010; Goll et al.
2012; Yang et al. 2013) or the K cycle (Cornut et al. 2023a, b) and are thus potentially able to evaluate
their feedback on forests C balance. None of these models has been thoroughly validated due to the
scarcity of data on the cycles of minerals in forest ecosystems. Hence there is a clear and urgent need
to develop and carefully evaluate process-based models coupling C, water and mineral cycles in forest
ecosystems.
Tropical forest plantations managed in short rotations are interesting study cases for the development
and evaluation of such process-based models, since they are simplified ecosystems (only one plant
species growing on homogeneous soils) with high nutrient removals, and the effects of ecosystem
manipulations and fertilizations on the biogeochemical cycles can be studied over a complete rotation(Laclau et al., 2010). Moreover, these plantations represent significant area worldwide with ~20MHa,
and in particular in Brazil with ~5 MHa, where the planted area is continuously increasing since 15
years and predicted to double by 2020. The sustainability of these highly productive plantations is
however questioned because they require important fertilization inputs and they may be vulnerable
to the increase in the frequency and intensity of drought periods expected under climate change. The
interaction between water, carbon and nutrient cycles is critical in this system: for instance, reducing
the fertilization may limit the water loss and maintain wood production in a dryer climate, thus
reducing the risk of dieback (Battie-Laclau et al. 2014, Christina et al. 2015).
Research objectives
The general objective of the present project is to develop an unprecedented model, coupling process-
based representations of the carbon, water and nutrient (N, P, K) cycles in forest stands. Indeed, we
postulate that interactions among three target nutrients (N, P and K) and the C and water cycles are
key to understand the dynamics and intensity of growth in forests.
Hence our fundamental working hypothesis is that a representation of the combined impacts of the
target nutrients (NPK) on elementary C and water processes (e.g. photosynthesis, respiration, C
allocation, stomatal conductance, soil processes) will provide a powerful, unprecedented tool for
identifying the synergistic and antagonistic influences of N, P and K on forest growth. The model will
be developed based on the case-study of tropical Eucalypt plantations, which offers a unique
combination of a simplified forest ecosystem, numerous ecophysiological measurements, and clearly
visible effects of N, P and/or K deficit on tree growth in different pedoclimatic conditions. The model
development targets a broader perspective than Eucalyptus plantations. Indeed, the application on
this ecosystem is the first step towards its implementation in other tropical and temperate forest
ecosystems.
The development of the model will be based on the CASTANEA framework developed by ESE and the
MAESPA framework developed on Eucalyptus plantations by Eco&sols. CASTANEA is a forest
ecosystem process-based model, initially designed for simulating the growth and C and water
exchanges of even-aged forest stands at scales of hours to decades (Dufrêne et al. 2005). The MAESPA
(Duursma and Medlyn 2012) carbon-water model has already been used in tropical Eucalypt
plantations by Eco&Sols and pointed to the identification of key constraints (importance of the
seasonality of Leaf Area Index, and of the access to deep soil water) on Eucalypt production (Christina
et al. 2015; Christina et al. 2017). On the other hand, the MAESPA model, though very detailed in its
simulation of the leaf-to-canopy photosynthesis and latent heat exchange with the atmosphere and
soil water flow in particular in very deep sandy soils does not simulate the other plant carbon processes
(e.g. allocation to organs, evolution of the compartment sizes).
A first work has been successfully conducted during the PhD of Ivan CORNUT (defended May 2022,
codirection N. Delpierre, ESE, G. le Maire, Eco&Sols): 1) the CASTANEA and MAESPA models have been
merged in a common Carbon-Water-Energy budget process-based Eucalyptus ecosystem model (soil-
plant-atmosphere continuum), simulating the growth of an eucalyptus plantation from planting to
harvest and carbon sequestration at the ecosystem level; 2) the development of a leaf-cohort model
for accounting for continuous leaf production and leaf fall across the year, and 3) the development of
the K-cycle within the coupled CASTANEA-MAESPA-K model (Cornut et al., 2023 a,b ; Cornut et al.
2021). This model was useful in understanding and quantifying the effects of K availability on the
acquisition of C and the resource-use of tropical eucalypt plantations.
The coupling of the representations of 5 biogeochemical cycles (CNPK+water) in an ecosystem is not
straightforward, and cannot be done without a detailed knowledge of the processes driving the cycles
and a significant amount of data. We will develop the new model from CASTANEA-MAESPA-K, into
which we will incorporate representations of the coupled N and P cycles, that have been implemented
in the CASTANEA-NP model (Delpierre & Cornut, 2018, unpublished). CASTANEA-NP simulates the
bioavailability of N and P in the soil, processes of nitrification/denitrification (N) and P interaction with
the soil matrix, the absorption of N and P by roots (reaction-diffusion model considering the buffer
power of soil particles), their allocation to forming tissues and the resorption and shedding of N and P
from senescing leaves. The model development effort will be continued, starting with a new
bibliographical study on recent advances in other models for some processes (e.g. Goll et al. 2017; Sun
et al. 2021), and developing new sub-processes as was done in Ivan CORNUT PhD (Cornut 2022, Cornut
et al. 2023a,b). This modelling challenge will be addressed through the implication of specialists of
geochemical processes (interactions of nutrients with the soil matrix), specialists of the decomposition
of organic matter and specialists of the plant nutrient uptake (from ESE, Eco&Sols and Brazilian
partners).
Datasets
The project will rely on three existing experimental datasets on Eucalyptus plantations:
(1) The E. grandis nutrient omission and water-reduction trials conducted by Eco&Sols and the
University of São Paulo at the Itatinga Experimental Station (São Paulo state, Brazil) are particularly
relevant model systems, since a unique amount of CNPK+water data under multiple interactions of
potassium and water supply were gathered (Laclau et al. 2010, Christina et al. 2015). This site was used
to develop the K cycle model described above, and will be used here as a test case for N and P cycles.
(2) The Eucflux flux-tower site, conducted by Eco&Sols and the Instituto de Pesquisa e Ensino Florestal
(IPEF), where carbon and water fluxes are studied in detail through eddy-covariance techniques,
together with frequent biomass inventories and a continuous measurements of soil water content and
soil respiration (Christina et al. 2017). This site will be used because it allows to test a large range of
processes associated to carbon and water cycling, at different time scales from intra-day to a rotation-
length.
(3) A large experiment across Brazil, from North (tropical) to South (subtropical) regions, conducted by
the Suzano company, which tests N, P and K omissions on 18 sites with constrasting climate and soil
types. An identical subset of Eucalyptus clones was planted on all sites. One of these clones being the
same as at the Eucflux site (Stape et al. 2018). These largescale experiments will be the basis for
developing the model and assessing its robustness to changes in pedoclimatic conditions, as some sites
are mostly P-limited while others are more N or K limited. New measurements will have to be acquired
in the context of the present project. These new measurements will target key processes identified by
the bibliographical and preliminary modelling works.
Selected references
Christina, M., Le Maire, G., Battie‐Laclau, P., Nouvellon, Y., Bouillet, J. P., Jourdan, C., … & Laclau,
J. P. (2015). Measured and modeled interactive effects of potassium deficiency and water deficit on
gross primary productivity and light‐use efficiency in E ucalyptus grandis plantations. Global change
biology, 21(5), 2022-2039.
Cornut, I., Le Maire, G., Laclau, J. P., Guillemot, J., Mareschal, L., Nouvellon, Y., & Delpierre, N.
(2021). Potassium limitation of wood productivity: A review of elementary processes and ways
forward to modelling illustrated by Eucalyptus plantations. Forest Ecology and Management, 494,
119275.
Cornut, I., Delpierre, N., Laclau, J. P., Guillemot, J., Nouvellon, Y., Campoe, O., … & le Maire, G.
(2023a). Potassium-limitation of forest productivity, part 1: A mechanistic model simulating the
effects of potassium availability on canopy carbon and water fluxes in tropical eucalypt stands.
EGUsphere, 1-37.
Cornut, I., le Maire, G., Laclau, J. P., Guillemot, J., Nouvellon, Y., & Delpierre, N. (2023b).
Potassium-limitation of forest productivity, part 2: CASTANEA-MAESPA-K shows a reduction in
photosynthesis rather than a stoichiometric limitation of tissue formation. EGUsphere, 2022, 1-27.
Laclau, J. P., Ranger, J., de Moraes Gonçalves, J. L., Maquère, V., Krusche, A. V., M’Bou, A. T., … &
Deleporte, P. (2010). Biogeochemical cycles of nutrients in tropical Eucalyptus plantations: main
features shown by intensive monitoring in Congo and Brazil. Forest ecology and management, 259(9),
1771-1785.
Candidate profile :
MSc in Ecology or Plant Biology with a strong interest in modelling
Strong interest in ecosystem functioning and / or plant ecophysiology
Prior knowledge of a programming language is essential (Fortran / C++ / Matlab / R / Java /
Python…)
Previous experience in ecological modelling would be an asset
Good command of scientific English (reading and writing articles)
The student will need to be rigorous, have a taste for interdisciplinarity, be able to work in a
team and be capable of integrating into two work teams with different specialities.
For applying :
send a CV, motivation letter and contacts of two referees to Nicolas DELPIERRE and Guerric LE MAIRE.
