Thesis: Impact of drought and heat stress on forest leaf phenology in relation to species strategy

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The French National Research Institute for Agriculture, Food, and Environment (INRAE) is a major player in research and innovation. It is a community of 12,000 people with 272 research, experimental research, and support units located in 18 regional centres throughout France. Internationally, INRAE is among the top research organisations in the agricultural and food sciences, plant and animal sciences, as well as in ecology and environmental science. It is the world’s leading research organisation specialising in agriculture, food and the environment. INRAE’s goal is to be a key player in the transitions necessary to address major global challenges. Faced with a growing world population, climate change, resource scarcity, and declining biodiversity, the Institute has a major role to play in building solutions and supporting the necessary acceleration of agricultural, food and environmental transitions.

Work environment, missions and activities

You will be welcomed in the unit Interactions Sol Plante ATmosphère (UMR 1391 ISPA, Bordeaux, France) for PhD position

Project description:

Leaf phenology is the study of the drivers and dynamics of leaf bud burst, unfolding, maturation and senescence. It plays a key role in carbon, water and energy exchanges between forests and the atmosphere. By directly affecting many key biological and biophysical processes, the timing of leaf presence and absence impacts biomass production, local climate, community structure and forest resilience. However, global changes and associated extreme weather events are disrupting these processes, leading to anomalies such as increased late spring frosts, summer leaf loss due to drought/heat, and the development of new leaves in the middle of winter. 

Current approaches establish an empirical link between leaf phenology and standardised air temperature (the macroclimate). This does not capture the actual conditions experienced by trees (the microclimate), nor the interactions between leaf dynamics and underlying ecophysiological processes (e.g. photosynthesis, transpiration). The functional strategy of species is also largely overlooked in these approaches, even though we observe substantial inter-species differences in a given environment (e.g. mixed forest) related to their phenological dynamics (early vs. late species), the cost of construction and longevity of leaves, the remobilisation of nutrients before senescence or through litter decomposition, etc. This results in poor models for predicting the phenological stages of leaf emergence and senescence. Extrem events such as stress-induced defoliation or secondary flushing are rarely represented in these models, even though their frequency is increasing with global change. This leads to a bias in the interpretation of leaf dynamics and tree response to global warming. Vegetation models such as those used to study the carbon cycle and forest resilience are therefore subject to considerable uncertainty. 

This thesis project aims to fill this gap by studying the interactions between temperature, activity (carbon, water and nutrient management) and the phenological dynamics of the leaves of European temperate tree species at different scales (leaf, individual, ecosystem) in relation to their functional strategy. By combining in situ measurements, experiments and observations, visible imaging and surface temperature data (camera network, satellite remote sensing) and process modelling, this project will 1) quantify the role of leaf temperature in controlling leaf growth and senescence, 2) study the links between temperature, leaf activity and the senescence process under different environmental conditions 3) quantify how thermal and water stresses influence leaf growth and senescence processes.

Research Topic: 

Forest dynamics in the context of climate change; responses of phenology and the dynamics of carbon and water cycles under water and heat stress

Research Field: 

Forest macroecology, functional ecology, ecophysiology, environmental physics

Objectives: The overall objective of this thesis project is to study the relationships between leaf temperature, leaf activity, and the dynamics of growth and senescence in relation to their functional strategy. More specifically, you will focus on European forest tree species and seek to test the following two hypotheses:

(H1) Leaf senescence results from a balance between leaf activity (carbon assimilation, transpiration, nutrient allocation) and risk management (water stress, light stress, thermal stress), which can be interpreted through a cost-benefit mechanism.

(H2) Different species with different functional strategies (i.e., functional traits, late-maturing vs. early-maturing species) do not exhibit the same phenological dynamics, resulting in different responses to thermal and water stress. 

Method: To achieve these objectives, you will be required to develop a multidisciplinary approach combining field observations, data analysis, and process modeling through three tasks: 

1) Analyze the relationships between leaf temperature, leaf activity, and phenology in relation to environmental conditions. For this task, you will combine and analyze multiple datasets acquired at different spatial and temporal scales, such as: i) measurements of photosynthesis, temperature, and functional traits at the leaf scale, ii) temperature observations from thermal imaging (camera, radiometric thermometer), phenology (field observations, Phenocam), and carbon, energy, and water fluxes (eddy covariance, sap flow) at the individual and ecosystem scales; and iii) surface temperature and phenology observations from remote sensing (drone, satellite) at the ecosystem and global scales. You will also be required to conduct additional field measurements as well as ad hoc experiments to test certain hypotheses.

2) Develop a phenological model of senescence that accounts for leaf temperature and physiological activity. This task will involve evaluating existing models and developing a new phenological model based on the results of Task 1. 

3) Specifically investigate the links between leaf dynamics and water and heat stress in relation to the functional strategy of plant species. This work will focus in particular on stress-induced senescence and the subsequent resumption of growth and leaf activity.

Expected results: 

This project will generate fundamental knowledge about the relationships between leaf activity (photosynthesis, transpiration), leaf temperature, and the dynamics of leaf phenology, with the aim of developing a new phenology model that integrates biophysics and ecophysiology. This model will be developed for integration into terrestrial biosphere models to quantify the impact of leaf dynamics on biogeochemical cycles, as well as to address specific applications such as the response of ecosystems and/or certain species to late frosts and summer droughts.

References:

Peaucelle, M. et al. Spatial variance of spring phenology in temperate deciduous forests is constrained by background climatic conditions. Nature communications 10, 5388; 10.1038/s41467-019-13365-1 (2019).

Peaucelle, M., Peñuelas, J. & Verbeeck, H. Accurate phenology analyses require bud traits and energy budgets. Nature plants; 10.1038/s41477-022-01209-8 (2022).

Peaucelle, M. et al. Representing explicit budburst and senescence processes for evergreen conifers in global models. Agricultural and Forest Meteorology 266-267, 97–108; 10.1016/j.agrformet.2018.12.008 (2019)

Kloos, S. et al. The linkage between functional traits and drone-derived phenology of 74 Northern Hemisphere tree species. Sci. Total Environ. 952, 175753 (2024).

Hänninen, H. & Tanino, K. Tree seasonality in a warming climate. Trends in plant science 16, 412–416; 10.1016/j.tplants.2011.05.001 (2011)

Fu, Y. S. H. et al. Variation in leaf flushing date influences autumnal senescence and next year's flushing date in two temperate tree species. Proceedings of the National Academy of Sciences of the United States of America 111, 7355–7360; 10.1073/pnas.1321727111 (2014).

Xie, Y., Wang, X. & Silander, J. A. Deciduous forest responses to temperature, precipitation, and drought imply complex climate change impacts. Proceedings of the National Academy of Sciences of the United States of America 112, 13585–13590; 10.1073/pnas.1509991112 (2015).

Marchin, R., Zeng, H. & Hoffmann, W. Drought-deciduous behavior reduces nutrient losses from temperate deciduous trees under severe drought. Oecologia 163, 845–854; 10.1007/s00442-010-1614-4 (2010).

Cole, E. F. & Sheldon, B. C. The shifting phenological landscape: Within- and between-species variation in leaf emergence in a mixed-deciduous woodland. Ecology and evolution 7, 1135–1147; 10.1002/ece3.2718 (2017).

Zhao, H. et al. Diverging models introduce large uncertainty in future climate warming impact on spring phenology of temperate deciduous trees. The Science of the total environment 757, 143903; 10.1016/j.scitotenv.2020.143903 (2021).

Wright, I. J. et al. The worldwide leaf economics spectrum. Nature 428, 821–827; 10.1038/nature02403 (2004).

Caldararu, S., Purves, D. W. & Palmer, P. I. Phenology as a strategy for carbon optimality: a global model. Biogeosciences 11, 763–778; 10.5194/bg-11-763-2014 (2014).

Training and skills

• Recommended training: Master’s degree, engineering school, or equivalent in environmental science
• Knowledge required: Computer programming (R, Python), data analysis and statistics, plant ecophysiology, functional ecology 
• Appreciated experience: Theoretical or experimental research.
• Skills sought: Independence (ability to propose, develop, and test approaches, as well as organize one’s own work), ability to work in a team, and proficiency in writing (scientific articles) and speaking (scientific presentations) in English. A strong interest in modeling and data analysis.

INRAE's life quality

By joining our teams, you benefit from (depending on the type of contract and its duration):

- up to 30 days of annual leave + 15 days "Reduction of Working Time" (for a full time);
- parenting support: CESU childcare, leisure services;
- skills development systems: training, career advise;
- social support: advice and listening, social assistance and loans;
- holiday and leisure services: holiday vouchers, accommodation at preferential rates;
- sports and cultural activities;
- collective catering.

How to apply

I send my CV and my motivation letter