Li et al. (2025) Atmospheric dryness effects on canopy chlorophyll fluorescence and Gross Primary Production (GPP) in a deciduous forest during heat waves

Identification

• Journal: Remote Sensing of Environment
• Year: 2025
• Date: 2025-11-20
• Authors: Zhaohui Li, Gabriel Hmimina, Gwendal Latouche, Daniel Berveiller, Abderrahmane Ounis, Yves Goulas, Kamel Soudani
• DOI: 10.1016/j.rse.2025.115148

Research Groups

• Ecologie Société Evolution (ESE), Université Paris-Saclay, CNRS, AgroParisTech, Gif-sur-Yvette, France
• Laboratoire de Météorologie Dynamique (LMD), Sorbonne Université, IPSL, CNRS, École polytechnique, Palaiseau Cedex, France

Short Summary

This study investigated how atmospheric dryness during heat waves affects Sun-Induced chlorophyll Fluorescence (SIF), LED-induced chlorophyll fluorescence (FyieldLIF), and Gross Primary Production (GPP) in a temperate deciduous forest. It found that FyieldLIF provided a more robust and physiologically sensitive proxy for GPP and photosynthetic capacity (Amax) under high atmospheric dryness conditions compared to SIF, particularly at daily temporal scales.

Objective

• To investigate how heat-wave induced high atmospheric dryness (elevated Vapor Pressure Deficit, VPD) affects canopy chlorophyll fluorescence (both SIF and FyieldLIF) and GPP, as well as their relationships.
• To assess how atmospheric dryness affects the relationships among SIF, GPP, and FyieldLIF.
• To clarify physiological versus radiative controls on ChlF–GPP relationships by normalizing SIF and GPP by incident Photosynthetically Active Radiation (PAR) to obtain SIFy and GPPy.
• To evaluate whether FyieldLIF and/or SIFy can effectively capture photosynthetic capacity limitations under stress, as reflected in the maximum CO₂ assimilation rate (Amax).

Study Configuration

• Spatial Scale: A mixed temperate deciduous broadleaf forest (Fontainebleau-Barbeau station, FR-Fon, 48°28′26″ N, 2°46′57″ E, 53 km southeast of Paris, France). Dominant species are sessile oaks (79%) and hornbeam (18%). Canopy height is approximately 25 meters, with an average Leaf Area Index (LAI) of 5.8 m² m⁻².
• Temporal Scale: Measurements and analysis were conducted from April to mid-September 2022, encompassing the summer 2022 European heat waves. Data were analyzed at both half-hourly and daily temporal scales.

Methodology and Data

• Models used:
  • Three-band Fraunhofer Line Discrimination (3FLD) algorithm for SIF retrieval at the O₂-A band.
  • Photosynthetic light-response curve fitting model (Baly, 1935) to derive Amax and α (slope of the light-response curve at low light intensity).
  • Night-time algorithm for partitioning Eddy Covariance fluxes into GPP and ecosystem respiration (Reco).
• Data sources:
  • Tower-based observations:
    • Continuous measurements of carbon dioxide (CO₂), water, and energy exchanges using an Eddy Covariance (EC) flux observation system (Li7500, Li7200 infrared gas analyzers; R3–50, HS-50 sonic anemometers).
    • Sun-Induced chlorophyll Fluorescence (SIF) using the SIF3 instrument (HR1-T spectrometer, spectral range 658–800 nm, ~0.3 nm FWHM resolution; LR1-T spectrometer, 400–1000 nm, ~1.5 nm FWHM resolution).
    • Apparent chlorophyll fluorescence yield (FyieldLIF) using the LIF instrument (6-W-modulated blue array LED, peak wavelength 455 nm; PIN-photodiode with optical filters for far-red fluorescence emission above 725 nm).
    • Ancillary environmental variables: Air temperature (Ta) and relative humidity (HMP155Ax6 thermohygrometer) for Vapor Pressure Deficit (VPD) calculation; Photosynthetically Active Radiation (PAR, PARreflected, PARtransmitted, PARdiffuse, PARtotal using PQS1 PAR Quantum Sensor and BF5 sunshine sensor); precipitation (ARG100 rain gauge); soil water content (SWC) (Enviroscanx4).
    • Reflectance-based vegetation indices: Normalized Difference Vegetation Index (NDVI) and modified red-edge Normalized Difference Index (mNDI705) calculated from spectrometer data.
  • Site Infrastructure: The Fontainebleau-Barbeau forest site (FR-Fon) is part of the European ICOS (Integrated Carbon Observation System) research infrastructure.

Main Results

• At the half-hourly scale, the correlation between SIF and GPP significantly decreased under high atmospheric dryness (R² from 0.49 to 0.17). In contrast, the correlation between FyieldLIF and GPP significantly increased under high atmospheric dryness (R² from 0.07 to 0.43).
• At the daily scale, correlations between FyieldLIF and SIF, and FyieldLIF and GPP improved markedly under high atmospheric dryness conditions (R² increased from 0.17 to 0.78 and from 0.43 to 0.90, respectively). Overall, correlations between SIF and GPP, and FyieldLIF and GPP increased at the daily scale compared to the half-hourly scale.
• Under high atmospheric dryness, GPP exhibited an advanced diurnal peak (around 08:30 UTC) compared to PAR and SIF, likely due to stomatal closure. FyieldLIF showed a continuous decrease from morning to late afternoon under high atmospheric dryness, while SIFy exhibited more fluctuating variations.
• FyieldLIF demonstrated significantly stronger and more consistent correlations with the maximum photosynthetic rate (Amax) than SIFy across various conditions (cloudy/low VPD, clear sky/medium VPD, clear sky/high VPD). The highest correlation was observed between Amax and FyieldLIF (R² = 0.85) under clear sky and high VPD conditions, compared to Amax and SIFy (R² = 0.56) under the same conditions.
• Environmental conditions during High Atmospheric Dryness (HAD) periods showed significantly higher PAR, air temperature, and VPD, but no significant difference in soil water content compared to No Atmospheric Dryness (NAD) periods, confirming atmospheric dryness as the primary stressor.
• Canopy structure and chlorophyll content, as indicated by NDVI, mNDI705, and fAPAR, remained stable throughout the study period, minimizing their confounding effects.

Contributions

• This study is among the first to integrate canopy-level active (FyieldLIF) and passive (SIF) chlorophyll fluorescence measurements to investigate the effects of atmospheric dryness on ChlF-GPP relationships during heat waves in a temperate deciduous forest.
• It demonstrates the superior capability of FyieldLIF to detect plant physiological responses to high atmospheric dryness and track photosynthetic capacity (Amax) more robustly than SIF, especially under stress conditions.
• The research highlights the time-scale dependency of ChlF-GPP relationships under atmospheric dryness, with stronger correlations observed at daily scales.
• It emphasizes the potential of canopy-level active chlorophyll fluorescence as a valuable tool for monitoring photosynthesis and its relationship with chlorophyll fluorescence under extreme climatic events.
• The findings provide mechanistic insights into the decoupling of SIF-GPP relationships under stress, attributing FyieldLIF's strength to its direct link to physiological regulation (quantum yield of ChlF, ∅F) while SIF is more influenced by structural and radiative factors.

Funding

• "Centre National d’Études Spatiales” (CNES) through the FLEX/VELIF project.
• Climate change and land use (Cland) Convergence Institute.

Citation

@article{Li2025Atmospheric,
  author = {Li, Zhaohui and Hmimina, Gabriel and Latouche, Gwendal and Berveiller, Daniel and Ounis, Abderrahmane and Goulas, Yves and Soudani, Kamel},
  title = {Atmospheric dryness effects on canopy chlorophyll fluorescence and Gross Primary Production (GPP) in a deciduous forest during heat waves},
  journal = {Remote Sensing of Environment},
  year = {2025},
  doi = {10.1016/j.rse.2025.115148},
  url = {https://doi.org/10.1016/j.rse.2025.115148}
}