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Funding :

ANR Blanc

Contact EcoSys :

Michaël Chelle

Abstract :

Vegetation plays a fundamental role in the functioning of land surfaces. A better understanding of the interactions between vegetation and atmosphere that would allow predicting the response of vegetation to future climate change is mandatory to design mitigation strategies. In this framework, remote sensing of vegetation mainly focused on the analysis of reflected sunlight in the optical domain to derive canopy biophysical variables, such as leaf area index, the fraction of reflected (albedo) or absorbed (fAPAR) radiation, or the content in chlorophyll pigments.

Chlorophyll fluorescence is considered as a complementary observation compared to others based on reflectance. It is very promising since it is widely used to characterize the functioning of photosynthesis, which is a key parameter of the carbon cycle. Fluorescence emission occurs by a mechanism reverse to the absorption that directly competes with the photochemical conversion. This is the reason why fluorescence yield is closely linked to photosynthetic efficiency. Fluorescence yield variations can be directly monitored by active methods, because illumination conditions are well controlled. However, remote sensing of fluorescence with active techniques has been limited by the power of available excitation sources. During the last decade, a passive technique for measuring the sun-induced fluorescence (SIF) has been developed, and successfully applied for quantifying the fluorescence from space in the GOSAT mission. This technique is based on the analysis of the spectral absorption bands in the red and far red. Recently, ESA has selected the FLEX mission dedicated to fluorescence remote sensing. However, if the feasibility of measuring fluorescence from space has now been demonstrated, the interpretation of canopy fluorescence in terms of eco-physiological status still remains unclear.

The purpose of this proposal is to develop the experimental and theoretical tools that will remove this bottleneck. We propose to develop a new instrument for measuring simultaneously the sun-induced and the laser-induced fluorescence (LIF) on the same target. LIF will enable the assessment of changes in the fluorescence yield, which is the true variable directly linked to photosynthesis efficiency, thus filling the gap between SIF and physiological status. In addition, the instrument will also measure: (i) vegetation reflectance spectrum to derive leaf area index, fAPAR or chlorophyll content, and (ii) the canopy vertical structure by monitoring the backscattered laser pulse. In parallel, we propose to develop a canopy fluorescence model to interpret sun-induced and laser-induced fluorescence measurements.

Experimental studies will be carried on crops with different carbon metabolism (C3, C4). The obtained relationship between canopy fluorescence and photosynthesis will be tested against natural vegetation during airborne experiments. At the end of the project, the tools allowing to accurately measure and simulate canopy fluorescence signal will be made available to the scientific community. The outcomes of the project will from a strong scientific basis for the interpretation of fluorescence from space with data from FLEX, GOSAT or OCO-2. 

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