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ASSESSMENT OF CANOPY FLUORESCENCE YIELD WITH AIRBORNE PASSIVE AND ACTIVE MEASUREMENTS : THE CALSIF PROJECT

Current techniques for large scale remote sensing of fluorescence aim at quantifying the sun-induced fluorescence (SIF) in absorption features present in the incoming radiation. However, these techniques allow measurement of the emitted fluorescence flux but do not provide a direct assessment of the fluorescence yield, which is the true variable linked to the physiological status of the plant. To overcome these limitations, the authors initiated, with the support from CNES and ANR, the CALSIF project whose main purpose is to build an airborne instrument for simultaneous measurement of laser induced fluorescence (LIF) and SIF. By combining fluorescence yield and radiance measurements over different vegetation types, a partitioning of structure and physiological effects on fluorescence can be derived. Additionally, by analyzing the lidar waveform, the vertical structure of the canopy will also be resolved. In parallel, a new fluorescence canopy model is developed. The new model will simulate LIF and SIF, and takes into account 3D effects such as clumping or hot spot using a mixed approach: the irradiance due to close leaves is calculated with the radiosity equation and the irradiance due to distant leaves is approximated using the radiative transfer equation of turbid media. Expected outcomes of the CALSIF project are a better understanding of canopy structure effects in the measured fluorescence signal and improvements of algorithms to derive photosynthesis products from space measurements of vegetation fluorescence.