What was the purpose of the PhotoProxy 2019 campaign?

The Technical Assistance for the Photosynthetic-Proxy Experiment, also known as PhotoProxy, focused on subjects needed to develop proposed Level-2D data products for the FLuorescence EXplorer (FLEX) mission, an upcoming ESA Earth Explorer satellite.

ESA's Biomass and FLEX Earth Explorer satellite missions will open new opportunities to enhance our knowledge of the global carbon cycle. In particular, the scientific exploitation of Biomass and FLEX in synergy with the Copernicus Sentinel satellite series and other existing and future missions (e.g. CMOS, GEDI, NISAR, TanDEM-X/L) will provide an unprecedented opportunity to better understand and characterise the different components of the carbon cycle and its dynamics. Preparing for the fast exploitation of this unique and unprecedented observational capacity, ESA launched the Carbon Science Constellation Initiative. This initiative will be implemented through a cluster of different studies, research activities, campaigns and tool development efforts dedicated to support the scientific community in exploring the potential synergies between different Earth Observation approaches and maximise the scientific impact of this unique set of sensors for carbon cycle research.

The PhotoProxy campaign addressed relevant open aspects that are related to the quantitative assessment of vegetation photosynthesis and vegetation stress from space. In recent years, the fluorescence signal that is emitted from the core of the photosynthetic apparatus during photosynthetic energy conversion, has become the most promising indicator of actual photosynthetic rates. The FLEX mission will use a novel technique to measure chlorophyll fluorescence, rather than the current techniques that detect potential photosynthesis dervied from passive reflectance measurements. PhotoProxy aimed to investigate whether a combination of the two techniques may be a better approach for assessment of photosynthesis and gross primary productivity (GPP).

PhotoProxy used the dataset created during the FLEXSense 2018 campaign, which combined satellite, airborne, and ground-based measurements to simulate future FLEX products. PhotoProxy combined the FLEXSense dataset with local flux measurements to derive optical remote sensing parameters and GPP.

What was the outcome of the PhotoProxy 2019 campaign?

Sun-Induced Fluorescence (SIF) represents an invaluable tool to track and predict photosynthesis and transpiration (Shan et al., 2019), which has been successfully applied at both canopy (Damm et al., 2010;) and global scale (Guanter et al., 2014). However, the influence of stress events such as extreme temperature or drought on the relationship between GPP and fluorescence signal F760 is not currently understood with sufficient detail.

During the PhotoProxy campaign, the 2018 heatwave in Spain was analysed, revealing that signals F760 and F680 were strongly affected by the heatwave and that GPP and SIF respond differently to extreme temperature. During the heatwave the usually positive relationship between GPP and F760 became negative, highlighting the non-linear nature of the relationship between GPP and F760. The two peaks of SIF (F760 and F680) reacted differently to the heatwave, indicating that the two peaks of SIF carry out different information and can be used together to understand vegetation stress. Assessment indicated that Photochemical Reflectance Index (PRI) was the best predictor of GPP during the heatwave, which indicates that photosynthetic activity was highly controlled by heat dissipation processes. These results suggest that including PRI in an empirical model together with F760 might improve GPP prediction, especially during stress conditions.

Download the Photosynthetic-Proxy Experiment Final Report

Digital Object Identifier: European Space Agency, 2019, PhotoProxy 2019 campaign, https://doi.org/10.57780/esa-bb0ea39

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