Assessing SIF-GPP Relationships in Peatlands: Temporal Insights for FLEX Mission Validation

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The Fluorescence Explorer (FLEX) mission aims to monitor vegetation sun-induced chlorophyll fluorescence (SIF), a proxy for ecosystem health. Studying photosynthesis and its relationship with SIF provides valuable insights into the physiological responses of ecosystems to environmental stress. Peatlands are among the most valuable ecosystems in the carbon cycle, acting as carbon storage and sinks in normal conditions but becoming carbon sources in drought conditions.This study investigates the correlation between SIF and Gross Primary Productivity (GPP) in the temperate peatland in Poland. Measurements by the FloX system with a temporal resolution of ~30 seconds between June and October 2019 and CO2 fluxes measured by manual chambers were used for this purpose. GPP was calculated from the subsequent net ecosystem exchange and respiration measurements, and the gaps between measurements were modelled with a Michaelis-Menten rectangular hyperbola. SIF was retrieved using the Improved Fraunhofer Line Depth (iFLD) Spectral Fit Method (SFM) and Spectrum Fitting (SpecFit) algorithms in the O2-A and O2-B bands. The data were analysed at different time intervals (30 min, 1 h, 3 h, whole day, and the entire dataset).Our results show that the strength of the SIF-GPP relationships changes significantly with time interval. Correlations tend to weaken or break (r2

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