Using Sun Induced Chlorophyll fluorescence (SIF) and carotenoid seasonal dynamics composition to detect spring activation in evergreen conifers

Conifer needles remain green year-round, making the photosynthetic transition between spring and winter invisible to classical remote sensing (RS) monitoring techniques such as Normalized Difference Vegetation Index (NDVI), causing bias in light use efficient models (LUE). Previous studies have shown that non-photochemical quenching dissipation process such as solar-induced chlorophyll fluorescence (SIF), carotenoids involved in the xanthophyll cycle and chlorophyll pigments concentrations are closer related to photosynthetic seasonal dynamics in evergreen conifers. Nowadays, these dynamics can be detected using RS tools such as the Photochemical Reflectance Index (PRI), the Chlorophyll/Carotenoid Index (CCI) and the sun induced chlorophyll fluorescence signal (SIF). To better understand the mechanisms and timing of this invisible transition, we conducted two needle-scale experiments to explore the link between SIF, PRI, CCI and chlorophyll/carotenoid pigment quantification. The first experiment was conducted under natural outdoors conditions (NC) and the second one was developed under an increased-temperature controlled environment (TC) on two conifer species: Lodgepole pine (Pinus contorta) and Black Spruce (Picea mariana). Results from NC experiment showed that PRI and CCI captured the seasonal transition of chlorophyll/carotenoid ratio changes and clear positive relationships were observed between PRI, CCI and SIF emission. TC experiment results displayed a positive correlation between temperature CCI, PRI and SIF. Non-linear sigmoidal model used to analyze kinetics showed that for both species in NC, the SIF signal started to increase before the CCI and PRI. This study showed that alternative RS techniques based on spectral reflectance of carotenoids, spectral monitoring of the SIF emission and the use of foliar pigment information can track the regulation relative timing of the spring activation of photosynthesis in evergreen conifer species. These metrics combined offer a powerful method to monitor photosynthetic phenology in boreal evergreens via RS and can improve efficiency in light use efficient models (LUE).