[A-CG39] Biogeochemical cycles in Land Ecosystem

[ACG39-09] Evaluation of Solar Induced chlorophyll Fluorescence using the sub-nanometer spectroscopy above the canopy of deciduous needleleaf forest in the north-slope of Mt. Fuji, central Japan

*Tomoki Morozumi¹, Tomomichi Kato¹, Hibiki M Noda², Yoshiyuki Takahashi² (1.Research Faculty of Agriculture, Hokkaido University, 2.Earth System Division, National Institute for Environmental Studies (NIES))

Keywords:Forest, Remote Sensing

Advances in remote sensing techniques are essential for assessing the CO₂ absorption function of the terrestrial ecosystems for climate change predictions. Recent sub-nanometer spectroscopy can estimate Solar Induced chlorophyll Fluorescence (SIF) which is related to the photosynthetic activity from leaf-scale to canopy-scale including gross primary productivity (GPP) of terrestrial ecosystems. SIF is the red and far-red fluorescence signal mostly emitted from the chlorophyll-a in a photosystem II after the excitation by solar photons. SIF has been measured by satellites on a global scale, while in-situ measurements from flux towers (i.e. ground-based SIF observations) did not cover deciduous needleleaf forest (DNF) ecosystems. We think that it is important to know the dynamics of functions in the DNF ecosystem because it is one of the common ecosystem types in high-latitude regions under the severe influences of climate change. The purpose of this study is to evaluate the seasonal and diurnal variations of ground-based SIF in the DNF ecosystem. We continuously observed sub-nanometer (full width at half maximum < 0.4 nm) spectral radiation above the canopy of the forest, dominated by Japanese Larch (Larix kampferi), at Fuji-Hokuroku (FHK) flux site (35.4°N, 138.8°E) Yamanashi, Japan since March 2021. A spectrometer (QEpro, Ocean Insight, Dunedin, FL, USA) was equipped with 3 optical fiber cables via optical fiber splitter and captured the upward and downward spectral radiation at 32 m with a field of view (FOV) of 180° (nadir view) and FOV of 25° (northward zenith of 45° view). SIF was retrieved using the spectral fitting method for O₂-A absorption band at wide (759-767 nm) and narrow (759.5-761.5 nm) windows to test the response against photosynthetic active radiation (PAR) under various sky conditions. Half-hourly SIF varied 0-0.1 to 0.1-1.1 (mW m^-2 nm^-1 sr^-1) from March to August. Increasing SIF were detected soon after the increasing NDVI linked to the start of leaf flushing and leaf growth in late April to May. After the saturation of NDVI, SIF varied dynamically from May to August. Correlation coefficients between SIF and PAR were varied between clear-sky and overcast in the wide window, although they were relatively consistent between clear-sky and overcast in the narrow window of the spectral fitting. The result also suggests that there was a suppression of SIF in rainy periods in late June to early July. Our new findings help us to understand global carbon cycling.