B006-0007
Variations in spatiotemporal averaging, canopy illumination, and surface coverage and their impact on the interpretation of SIF and vegetation index observations

Monday, 7 December 2020
Poster
Katja Grossmann1, Jochen Stutz2, Madeline Young3, Zoe Pierrat3, Troy Magney4, Philipp Köhler5, Christian Frankenberg6 and Andre Butz1, (1)Heidelberg University, Institute of Environmental Physics, Heidelberg, Germany, (2)University of California Los Angeles, Atmospheric and Oceanic Sciences, Los Angeles, CA, United States, (3)University of California Los Angeles, Los Angeles, CA, United States, (4)University of California Davis, Plant Sciences, Davis, CA, United States, (5)California Institute of Technology, Pasadena, CA, United States, (6)California Institute of Technology, Environmental Science and Engineering, Pasadena, CA, United States
Abstract:
Spatiotemporally averaged observations of solar-induced chlorophyll fluorescence (SIF) and vegetation indices (VIs) at the canopy scale are increasingly used as a proxy for Gross Primary Productivity (GPP) and to study photosynthetic activity. Several studies indicate that canopy SIF measurements are highly sensitive to canopy structure and illumination geometry. This is particularly important for agricultural croplands, which cover different light conditions and plant growing stages during one single growing season. Thus, it is necessary to determine the fraction of the surface that is vegetated to reconstruct the signal originating solely from vegetation and to examine the influence of surface coverage and canopy illumination on SIF and VIs.

We use ground-based simultaneous co-centered measurements of red and far-red SIF, canopy reflectances, and VIs at a soybean field in the Midwestern U.S. during the 2017 growing season collected by the PhotoSpec instrument. The PhotoSpec instrument has a 2D scanning telescope unit with a narrow field-of-view of 0.7° and a 20 second time resolution to investigate the spatiotemporal averaging of SIF and VIs at different viewing angles and light conditions and its impact on the seasonal dynamics. We determine the surface coverage using canopy reflectances and VIs and validate it with independent camera images and analyze how these factors regulate the SIF/GPP relationship.

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