B006-0018
Radiative transfer and viewing geometry considerations for the SIF/GPP relationship
Monday, 7 December 2020
Poster
Zoe Pierrat, University of California Los Angeles, Los Angeles, CA, United States, Alexander Norton, University of Melbourne, Earth Sciences, Parkville, Australia, Nicholas Parazoo, University of California Los Angeles, JIFRESSE, Los Angeles, CA, United States, Andrew Maguire, University of Idaho, Moscow, ID, United States, Katja Grossmann, Heidelberg University, Institute of Environmental Physics, Heidelberg, Germany, Troy Magney, University of California Davis, Plant Sciences, Davis, CA, United States, Alan Barr, University of Saskatchewan, Saskatoon, SK, Canada, Bruce Johnson, University of Saskatchewan, Saskatoon, Canada and Jochen Stutz, University of California Los Angeles, Department of Atmospheric and Oceanic Sciences, Los Angeles, CA, United States
Abstract:
Solar-Induced chlorophyll Fluorescence (SIF) provides a powerful proxy for determining forest gross primary production (GPP), particularly in evergreen ecosystems where traditional measures of greenness fail. The dynamics of the SIF/GPP relationship, however, are poorly understood under varying viewing directions and light conditions. This is, in large part, due to challenges in measuring SIF at the spatiotemporal scale that is necessary to understand these effects. Therefore, the aim of this work is to utilize high-temporal and spatial resolution SIF measurements to better constrain the response of SIF to ambient canopy illumination and viewing geometry.
We use a PhotoSpec instrument and eddy covariance measurements to explore the SIF/GPP relationship under various viewing directions and light conditions during the 2019 and 2020 growing seasons at the Old Black Spruce site in Saskatchewan, Canada. PhotoSpec is a tower-based 2-D scanning spectrometer system capable of taking Fraunhofer-line based SIF retrievals in the red and far-red wavelength ranges with a 0.7 degree field of view at a ~30 second time resolution. Measured SIF and GPP are combined with SCOPE modelling results to provide a mechanistic understanding of the physical and ecophysiological drivers for the SIF/GPP relationship in the Boreal Forest.
Our results show that viewing direction and solar zenith/azimuth angles are important for the SIF signal under direct light conditions, but not under diffuse. Furthermore, the SIF/GPP relationship changes under direct and diffuse light conditions at a 30 minute, daily, and monthly resolution. Our ability to use SIF as a proxy for GPP depends on a quantitative understanding of radiative transfer within the canopy and how scanning geometry impacts SIF measurements. These results provide an important insight into these relationships in the Boreal forest, a region where GPP has been traditionally difficult to track using remote sensing.
