Structural and Physiological Effects on the Relationship between Solar-Induced Fluorescence and Gross Primary Production: A Comparison Study between Nadir and Hemispherical Fluorescence Observations

Previous studies have observed quasi-linear relationships between solar-induced fluorescence (SIF) and gross primary production (GPP) in different ecosystems across multiple spatial scales. However, the mechanisms associated with the relationship remains largely unclear. Given the light use efficiency (LUE) model (GPP=LUE*APAR, SIF=SIF_y*APAR, where SIF_y refers to SIF yield), the energy source, i.e. the absorbed photosynthetically active radiation (APAR) is the primary driver of SIF, GPP, and SIF:GPP relationship. After accounting for APAR in the LUE model, previous field studies also showed different patterns of SIF_y:LUE relationships across various ecosystems at the canopy scale. Different patterns of observed SIF_y:LUE relationships at the canopy scale could be associated with differences in: (1) canopy structure affecting the scattering and reabsorption of emitted SIF; (2) plant physiology affecting the leaf-level SIF:GPP and SIF_y-LUE relationships; (3) sensor field-of-view that are used to collect optical signals for SIF retrieval. Understanding and quantifying the effects of these differences on the SIF_y:LUE relationship is crucial to develop optimal and generally applicable algorithms to estimate GPP from SIF. In July-August 2018, we collected leaf-level gas exchange and pulse-amplitude-modulated fluorescence data (light response curves of photosynthesis and steady state fluorescence) on sun and shade leaves in an irrigated soybean field at Mead, Nebraska (AmeriFlux US-Ne2). In addition, we collected canopy SIF with a bi-hemispherical (BH) system and a hemispherical-nadir (HN) system. A radiative transfer model will be used to simulate instantaneous light distribution within the canopy using leaf gas exchange and reflectance data. By combining instantaneous light distribution and leaf-level light responses of photosynthesis and fluorescence, we attempt to quantify the contribution of sun and shade leaves to the BH and the HN canopy level SIF, GPP, SIFy, and LUE as well as the relationships between them (e.g. SIF:GPP and SIF_y:LUE relationships). Comparison of the sun-shade-leaf effects on signals from BH and HN systems will provide us important insights on the structural and physiological contributions on the SIF:GPP relationship.