Unpacking the drivers of diurnal dynamics of sun-induced chlorophyll fluorescence (SIF): Canopy structure, plant physiology, instrument configuration and retrieval methods

Authors: Chang, Christine; WEN, JIAMING - Cornell University; HAN, JIMEI - Cornell University; KIRA, OZ - Cornell University; LEVONNE, JULIE - Cornell University; MELKONIAN, JEFFREY - Cornell University; RIHA, SUSAN - Cornell University; SKOVIRA, JOSEPH - Cornell University; NG, SHARON - Cornell University; GU, LIANHONG - Oak Ridge National Laboratory; WOOD, JEFFREY - University Of Missouri; NAETHE, PAUL - Jb Hyperspectral Devices Gmbh; SUN, YING - Cornell University

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2021
Publication Date: 8/31/2021
Citation: Chang, C.Y., Wen, J., Han, J., Kira, O., Levonne, J., Melkonian, J., Riha, S.J., Skovira, J., Ng, S., Gu, L., Wood, J.D., Naethe, P., Sun, Y. 2021. Unpacking the drivers of diurnal dynamics of sun-induced chlorophyll fluorescence (SIF): Canopy structure, plant physiology, instrument configuration and retrieval methods. Remote Sensing of Environment. 265:112672. https://doi.org/10.1016/j.rse.2021.112672.
DOI: https://doi.org/10.1016/j.rse.2021.112672

Interpretive Summary: Sun-induced chlorophyll fluorescence (SIF) is an emerging remote sensing technology with potential applications for monitoring the global carbon cycle, tracking climate stress impacts on crops and forests, and forecasting crop yield. Within the past decade, SIF has been collected from satellites and a growing international network of ground-based towers and airborne systems. However, use of these data is currently limited by the temporal and spatial scale mismatch between ground systems and satellites, and the need for a better understanding of the factors driving the diurnal dynamics of SIF. We disentangled biological and physical factors contributing to measured diurnal SIF signals including canopy structure, plant physiology, instrument configuration and retrieval method. We also reported the influence of agricultural row orientation on diurnal SIF dynamics. Our study highlights sources of error that influence both ground and satellite SIF data, and informs efforts to synthesize SIF data across platforms and sites.

Technical Abstract: Sun-induced chlorophyll fluorescence (SIF) from spaceborne sensors is a promising tool for global carbon cycle monitoring, but its application is constrained because of insufficient understanding of the diurnal SIF dynamics and underlying drivers. Rapidly accumulated SIF measurements from ground-based towers offer opportunities to reveal diurnal SIF dynamics across biomes and environmental conditions; however, meaningful interpretation of such diurnal variations requires disentangling impacts from canopy structure, plant physiology, instrument configuration and retrieval methods, which often interact with and confound each other. This study aims to unpack these drivers using 1) concurrent canopy-scale (ground towers, augmented by unmanned aerial vehicle UAV campaigns) and leaf-scale measurements at an agricultural corn field, 2) a mechanistic SIF model that explicitly considers the dynamics of photochemistry (via the fraction of open photosystem II reaction centers, qL) and photoprotection (via nonphotochemical quenching, NPQ) as well as their interactive dependence on the sub-canopy light environment, and 3) cross-comparison of SIF instrument configuration and retrieval methods. We found that agricultural row orientations can introduce a distinctive midday dip in SIF in absence of stress, because of a midday drop of absorbed photosynthetically active radiation (APAR) when crop row structures are north-south oriented. Such structures consequently influenced sub-canopy physiology, causing distinctive responses in both qL and NPQ at different positions within the vertical canopy that collectively influenced fluorescence quantum yield (FF) at the leaf scale. Once integrated at the canopy scale, diurnal dynamics of both APAR and e are critical for accurately shaping diurnal SIF variations. While sub-canopy qL and NPQ exhibited strong diurnal dynamics at the leaf level, their influence was greatly attenuated at the canopy scale due to opposing effects on SIF at different canopy layers. Furthermore, different system configurations and retrieval methods can bias the SIF magnitude and distort its diurnal shapes to different extents, therefore confounding the interpretation of inherent strength and dynamics of SIF emission. Our findings demonstrate the importance of agricultural row structures, the interactive variations in canopy structure and plant physiology, the instrument configuration, and the retrieval method in shaping the measured dynamics of diurnal SIF. This study highlights the necessity to account for these factors to accurately upscale satellite SIF from instantaneous to daily integrals and informs future synthesis work with different SIF instrumentation and retrieval methods across sites.