Can upscaling ground nadir SIF to eddy covariance footprint improve the relationship between SIF and GPP in croplands

Authors: WU, GENGHONG - University Of Illinois; GUAN, KAIYU - University Of Illinois; JIANG, CHONGYA - University Of Illinois; KIMM, HYUNGSUK - University Of Illinois; MIAO, GUOFANG - University Of Illinois; YANG, XI - University Of Virginia; Bernacchi, Carl; SUN, XIANGMIN - University Of Illinois; SUYKER, ANDREW - University Of Illinois; MOORE, CAITLIN - University Of Western Australia

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2023
Publication Date: 5/27/2023
Citation: Wu, G., Guan, K., Jiang, C., Kimm, H., Miao, G., Yang, X., Bernacchi, C.J., Sun, X., Suyker, A.E., Moore, C.E. 2023. Can upscaling ground nadir SIF to eddy covariance footprint improve the relationship between SIF and GPP in croplands. Agricultural and Forest Meteorology. 338. Article 109532. https://doi.org/10.1016/j.agrformet.2023.109532.
DOI: https://doi.org/10.1016/j.agrformet.2023.109532

Interpretive Summary: Plants absorb sunlight to drive photosynthesis, which is the process by which plants take up carbon dioxide and convert it to growth. Measuring photosynthesis is difficult, especially for fields of plants such as crops. One method to perform this measurement is to use sensors that directly measure carbon dioxide going into or out of fields. This is expensive and difficult to perform. A newer method is through the use of sensors that measure how much light plants in a field emit, which is called solar induced fluorescence. These measurements can be done with satellites, but there are many corrections, errors, and uncertainties that must be resolved before it can be used widely. Comparing these two methods can help to overcome these challenges, and that is what this research seeks to do. Specifically, the carbon dioxide measurements cover large fields and the light measurements represent very small areas. This work addresses this mismatch in measurement scale to quantify how important it is. Our results show that this mismatch in measurement scale is not terribly important in fields that are highly homogeneous, such as crop canopies.

Technical Abstract: Ground solar-induced chlorophyll fluorescence (SIF) is important for the mechanistic understanding of the dynamics of vegetation gross primary production (GPP) at fine spatiotemporal scales. However, eddy covariance (EC) observations generally cover larger footprint areas than ground SIF observations (a bare fiber with nadir), and this footprint mismatch between nadir SIF and GPP could complicate the canopy SIF-GPP relationships. Here, we upscaled nadir SIF observations to EC footprint and investigated the change in SIF-GPP relationships after the upscaling in cropland. We included 13 site-years data in our study, with seven site-years corn, four site-years soybeans, and two site-years miscanthus, all located in the US Corn Belt. All sites’ crop nadir SIF observations collected from the automated FluoSpec2 system (a hemispheric-nadir system) were upscaled to the GPP footprint-based SIF using vegetation indices (VIs) calculated from high spatiotemporal satellite reflectance data. Compared to Chapter 2, the performances of different factors for the upscaling are compared in this chapter, including (1) satellite vegetation indices: Enhanced Vegetation Index (EVI) vs. Near-infrared Reflectance of Vegetation (NIRv), (2) EC footprint methods: Flux Footprint Prediction (FFP) vs. Simple Analytical Footprint model on Eulerian coordinates (SAFE)), and (3) satellite data sources: Sentinel-2 (S2) vs. PlanetScope (PS). We found that SIF-GPP relationships were not substantially changed after upscaling nadir SIF to GPP footprint at our crop sites planted with corn, soybean, and miscanthus, with R2 change after the upscaling ranging from -0.007 to 0.051 and root mean square error (RMSE) difference from -0.658 to 0.095 umol m-2 s-1 relative to original nadir SIF-GPP relationships across all the site-years. The variation of the SIF-GPP relationship within each species across different site-years was similar between the original nadir SIF and upscaled SIF. Overall, soil-adjusted NIRv and EVI calculated from PS data combined with EC footprint calculated from the SAFE model showed a larger increase of R2 compared to other cases for individual site-year. Our study provided a methodological framework to correct this spatial mismatch between ground nadir SIF and GPP observations for croplands and other ecosystems. Our results also demonstrated that the spatial mismatch between ground nadir SIF and GPP might not significantly affect the SIF-GPP relationship in cropland that are largely homogeneous.