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

Authors

Genghong Wu, University of Illinois at Urbana-Champaign
Kaiyu Guan, University of Illinois at Urbana-Champaign, DOE Center for Advanced Bioenergy and Bioproducts Innovation
Chongya Jiang, University of Illinois at Urbana-Champaign, DOE Center for Advanced Bioenergy and Bioproducts Innovation
Hyungsuk Kimm, University of Illinois at Urbana-Champaign, Seoul National University
Guofang Miao, University of Illinois at Urbana- Champaign
Xi Yang, University of Virginia
Carl J. Bernacchi, University of Illinois at Urbana-Champaign, USDA-ARS
Xiangmin Sun, University of Illinois at Urbana-Champaign
Andrew E. Suyker, University of Nebraska - LincolnFollow
Caitlin E. Moore, University of Illinois at Urbana-Champaign, University of Western Australia

Date of this Version

5-27-2023

Citation

Agricultural and Forest Meteorology 338 (2023) 109532. https://doi.org/10.1016/j.agrformet.2023.109532

Comments

Used by permission.

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 siteyears 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. 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. Different VIs, EC footprint models, and satellite data led to marginal differences in the SIF-GPP relationships when upscaling nadir SIF to EC footprint. Our study provided a methodological framework to correct this spatial mismatch between ground nadir SIF and GPP observations for croplands and potentially for 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.