Difference in seasonal peak timing of soybean far-red SIF and GPP explained by canopy structure and chlorophyll content

Authors

Genghong Wu, University of Illinois Urbana-Champaign
Chongya Jiang, University of Illinois Urbana-Champaign
Hyungsuk Kimm, University of Illinois Urbana-Champaign
Sheng Wang, University of Illinois Urbana-Champaign
Carl Bernacchi, USDA ARS
Caitlin E. Moore, University of Illinois Urbana-Champaign
Andy Suyker, School of Natural Resources
Xi Yang, University of Virginia
Troy Magney, University of California, Davis
Christian Frankenberg, Division of Geological and Planetary Sciences
Youngryel Ryu, Research Institute of Agricultural and Life Science
Benjamin Dechant, Research Institute of Agricultural and Life Science
Kaiyu Guan, University of Illinois Urbana-Champaign

Document Type

Article

Date of this Version

9-15-2022

Citation

Remote Sensing of Environment 279 (2022) 113104

doi:10.1016/j.rse.2022.113104

Comments

U.S. government work

Abstract

Recent advances in remotely sensed solar-induced chlorophyll fluorescence (SIF) have provided an exciting and promising opportunity for estimating gross primary production (GPP). Previous studies mainly focused on the linear correlation between SIF and GPP and the slope of the SIF-GPP relationship, both of which lack rigorous consideration of the seasonal trajectories of SIF and GPP. Here, we investigated the timing of seasonal peaks of far-red SIF and GPP in soybean fields by integrating tower data, satellite data, and process-based Soil Canopy Observation of Photosynthesis and Energy (SCOPE, v2.0) model simulations. We found inconsistency between the seasonal peak timing of far-red SIF and GPP in three of four soybean fields based on tower far-red SIF and eddy-covariance measurements. In particular, far-red SIF reached its seasonal maximum 14–17 days earlier than GPP. This far-red SIF-GPP difference in peak timing degraded the correlation between sunny-day far-red SIF and GPP at daily scale (Pearson r = 0.83–0.87 at the site with 14–17 days difference and Pearson r = 0.96 at the site with no difference), and it can be explained by a divergence in the seasonality between absorbed photosynthetic active radiation (APAR) and canopy chlorophyll content (ChlCanopy). We found that the seasonality of far-red SIF - a byproduct of the light reactions of photosynthesis - was primarily controlled by APAR, whereas GPP seasonality was dominated by ChlCanopy. Further, SCOPE model simulations showed that the seasonal patterns of leaf area index (LAI), leaf chlorophyll content (ChlLeaf) and leaf angle distribution (LAD) could affect the different peak timing of SIF and GPP and consequently the seasonal relationship between far-red SIF and GPP. A further increase in LAI after the fraction of light absorption (FPAR) saturates and a later peak of ChlLeaf compared to LAI results in a later peak of GPP compared to far-red SIF. More horizontal leaf angles can further exacerbate this difference. Our results advance mechanistic understanding of the SIF-GPP relationships and combining chlorophyll content information with SIF could potentially improve remote-sensing-based GPP estimation.