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

Authors: WU, GENGJONG - University Of Illinois; JIANG, CHONGYA - University Of Illinois; KIMM, HYUNGSUK - University Of Illinois; WANG, SHENG - University Of Illinois; Bernacchi, Carl; MOORE, CAITLIN - University Of Western Australia; SUYKER, ANDY - University Of Nebraska; YANG, XI - University Of Virginia; MAGNEY, TROY - University Of California, Davis; FRANKENBERG, CHRISTIAN - California Institute Of Technology; RYU, YOUNGRYEL - Seoul National University; DECHANT, BENJAMIN - Seoul National University; GUAN, KAIYU - University Of Illinois

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2022
Publication Date: 9/15/2022
Citation: Wu, G., Jiang, C., Kimm, H., Wang, S., Bernacchi, C.J., Moore, C.E., Suyker, A., Yang, X., Magney, T., Frankenberg, C., Ryu, Y., Dechant, B., Guan, K. 2022. Difference in seasonal peak timing of soybean far-red SIF and GPP explained by canopy structure and chlorophyll content. Remote Sensing of Environment. 279. Article 113104. https://doi.org/10.1016/j.rse.2022.113104.
DOI: https://doi.org/10.1016/j.rse.2022.113104

Interpretive Summary: As sunlight is absorbed by plants, the first step in using photosynthesis to store the energy in sunlight to grow, some of that absorbed light is given off again by the leaves as light. This light given off by leaves is called chlorophyll fluorescence. While this light is impossible to observe with the naked eye, sensors now exist that can measure this light and use the measurements to understand how much photosynthesis the plants are doing. However, there remains a significant amount of uncertainty about how photosynthesis and fluorescence changes during the day. This research sought to measure photosynthesis from old techniques the measure carbon and from new techniques using fluorescence measurements, but to do so over a longer period of time and understand how the timing of both might change. The results indicate that there are differences in timing between the two, but determined that these differences were driven by how many leaves were present, the amount of the pigment that absorbs sunlight (chlorophyll) and how the leaves are angled in the plant field. These results advance understanding of how fluorescence measurements can be used to understand photosynthesis better.

Technical Abstract: Recent advances in remotely sensed solar-induced chlorophyll 'uorescence (SIF) have provided an unprecedented opportunity for studying 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 SIF and GPP in soybean fields by integrating tower data, satellite data, and process-based SCOPE model simulations. We found inconsistency between the seasonal peak timing of SIF and GPP in three of four soybean fields based on tower SIF and eddy-covariance measurements. In particular, SIF reached its seasonal maximum 12–25 days earlier than GPP. This SIF-GPP difference in peak timing degraded the correlation between sunny-day SIF and GPP at daily scale (Pearson r=0.70 at the site with 25 days difference and Pearson r=0.90 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 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 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 SIF. More horizontal 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.