Solar‐induced chlorophyll fluorescence is strongly correlated with terrestrial photosynthesis for a wide variety of biomes: First global analysis based on OCO‐2 and flux tower observations

Abstract

Solar-induced chlorophyll fluorescence (SIF) has been increasingly used as a proxy for terrestrial gross primary productivity (GPP). Previous work mainly evaluated the relationship between satellite-observed SIF and gridded GPP products both based on coarse spatial resolutions. Finer resolution SIF (1.3 km × 2.25 km) measured from the Orbiting Carbon Observatory-2 (OCO-2) provides the first opportunity to examine the SIF-GPP relationship at the ecosystem scale using flux tower GPP data. However, it remains unclear how strong the relationship is for each biome and whether a robust, universal relationship exists across a variety of biomes. Here we conducted the first global analysis of the relationship between OCO-2 SIF and tower GPP for a total of 64 flux sites across the globe encompassing eight major biomes. OCO-2 SIF showed strong correlations with tower GPP at both midday and daily timescales, with the strongest relationship observed for daily SIF at the 757 nm (R = 0.72, p < 0.0001). Strong linear relationships between SIF and GPP were consistently found for all biomes (R = 0.57-0.79, p < 0.0001) except evergreen broadleaf forests (R = 0.16, p < 0.05) at the daily timescale. A higher slope was found for C grasslands and croplands than for C ecosystems. The generally consistent slope of the relationship among biomes suggests a nearly universal rather than biome-specific SIF-GPP relationship, and this finding is an important distinction and simplification compared to previous results. SIF was mainly driven by absorbed photosynthetically active radiation and was also influenced by environmental stresses (temperature and water stresses) that determine photosynthetic light use efficiency. OCO-2 SIF generally had a better performance for predicting GPP than satellite-derived vegetation indices and a light use efficiency model. The universal SIF-GPP relationship can potentially lead to more accurate GPP estimates regionally or globally. Our findings revealed the remarkable ability of finer resolution SIF observations from OCO-2 and other new or future missions (e.g., TROPOMI, FLEX) for estimating terrestrial photosynthesis across a wide variety of biomes and identified their potential and limitations for ecosystem functioning and carbon cycle studies.

A global spatially Continuous Solar Induced Fluorescence (CSIF) dataset using neural networks

“…This higher \77/^_`2 value for ENF was also suggested by the comparison between OCO-2 SIF and FluxCom GPP dataset (Sun et al, 2018) and other comparisons using GOSAT SIF (Guanter et al, 2012). In consistent with (Li et al, 2018b), we also found small cross-biome 20 variation of GPP-SIF relationship. However, a large within-biome variation of \77/^_`2 is also found, which contributes to a large proportion of the observed cross-sites variations rather than the cross-biome variation.…”

High‐Resolution Global Contiguous SIF of OCO‐2

“…, Li, Xiao, He, Altaf Arain, et al (2018), and Sun et al (2017) conducted such investigations but used a limited number of EC sites, because the concurrent availability and consistent spatial footprints between OCO-2 SIF and EC sites were strongly limited by OCO-2's orbital gaps. First, its high-resolution and contiguous coverage will improve synergistic use of OCO-2 SIF with GPP from EC towers, such as Fluxnet and Long-Term Agroecosystem Research (LTAR) networks, facilitating mechanistic investigation of SIF-GPP relationships.…”

Varying Contributions of Drivers to the Relationship Between Canopy Photosynthesis and Far‐Red Sun‐Induced Fluorescence for Two Maize Sites at Different Temporal Scales

“…SIF has been found strongly correlated with the gross primary production (GPP) in a quasi‐linear pattern at large spatial and temporal scales (Frankenberg et al, ; Guanter et al, ; Li et al, ; Miao et al, ; Wood et al, ; Yang et al, ; K. Yang, Ryu, et al, ; Yang et al, ). Some studies suggest the potential of a universal GPP:SIF relationship across various biomes (Frankenberg et al, ; Li et al, ; Sun et al, ; but see Zhang et al, ), and others used SIF to constrain photosynthetic parameters (Guan et al, ; Perez‐Priego et al, ; Zhang et al, ; Zhang et al, ).…”

“…Likewise, SIF is the product of PAR in , fPAR, and fluorescence yield (SIF y ), in which the SIF y is an apparent parameter (equation ). Theoretically, SIF y can be defined as the product (SIF y = Φ f × f esc ) of the actual SIF y at the photosystem scale (Φ f ) and the escape ratio (f esc ) from the canopy (Guanter et al, ; Li et al, ). Practically, it is usually calculated by dividing SIF by PAR in ×fPAR (i.e., APAR).…”