Linking canopy scattering of far-red sun-induced chlorophyll fluorescence with reflectance

Remotely sensed sun-induced chlorophyll fluorescence (SIF) has been used as an indicator of global terrestrial vegetation photosynthesis. The connection between SIF and photosynthesis allows its use for improving estimates of gross primary production (GPP) and monitoring plant stress. In these analyses, up-scaling of the relationship between SIF and photosynthesis from the photosynthetic level to the canopy, regional or global scale has been one of the main challenges. The scaling is strongly affected by the radiative transfer of emitted SIF, notably scattering and re-absorption of SIF. It is essential to understand these processes in order to differentiate effects of canopy structural variation from effects of photosynthesis functional variation on SIF. In this study, we derive the relationship between canopy scattering of SIF and top-of-canopy (TOC) reflectance analytically, by investigating the radiative transfer of incident light and emitted SIF. The similarity of radiative transfer of intercepted incident light and emitted SIF results in a simple relationship between reflectance and canopy scattering of SIF. In particular, we find that the ratio of far-red reflectance (R) to the product of canopy interceptance (i0) and leaf albedo (ω) is an accurate estimate of canopy scattering of far-red SIF (i.e., σFC=Ri0ω). SCOPE model simulations are used to validate our findings. The relationship we found provides an easy and accurate approach for rapid decoupling canopy structural and functional regulation of SIF, and correction of SIF for bidirectional effects. This will improve estimates of canopy photosynthesis from SIF.