Abstract
Solar-induced chlorophyll fluorescence (SIF), an electromagnetic signal that can potentially indicate vegetation
photosynthetic activity, can be retrieved from ground-based, airborne and satellite measurements. However, due
to the scattering and re-absorption effects inside the leaves and canopy, SIF measured at the canopy level is only
a small part of the total SIF emission at the photosystem level. Therefore, a downscaling mechanism of SIF from
the canopy level to the photosystem level is important for better understanding the relationship between SIF and
the vegetation gross primary production (GPP). In this study, firstly, we analyzed the canopy scattering effects
using a simple parameterization model based on the spectral invariant theory. The probability for SIF photons to
escape from the canopy was found to be related to the anisotropic spectral reflectance, canopy interception of the
upward solar radiation, and leaf absorption. An empirical approach based on a Random Forest (RF) regression
algorithm was applied to downscale SIF constrained by the red, red-edge and far-red anisotropic reflectance. The
RF was trained using simulations conducted with the Soil Canopy Observation, Photochemistry and Energy
fluxes (SCOPE) model. The performance of the SIF downscaling method was evaluated with SCOPE and Discrete
Anisotropic Radiative Transfer (DART) model simulations, ground measurements and airborne data. Results
show that estimated SIF at the photosystem level matches well with simulated reference data, and the relationship
between SIF and photosynthetically active radiation absorbed by chlorophyll is improved by SIF
downscaling. This finding in combination with other evaluation criteria suggests the downscaling of canopy SIF
as an efficient strategy to normalize species dependent effects of canopy structure and varying solar-view geometries.
Based on our results for the SIF-APAR relationship, we expect that such normalization approaches can
be helpful to improve estimates of photosynthesis using remote sensing measurements of SIF.
Liu, Xinjie