Response of Canopy Solar-Induced Chlorophyll Fluorescence to the Absorbed Photosynthetically Active Radiation Absorbed by Chlorophyll

Solar-induced chlorophyll fluorescence (SIF), which can be used as a novel proxy for estimating gross primary production (GPP), can be effectively retrieved using ground-based, airborne and satellite measurements. Absorbed photosynthetically active radiation (APAR) is the key bridge linking SIF and GPP. Remotely sensed SIF at the canopy level (SIFcanopy) is only a part of the total SIF emission at the photosystem level. An SIF-based model for GPP estimation would be strongly influenced by the fraction of SIF photons escaping from the canopy ( fesc). Understanding the response of SIFcanopy to the absorbed photosynthetically active radiation absorbed by chlorophyll (APARchl) is a key step in estimating GPP but, as yet, this has not been well explored. In this study, we aim to investigate the relationship between remotely sensed SIFcanopy and APARchl based on simulations made by the Soil Canopy Observation Photosynthesis Energy fluxes (SCOPE) model and field measurements. First, the ratio of the fraction of the absorbed photosynthetically active radiation absorbed by chlorophyll (fPARchl) to the fraction of absorbed photosynthetically active radiation absorbed by green leaves (fPARgreen) is investigated using a dataset simulated by the SCOPE model. The results give a mean value of 0.722 for Cab at 5 μg cm−2, 0.761 for Cab at 10 μg cm−2 and 0.795 for other Cab content (ranging from 0.71 to 0.81). The response of SIFcanopy to APARchl is then explored using simulations corresponding to different biochemical and biophysical conditions and it is found that SIFcanopy is well correlated with APARchl . At the O2-A band, for a given plant type, the relationship between SIFcanopy and APARchl can be approximately expressed by a linear statistical model even for different values of the leaf area index (LAI) and chlorophyll content, whereas the relationship varies with the LAI and chlorophyll content at the O2-B band. Finally, the response of SIFcanopy to APARchl for different leaf angle distribution (LAD) functions is investigated using field observations and simulations; the results show that fesc is larger for a planophile canopy structure. The values of the ratio of SIFcanopy to APARchl are 0.0092 ± 0.0020, 0.0076 ± 0.0036 and 0.0052 ± 0.0004 μm−1 sr−1 for planophile vegetables/crops, planophile grass and spherical winter wheat, respectively, at the O2-A band. At the O2-B band, the ratios are 0.0063± 0.0014, 0.0049± 0.0030 and 0.0033 ± 0.0004 μm−1 sr−1, respectively. The values of this ratio derived from observations agree with simulations, giving values of 0.0055 ± 0.0002 and 0.0068 ± 0.0001 μm−1 sr−1 at the O2-A band and 0.0032 ± 0.0002 and 0.0047 ± 0.0001 μm−1 sr−1 at the O2-B band for spherical and planophile canopies, respectively. Therefore, both the simulations and observations confirm that the relationship between SIFcanopy and APARchl is species-specific and affected by biochemical components and canopy structure, especially at the O2-B band. It is also very important to correct for reabsorption and scattering of the SIF radiative transfer from the photosystem to the canopy level before the remotely sensed SIFcanopy is linked to the GPP. Remote Sens. 2017, 9, 911; doi:10.3390/rs9090911 www.mdpi.com/journal/remotesensing Remote Sens. 2017, 9, 911 2 of 19