Diurnal and Seasonal Variations in Chlorophyll Fluorescence Associated with Photosynthesis at Leaf and Canopy Scales

Abstract

There is a critical need for sensitive remote sensing approaches to monitor the parameters governing photosynthesis, at the temporal scales relevant to their natural dynamics. The photochemical reflectance index (PRI) and chlorophyll fluorescence (F) offer a strong potential for monitoring photosynthesis at local, regional, and global scales, however the relationships between photosynthesis and solar induced F (SIF) on diurnal and seasonal scales are not fully understood. This study examines how the fine spatial and temporal scale SIF observations relate to leaf level chlorophyll fluorescence metrics (i.e., PSII yield, YII and electron transport rate, ETR), canopy gross primary productivity (GPP), and PRI. The results contribute to enhancing the understanding of how SIF can be used to monitor canopy photosynthesis. This effort captured the seasonal and diurnal variation in GPP, reflectance, F, and SIF in the O2A (SIFA) and O2B (SIFB) atmospheric bands for corn (Zea mays L.) at a study site in Greenbelt, MD. Positive linear relationships of SIF to canopy GPP and to leaf ETR were documented, corroborating published reports. Our findings demonstrate that canopy SIF metrics are able to capture the dynamics in photosynthesis at both leaf and canopy levels, and show that the relationship between GPP and SIF metrics differs depending on the light conditions (i.e., above or below saturation level for photosynthesis). The sum of SIFA and SIFB (SIFA+B), as well as the SIFA+B yield, captured the dynamics in GPP and light use efficiency, suggesting the importance of including SIFB in monitoring photosynthetic function. Further efforts are required to determine if these findings will scale successfully to airborne and satellite levels, and to document the effects of data uncertainties on the scaling.

A Statistical Model for Estimating Midday NDVI from the Geostationary Operational Environmental Satellite (GOES) 16 and 17

“…Sites with sparse vegetation (e.g., shrubland systems) appeared to not as consistently have the same diurnal shape ( Figure S9). This could potentially be due to the low leaf area index (LAI) in shrublands contributing to a higher influence from the background soil, which has been found to decrease the midday NDVI values [27] and is shown in supplementary Figure S9. Campbell et al [27] found, however, that this decrease in midday NDVI values did not occur for areas with higher LAI values.…”

“…This could potentially be due to the low leaf area index (LAI) in shrublands contributing to a higher influence from the background soil, which has been found to decrease the midday NDVI values [27] and is shown in supplementary Figure S9. Campbell et al [27] found, however, that this decrease in midday NDVI values did not occur for areas with higher LAI values. NDVI values at shrubland sites appear to be less noisy due to having characteristically less clouds and precipitation and a more comprehensive statistical framework, like the one presented here, might not be as necessary, though further work needs to be done on this.…”

“…Radiances were converted to reflectance factors using information in the GOES R Product Definition and Users' Guide (PUG; pp [27][28]. In summary, radiances were converted to reflectance factors by multiplying by the kappa factor, which is provided in each data netcdf file and differs between channels and different satellite orbits (i.e., pre and post operational orbits).…”

A Spectral Fitting Algorithm to Retrieve the Fluorescence Spectrum from Canopy Radiance

“…The F INT ranges between an average value of 14 mW m −2 sr −1 up to more than 250 mW m −2 sr −1 nm −1 when the canopy is mature. The overall evolution of F across the growing season primarily responds to the illumination level and to the amount of green biomass in the canopy, especially to the chlorophyll content during the different growing stages [14,53,60]. The average value of downwelling radiance computed in the 400-700 nm spectral windows shows an increasing trend, related to changes in solar zenith angle across the season (from February to May).…”

“…Joiner et al, (2016) [10] and Wolanin et al, (2015) [11] were the only attempts to retrieve red F from GOSAT and SCIAMACHY, respectively. Conversely, studies based on measurements collected by field spectrometers [12][13][14][15][16] and airborne high-resolution imaging spectroscopy [17][18][19][20][21][22] more frequently considered both the red and far-red F.…”