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Downscaling of far-red solar-induced chlorophyll fluorescence of different crops from canopy to leaf level using a diurnal data set acquired by the airborne imaging spectrometer HyPlant - PubMed

Downscaling of far-red solar-induced chlorophyll fluorescence of different crops from canopy to leaf level using a diurnal data set acquired by the airborne imaging spectrometer HyPlant

Bastian Siegmann et al. Remote Sens Environ. 2021 Oct.

Abstract

Remote sensing-based measurements of solar-induced chlorophyll fluorescence (SIF) are useful for assessing plant functioning at different spatial and temporal scales. SIF is the most direct measure of photosynthesis and is therefore considered important to advance capacity for the monitoring of gross primary production (GPP) while it has also been suggested that its yield facilitates the early detection of vegetation stress. However, due to the influence of different confounding effects, the apparent SIF signal measured at canopy level differs from the fluorescence emitted at leaf level, which makes its physiological interpretation challenging. One of these effects is the scattering of SIF emitted from leaves on its way through the canopy. The escape fraction ( f esc ) describes the scattering of SIF within the canopy and corresponds to the ratio of apparent SIF at canopy level to SIF at leaf level. In the present study, the fluorescence correction vegetation index (FCVI) was used to determine f esc of far-red SIF for three structurally different crops (sugar beet, winter wheat, and fruit trees) from a diurnal data set recorded by the airborne imaging spectrometer HyPlant. This unique data set, for the first time, allowed a joint analysis of spatial and temporal dynamics of structural effects and thus the downscaling of far-red SIF from canopy ( SIF 760 canopy ) to leaf level ( SIF 760 leaf ). For a homogeneous crop such as winter wheat, it seems to be sufficient to determine f esc once a day to reliably scale SIF760 from canopy to leaf level. In contrast, for more complex canopies such as fruit trees, calculating f esc for each observation time throughout the day is strongly recommended. The compensation for structural effects, in combination with normalizing SIF760 to remove the effect of incoming radiation, further allowed the estimation of SIF emission efficiency ( ε SIF ) at leaf level, a parameter directly related to the diurnal variations of plant photosynthetic efficiency.

Keywords: Diurnal course; FCVI; Fluorescence correction vegetation index; Fluorescence escape fraction; HyPlant; Photosynthetically active radiation; SIF; Solar-induced chlorophyll fluorescence.

© 2021 The Authors.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1

Airborne image of the agricultural research station Campus Klein-Altendorf and meteorological measurements recorded at the time of the overflights. (a) HyPlant DUAL top-of-canopy (TOC) true-colour composite (RGB 640/550/460 nm) of the campus acquired on June 29th 2018 with dashed lines highlighting the locations of the sugar beet (SB-I – IV, red) and winter wheat fields (WW-I – V, orange) as well as the fruit tree parcels (1–8, blue). Background: Sentinel-2 (Band 8) from June 27th 2018. (b) Photosynthetic active radiation (PAR), (c) air temperature and (d) relative humidity measurements recorded by the campus-internal weather station in the period from June 26th–29th 2018. The vertical red dashed lines indicate the time points of the six HyPlant overflights. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2
Fig. 2

Spatial dynamics of canopy SIF760 (SIF760canopy) of different crops in the course of the day. The dashed lines highlight the locations of the investigated sugar beet (red) and winter wheat fields (orange) as well as the investigated parcels of the fruit orchard (blue). Background: Sentinel-2 (Band 8) from June 27th 2018. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3
Fig. 3

Box plot of the median, 0th, 25th, 75th and 100th percentiles showing the diurnal course of canopy (SIF760canopy) and leaf SIF760 (SIF760leaf) (a), fluorescence correction vegetation index (FCVI) (b), fraction of absorbed photosynthetically active radiation by leaf chlorophyll (fAPARchl) (c), SIF760 escape fraction (fesc) (d) and SIF760 emission efficiency as a function of APARchl (εSIF(FCVI)) (e) of sugar beet (red), winter wheat (orange) and fruit trees (blue). The vertical dashed grey lines indicate the time of solar culmination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4
Fig. 4

Spatial dynamic of the SIF760 escape fraction (fesc) of different crops in the course of the day. The dashed lines highlight the locations of the investigated sugar beet (red) and winter wheat fields (orange) as well as the investigated parcels of the fruit orchard (blue). Background: Sentinel-2 (Band 8) from June 27th 2018. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5
Fig. 5

Spatial dynamics of leaf SIF760 (SIF760leaf) of different crops in the course of the day. The dashed lines highlight the locations of the investigated sugar beet (red) and winter wheat fields (orange) as well as the investigated parcels of the fruit orchard (blue). Background: Sentinel-2 (Band 8) from June 27th 2018. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6
Fig. 6

Correlation coefficients calculated for corresponding pixels of the canopy (SIF760canopy) and leaf SIF760 (SIF760leaf) maps derived from the six HyPlant overflights. Averaged correlation coefficients and standard deviations calculated for the three observed crops (sugar beet, winter wheat and fruit trees) (a), correlation coefficients for the individual sugar beet fields (SB-I – SB-IV) (b), correlation coefficients for the individual winter wheat fields (WW-I – WW-V) (c) and correlation coefficients for the individual fruit tree parcels (FT-1 – FT-8) (d). The vertical grey dashed lines indicate the time points of the six HyPlant overflights. The symbols used for the different crop fields and parcels were plotted with slight time offsets for a better overview. The light grey bars indicate to which overflights the different symbols belong.

Fig. 7
Fig. 7

Spatial and temporal dynamics of sugar beet field SB-I. Leaf chlorophyll content (LCC) (a) and leaf area index (LAI) map (b) derived from HyPlant DUAL data recorded at 12:30. SIF760 escape fraction (fesc) derived from HyPlant DUAL data recorded at 12:30, 14:40 and 15:50 (c). Canopy (SIF760canopy) (d) and leaf SIF760 (SIF760leaf) maps (e) derived from HyPlant FLUO data recorded at 12:30, 14:40 and 15:50. Scatterplots of SIF760canopy and SIF760leaf for the three time points (f). Background of the maps: Sentinel-2 (Band 8) from June 27th 2018.

Fig. 8
Fig. 8

Plots of mean and standard deviation showing the diurnal dynamic of SIF760 emission efficiency as a function of PAR (εSIFPAR) (a-c) and as a function of APARchl (εSIFFCVI) (d-f) for sugar beet (red), winter wheat (orange) and fruit trees (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9
Fig. 9

Spatial dynamics of the SIF760 emission efficiency as a function of APARchl (εSIFFCVI) of different crops in the course of the day. The dashed lines highlight the locations of the investigated sugar beet (red) and winter wheat fields (orange) as well as the investigated parcels of the fruit orchard (blue). Background: Sentinel-2 (Band 8) from June 27th 2018. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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