Far-red Solar-induced Chlorophyll Fluorescence Research Articles

Global retrieval of the spectrum of terrestrial chlorophyll fluorescence: First results with TROPOMI

Solar-Induced chlorophyll Fluorescence (SIF) could be used as an indicator of photosynthetic status due to the close relationship between SIF and the photosynthetic apparatus. Terrestrial SIF is emitted throughout the red and near-infrared spectrum and is characterized by two peaks centered around 685 nm and 740 nm, respectively. In this study, we present a data-driven approach to reconstruct the terrestrial SIF spectrum from measurements by TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 precursor mission. This approach makes use of solar Fraunhofer lines in the combined spectral windows devoid of strong atmospheric absorption features to retrieve SIF signal from the solar radiation reflected by the surface and atmosphere system. Information contents are mainly from the two windows close to the red and far-red SIF peaks, 663–686 nm and 743–758 nm. A linear forward model represented as an addition of the SIF-free radiance spectrum and the SIF component is proposed with a proper selection of its parameter settings. The SIF component was simulated as linear combinations of 2 basis SIF spectra. Through inverting the linear forward model, the SIF spectrum was retrieved from the solar radiation reflected by the surface and atmosphere system. The evaluation of the retrieval results is performed by inter-comparison with other SIF datasets. The comparisons display similar spatial distributions for the weekly global SIF composites for the first two weeks in June and December of 2019 and July and December of 2021. Especially the comparison of the far-red SIF datasets with other dedicated far-red SIF retrievals demonstrates close agreement, indicating consistency among the retrieval approaches. The reconstructed TROPOMI red SIF shows improved and more reasonable spatiotemporal distributions. The retrieval uncertainty for the weekly global composite is about 12% and 2% of the peak red and far-red SIF values, respectively, which can be considered as satisfactory error thresholds for global composites of SIF observations. Different spectral features for several typical biomes from reconstructed SIF spectra suggest that red and far-red SIF may carry complementary information on photosynthetic function and biophysical properties of the plant. For the first time, the reconstruction of the SIF spectrum is achieved for spaceborne measurements with the potential to open new applications for better understanding of the ecosystem function.

Water stress significantly affects the diurnal variation of solar-induced chlorophyll fluorescence (SIF): A case study for winter wheat

Remote sensing of Solar-induced chlorophyll fluorescence (SIF) has been widely used in estimating Gross Primary Productivity (GPP) and detecting stress in terrestrial ecosystems. Water stress adversely impacts the growth, development, and productivity of a plant. Recently, the characterizing and understanding of the diurnal cycling of plant functioning and ecosystem processes has been explored using SIF. However, the diurnal response of SIF to different levels of water stress remains unclear. This study conducted field experiments on winter wheat by subjecting it to different levels of water stress including well-watered (CK) and, mild, moderate, and severe water stress (D1, D2, D3), and collected the spectral data using an automated SIF measurement system. The results observed the strong SIF-PAR (photosynthetically active radiation) correlations and that these relationships gradually decoupled with increasing water stress, which further decreased the accuracy of temporal upscaling of far-red SIF from an instantaneous to daily scale. To quantify the characteristics of diurnal far-red SIF, five indices including peak time, peak value, curve opening coefficient (leading coefficient of the parabola), and left/right slopes of the peak were proposed. The results demonstrated that diurnal far-red SIF was characterized by an earlier peak time, decreasing peak value, wider curve opening, and flattening right slope from the CK plot to the D3 plot. There were certain mechanisms linking the different indices, for example, between peak size and opening coefficient. Furthermore, the response of far-red SIF to water stress was most pronounced at noon. SIF/PAR exhibited a more significant response to varying water stress compared to far-red SIF, which mitigated the negative influence of PAR variations on diurnal SIF. These findings contribute to the monitoring of plant water dynamics at fine temporal scales.

Analysing far-red SIF directional anisotropy of three structurally contrasting forest canopies towards improved GPP estimation

Solar-induced chlorophyll fluorescence (SIF) is used to estimate terrestrial gross primary production (GPP) due to its physiological link to photosynthesis. However, strong angular variations in satellite-observed SIF (SIFobs), especially for forest canopies with a high structural heterogeneity, hampers robust estimation of GPP. Here, we use directional SIFobs datasets from OCO-2 and TROPOMI satellite sensors and the 3D discrete anisotropic radiative transfer model to investigate the directional properties of far-red SIFobs and their relationships with two GPP products obtained from MODIS and FluxCom datasets for three European forest types with contrasting canopy structures, i.e., boreal, temperate and Mediterranean forests. We found bowl-like angular distribution patterns of SIFobs in both observed and simulated boreal and temperate forests, but no generic distribution pattern of SIFobs was found for the Mediterranean forest. In our GPP estimation of boreal forest, oblique SIFobs performed better than the widely used nadir SIFobs. However, we did not find an optimal view angle suitable for all three forest types. Further, the angular dependencies and forest structure impacts present in SIFobs were found to still propagate into the estimation of total emitted SIF (SIFtotal), which generally regarded as free of these confounding effects. Finally, our results demonstrate that two commonly used variables, i.e., SIFobs with a constant viewing angle and the approximated SIFtotal, both fail to produce a robust relationship with GPP for the Mediterranean forest. These findings highlight the importance of forest canopy structural heterogeneity, including forest floor, on interpretation of directional satellite SIF observations, even when corrected and expressed as a total canopy SIF emission.

Red solar-induced chlorophyll fluorescence as a robust proxy for ecosystem-level photosynthesis in a rice field

Terrestrial ecosystem photosynthesis plays a crucial role in global carbon cycling. Solar-induced chlorophyll fluorescence (SIF) is a state-of-the-art proxy for ecosystem photosynthesis. Although satellite-derived SIF correlates with gross primary productivity (GPP), it needs to be complemented with ground-based measurements to better understand the dynamics of carbon uptake by plants, because of the coarse spatiotemporal resolution of satellite-based data. We used an ultrafine-resolution spectrometer to detect both red SIF (SIFred, 687 nm) and far-red SIF (SIFfar-red, 760 nm) at ground level. Then, we investigated how well they represented eddy-tower-derived GPP in a rice field of central Japan during the mid to late growing season in 2019. Half-hourly and daily averaged SIFred showed stronger correlation with GPP (R2 = 0.61 and 0.83, respectively) than SIFfar-red (R2 = 0.42 and 0.57, respectively). Stronger seasonal correlation with GPP was observed for canopy-escaping SIF than for total SIF. The SIFred/SIFfar-red ratio was not significantly correlated with seasonal changes in the normalized difference vegetation index or in the pigment-specific simple ratios for chlorophyll-a and -b. There were no clear differences in the SIFred–GPP relationships between sunny and cloudy conditions. We concluded that SIFred could be an equivalent or better proxy than SIFfar-red for ecosystem-level photosynthesis. We further postulated that SIF emission is mainly controlled by plant physiology during the mid to late growing season, when temporal variations of the canopy structure in rice ecosystems are small.

Exploring the Sensitivity of Solar-Induced Chlorophyll Fluorescence at Different Wavelengths in Response to Drought

Due to the mechanistic coupling between solar-induced chlorophyll fluorescence (SIF) and photosynthesis, SIF has an advantage over greenness-based vegetation indices in detecting drought. Since photosystem I (PSI) contributes very little to red SIF, red SIF is assumed to be more responsive to environmental stress than far-red SIF. However, in addition to affecting photosynthesis, drought also has an impact on vegetation chlorophyll concentration and thus affects the reabsorption process of red SIF. When these responses are entangled, the sensitivity of SIF in the red and far-red regions in response to drought is not yet clear. In this study, we conducted a water stress experiment on maize in the field and measured the upward and downward leaf SIF spectra by a spectrometer assembled with a leaf clip. Simultaneously, leaf-level active fluorescence was measured with a pulse-amplified modulation (PAM) fluorometer. We found that SIF, after normalization by photosynthetically active radiation (PAR) and dark-adapted minimal fluorescence (Fo), is a better estimation of SIF yield. By comparing the wavelength-dependent link between SIF yield and nonphotochemical quenching (NPQ) across the range of 660 to 800 nm, the results show that red SIF and far-red SIF have different sensitivities in response to drought. SIF yield in the far-red region has a strong and stable correlation with NPQ. Drought not only reduces red SIF due to photosynthetic regulation, but it also increases red SIF by reducing chlorophyll content (weakening the reabsorption effect). The co-existence of these two contradictory effects makes the red SIF of leaf level unable to reliably indicate NPQ. In addition, the red:far-red ratio of downward SIF and the ratio between the downward SIF and upward SIF at the red peak can be good indicators of chlorophyll content. These findings can help to interpret SIF variations in remote sensing techniques and fully exploit SIF information in red and far-red regions when monitoring plant water stress.

HyScreen: A Ground-Based Imaging System for High-Resolution Red and Far-Red Solar-Induced Chlorophyll Fluorescence

Solar-induced chlorophyll fluorescence (SIF) is used as a proxy of photosynthetic efficiency. However, interpreting top-of-canopy (TOC) SIF in relation to photosynthesis remains challenging due to the distortion introduced by the canopy's structural effects (i.e., fluorescence re-absorption, sunlit-shaded leaves, etc.) and sun-canopy-sensor geometry (i.e., direct radiation infilling). Therefore, ground-based, high-spatial-resolution data sets are needed to characterize the described effects and to be able to downscale TOC SIF to the leafs where the photosynthetic processes are taking place. We herein introduce HyScreen, a ground-based push-broom hyperspectral imaging system designed to measure red (F687) and far-red (F760) SIF and vegetation indices from TOC with single-leaf spatial resolution. This paper presents measurement protocols, the data processing chain and a case study of SIF retrieval. Raw data from two imaging sensors were processed to top-of-canopy radiance by dark-current correction, radiometric calibration, and empirical line correction. In the next step, the improved Fraunhofer line descrimination (iFLD) and spectral-fitting method (SFM) were used for SIF retrieval, and vegetation indices were calculated. With the developed protocol and data processing chain, we estimated a signal-to-noise ratio (SNR) between 50 and 200 from reference panels with reflectance from 5% to 95% and noise equivalent radiance (NER) of 0.04 (5%) to 0.18 (95%) mW m-2 sr-1 nm-1. The results from the case study showed that non-vegetation targets had SIF values close to 0 mW m-2 sr-1 nm-1, whereas vegetation targets had a mean F687 of 1.13 and F760 of 1.96 mW m-2 sr-1 nm-1 from the SFM method. HyScreen showed good performance for SIF retrievals at both F687 and F760; nevertheless, we recommend further adaptations to correct for the effects of noise, varying illumination and sensor optics. In conclusion, due to its high spatial resolution, Hyscreen is a promising tool for investigating the relationship between leafs and TOC SIF as well as their relationship with plants' photosynthetic capacity.

Contributions of the understory and midstory to total canopy solar-induced chlorophyll fluorescence in a ground-based study in conjunction with seasonal gross primary productivity in a cool-temperate deciduous broadleaf forest

Solar-induced chlorophyll fluorescence (SIF) can represent gross primary productivity (GPP) in many types of terrestrial vegetation. In principle, the chlorophyll-a fluorescence signal responds to the amount of light absorption and the fraction of energy distribution in photosystems. Therefore, it is mechanistically linked with CO2 assimilation. Recently, radiative transfer models have traced the processes of emission, interception and reabsorption within a canopy in heterogeneous tree stands. These processes influence the expected relationship between SIF and GPP. We inferred that the vertical profile of SIF could reveal those processes. In addition, the sum of SIF emission from different pathways within the canopy must be equivalent to the total SIF emission from all leaves. The purposes of this study were: (1) to clarify the seasonal and diurnal variations of ground-based SIF observed above the overstory, midstory and understory layers (strata); (2) to examine whether the sum of SIF observed from the three layers could represent total SIF emissions, and (3) to examine whether SIF above the understory could reveal the photosynthetic activity within the vertical profile of a forest. To estimate how much SIF was present after its emission from leaves within the canopy, we conducted spectral radiation observations in three vertical layers: above the understory, midstory and overstory (8, 14 and 18 m from the ground, respectively) to retrieve the red and far-red SIF in a deciduous broadleaf forest ecosystem in Japan from April to November 2020. We found that SIF above the overstory and the sum of SIF from the three layers increased sharply at the leaf-onset of the overstory and was correlated with each season's GPP. The sum of far-red SIF from the three layers was proportional to the total SIF estimated using the escape ratio approach for SIF above the overstory. In this study we demonstrated that SIF above the understory could detect the increased photosynthesis of the understory in spring, compared with the GPP and the vertical profile of the CO2 concentrations. In summary, SIF observed in three forest layers detected the seasonal change of GPP within the canopy. Thus, a multi-layer approach can be used to understand the relationship between SIF and photosynthesis within the canopy.

Temporally corrected long-term satellite solar-induced fluorescence leads to improved estimation of global trends in vegetation photosynthesis during 1995–2018

Terrestrial gross primary production (GPP) is one of the largest flux of carbon cycle and also introduces the largest uncertainties of the estimated global carbon budget. During the past decades, the emerging remotely-sensed solar-induced chlorophyll fluorescence (SIF) data have been rapidly developed as the novel proxy of GPP. Though satellite SIF datasets have been widely used for plenty of climatic and ecological studies, a lack of consistent long-term satellite SIF data has hindered its use for long-term assessments due to their temporal inconsistency induced by sensor degradation. Here, based on the time series of SIF data at the Sahara Desert as the benchmark, we present a practical approach to correct the temporal inconsistency of different satellites (GOME, SCIAMACHY and GOME-2) far-red SIF data. The temporally corrected SIF datasets were then spatially downscaled to a finer resolution of 0.05° with the light-use-efficiency (LUE) method. Then, the individual time series of SIF data from the three satellite sensors were fused together to generate a continuously long-term consistent global SIF dataset from July 1995 to December 2018 (termed as LT_SIFc*). We further validated and compared the spatial and temporal characteristics of LT_SIFc* with the ground-based long-term GPP observations from 15 flux sites, modelled global long-term GPP time series, and high-spatial resolution SIF from TROPOMI and OCO-2, respectively. Our results show that the LT_SIFc* can capture the temporal trends and inter-annual variations for both site- and global-GPP, and also the global spatial distributions of SIF at the spatial resolution of 0.05° validated with TROPOMI SIF (R2 = 0.76, RMSE = 0.11 mW m−2 nm−1 sr-1). Our temporally corrected and spatially downscaled global long-term SIF product has the potential for the better estimations of global spatial–temporal variations in GPP, and also could be helpful for the improvement in the predictions of future land carbon storage and climate change.

A precise method unaffected by atmospheric reabsorption for ground-based retrieval of red and far-red sun-induced chlorophyll fluorescence

Remote sensing employs solar-induced chlorophyll fluorescence (SIF) as a proxy for photosynthesis from field to airborne and satellite sensors. The investigation of SIF offers a unique way of studying vegetation functioning from the local to the global scale. However, the passive, optical retrieval of the SIF signal is still challenging. Common retrieval approaches extract the SIF infilling directly from atmospheric oxygen bands in down-welling and up-welling radiance. They often involve a complex signal correction to compensate for atmospheric reabsorption and require long computing time. In contrast, the exploitation of solar Fraunhofer lines is devoid of atmospheric disturbances. We propose a new retrieval method for red and far-red SIF directly from up-welling radiance spectra in the spectral range between 650 nm and 810 nm by applying Partial Least Squares (PLS) regression machine learning. Solar Fraunhofer lines are exploited for SIF retrieval with the PLS approach by excluding telluric absorption features. The PLS models are trained and tested on synthetic reflectance and SIF data modeled with SCOPE. We identified a logarithmic relationship of the retrieval error with respect to signal-to-noise ratio of the instrument. The approach has been tested with real-world data measured by the Fluorescence Box (FloX), and evaluated against two well-established retrieval methods: the spectral fitting method (SFM) and the singular value decomposition (SVD). PLS models exploiting solar Fraunhofer lines retrieved meaningful SIF values with high precision and demonstrated robustness against atmospheric reabsorption, including from a 100m tall tower. In addition, PLS retrieval requires no complex correction for atmospheric reabsorption and computes 37 times faster than SFM. Hence, PLS retrieval allows fast and robust exploitation of SIF from solar Fraunhofer lines with high precision under conditions in which other retrieval approaches require complex atmospheric correction.

Global assessment of partitioning transpiration from evapotranspiration based on satellite solar-induced chlorophyll fluorescence data

• A model is developed for terrestrial transpiration estimation from spaceborne SIF. • Global mean terrestrial transpiration to evapotranspiration ratio from SIF is 0.57. • Global terrestrial transpiration in 2018 is estimated using our model. An accurate assessment of terrestrial ecosystem transpiration (T) is important to understand the vegetation-atmosphere feedbacks under climate change. Solar-induced chlorophyll fluorescence (SIF) shows great potential to estimate T because of its mechanical linkage with photosynthesis and stomatal conductance. However, a global and spatially estimation of terrestrial T based on remotely sensed SIF remains unresolved and novel strategies are challenged to entail a precise partition of T from evapotranspiration (ET) across various biomes. Here, with far-red SIF from Sentinel-5 Precursor satellite and ground observations for a total of 30 sites encompassing ten primary plant functional types (PFTs), we extend a SIF-driven semi-mechanism canopy conductance (g c ) model for different plant functional types (PFTs), and use the optimized Penman-Monteith model (PM opt ) to calculate T and T/ET. We reveal that the relationship between SIF and the product of g c and 0.5 power of vapor pressure deficit (g c × VPD 0.5 ) is tighter than the relationship between SIF and ecosystem productivity. The SIF-g c × VPD 0.5 linear regressions show improved R 2 and increased magnitude in slopes across PFTs when aggregating daily to 16-day. Our T/ET results show high correlations with the results of the Ball-Berry-Leuning model combined with PM opt at the site scale (R 2 = 0.69), and with the results calculated by leaf area index in a previous study at the PFT scale (0.70). We further determine the global mean T/ET (0.57 ± 0.14), close to the ensemble mean of global averaged T/ET (0.55), using 36 different methods. The global T estimated using the SIF-based approach is compared with two other remote sensing products. Our method provides a valuable tool for T and ET estimation using remote sensing data and is critical to understanding ecohydrological processes under climate change.

Physiological dynamics dominate the response of canopy far-red solar-induced fluorescence to herbicide treatment

Solar-induced chlorophyll fluorescence (SIF) has shown great potential for detecting changes in vegetation function under herbicide stress. However, how physiological (ΦF, canopy SIF emission efficiency) and non-physiological (e.g., structure and illumination) dynamics regulate canopy SIF, and the coupling between SIF and gross primary production (GPP) under herbicide stress remains unclear. Here, we conducted continuous eddy covariance flux and far-red SIF measurements during the early stage of maize in an herbicide-resistant maize field, where herbicide exclusively affects weeds. We investigated the performance of SIF, GPP, and vegetation indices (VIs) in capturing herbicide stress and then explored the sensitivity of SIF to the effects of herbicide treatments by disentangling canopy SIF into the physiological (ΦF) and non-physiological components (NIRvP). We found that SIF rapidly increased in response to the herbicide and that GPP decreased, and that both were more responsive than VIs in capturing the early effects of herbicides. Thus, the opposing responses in SIF and GPP disrupted their otherwise linear relationship during herbicide treatment. More importantly, we found that the increased ΦF dominated the variation of SIF during the early stages of herbicide stress, while the influence of NIRvP was prominent in the variability of SIF in the absence of herbicide. By unraveling its physiological and non-physiological contributions, our findings advance our understanding of how SIF responds to herbicide stress in heterogeneous cropland and will improve our interpretation of SIF as a tool for monitoring photosynthesis.

Different Responses of Solar-Induced Chlorophyll Fluorescence at the Red and Far-Red Bands and Gross Primary Productivity to Air Temperature for Winter Wheat

Solar-induced chlorophyll fluorescence (SIF) is closely related to the light-reaction process and has been recognized as a good indicator for tracking gross primary productivity (GPP). Nevertheless, it has not been widely examined how SIF and GPP respond to temperature. Here, we explored the linkage mechanisms between SIF and GPP in winter wheat based on continuous measurements of canopy SIF (cSIF), GPP, and meteorological data. To separately explore the structural and physiological mechanisms underlying the SIF–GPP relationship, we studied the temperature responses of the estimated light use efficiency (LUEp), canopy-level chlorophyll fluorescence yield (cSIFyield) and photosystem-level chlorophyll fluorescence yield (ΦF) estimated using canopy-scale remote sensing measurements. We found that GPP, red canopy SIF (cSIF688) and far-red canopy SIF (cSIF760) all exhibited a decreasing trend during overwintering periods. However, GPP and cSIF688 showed relatively more obvious changes in response to air temperature (Ta) than cSIF760 did. In addition, the LUEp responded sensitively to Ta (the correlation coefficient, r = 0.83, p-value < 0.01). The cSIFyield_688 and ΦF_688 (ΦF at 688 nm) also exhibited significantly positive correlations with Ta (r > 0.7, p-value < 0.05), while cSIFyield_760 and ΦF_760 (ΦF at 760 nm) were weakly correlated with Ta (r < 0.3, p-value > 0.05) during overwintering periods. The results also show that LUEp was more sensitive to Ta than ΦF, which caused changes in the LUEp/ΦF ratio in response to Ta. By considering the influence of Ta, the GPP estimation based on the total SIF emitted at the photosystem level (tSIF) was improved (with R2 increased by more than 0.12 for tSIF760 and more than 0.05 for tSIF688). Therefore, our results indicate that the LUEp/ΦF ratio is affected by temperature conditions and highlights that the SIF–GPP model should consider the influence of temperature.

Investigating the Performance of Red and Far-Red SIF for Monitoring GPP of Alpine Meadow Ecosystems

Alpine meadow ecosystems are extremely vulnerable to climate change and serve an essential function in terrestrial carbon sinks. Accurately estimating their gross primary productivity (GPP) is essential for understanding the global carbon cycle. Solar-induced chlorophyll fluorescence (SIF), as a companion product directly related to plant photosynthesis process, has become an attractive pathway for estimating GPP accurately. To date, the quantitative SIF-GPP relationship in terrestrial ecosystems is not yet clear. Especially, red SIF and far-red SIF present differences in their ability to track GPP under different environmental conditions. In this study, we investigated the performance of SIF at both red and far-red band in monitoring the GPP of an alpine meadow ecosystem based on continuous tower-based observations in 2019 and 2020. The results show that the canopy red SIF (SIFRed) and far-red SIF (SIFFar-red) were both strongly correlated with GPP. SIFRed was comparable to SIFFar-red for monitoring GPP based on comparisons of both half-hourly averaged and daily averaged datasets. Moreover, the relationship between SIFRed and GPP was linearly correlated, while the relationship between SIFFar-red and GPP tended to be nonlinear. At a diurnal scale, dramatic changes in photosynthetically active radiation (PAR), air temperature (Ta), and vapor pressure deficit (VPD) all had effects on the slope of the linear fitted line with zero intercept for SIFRed-GPP and SIFFar-red-GPP, and the effect on the slope of the linear fitted line with zero intercept for SIFFar-red-GPP was obviously stronger than that for SIFRed-GPP. PAR was the dominant factor among the three environmental factors in determining the diurnal variation of the slope of SIF-GPP. At a seasonal scale, the SIFFar-red/GPP was susceptible to PAR, Ta, and VPD, while the SIFRed/GPP remained relatively stable at different levels of Ta and VPD, and it was only weakly affected by PAR, suggesting that SIFRed was more consistent than SIFFar-red with GPP in response to seasonal variations in environmental factors. These results indicate that SIFRed has more potential than SIFFar-red for monitoring the GPP of alpine meadow ecosystems and can also assist researchers in gaining a more comprehensive understanding of the diversity of SIF-GPP relationships in different ecosystems.

Representation of Leaf-to-Canopy Radiative Transfer Processes Improves Simulation of Far-Red Solar-Induced Chlorophyll Fluorescence in the Community Land Model Version 5.

Recent advances in satellite observations of solar‐induced chlorophyll fluorescence (SIF) provide a new opportunity to constrain the simulation of terrestrial gross primary productivity (GPP). Accurate representation of the processes driving SIF emission and its radiative transfer to remote sensing sensors is an essential prerequisite for data assimilation. Recently, SIF simulations have been incorporated into several land surface models, but the scaling of SIF from leaf‐level to canopy‐level is usually not well‐represented. Here, we incorporate the simulation of far‐red SIF observed at nadir into the Community Land Model version 5 (CLM5). Leaf‐level fluorescence yield was simulated by a parametric simplification of the Soil Canopy‐Observation of Photosynthesis and Energy fluxes model (SCOPE). And an efficient and accurate method based on escape probability is developed to scale SIF from leaf‐level to top‐of‐canopy while taking clumping and the radiative transfer processes into account. SIF simulated by CLM5 and SCOPE agreed well at sites except one in needleleaf forest (R 2 > 0.91, root‐mean‐square error <0.19 W⋅m−2⋅sr−1⋅μm−1), and captured the day‐to‐day variation of tower‐measured SIF at temperate forest sites (R 2 > 0.68). At the global scale, simulated SIF generally captured the spatial and seasonal patterns of satellite‐observed SIF. Factors including the fluorescence emission model, clumping, bidirectional effect, and leaf optical properties had considerable impacts on SIF simulation, and the discrepancies between simulate d and observed SIF varied with plant functional type. By improving the representation of radiative transfer for SIF simulation, our model allows better comparisons between simulated and observed SIF toward constraining GPP simulations.

Non-linearity between gross primary productivity and far-red solar-induced chlorophyll fluorescence emitted from canopies of major biomes

Solar-induced chlorophyll fluorescence (SIF) observed from vegetation has been considered as a promising proxy of gross primary productivity (GPP) in several studies, and recent work has shown advantages of using the total emitted SIF (SIFtotal) to capture the variation of GPP. The non-linearity between SIF and GPP at the canopy level was often observed in many previous studies, but the non-linearity between SIFtotal and GPP has not yet been systematically investigated. In this study, based on a theoretical analysis of the relationship between SIF and GPP at the leaf level and how it propagates to the canopy level with the consideration of sunlit and shaded leaf fractions in the canopy, we asserted that non-linear relationships between SIFtotal and GPP at the canopy level are general and physically sound. We derived SIFtotal from two different approaches using TROPOMI SIF data. One is based on the canopy escape ratio of SIF signals observed by satellite sensors (SIFobs), and the other is an angular normalization method. At the site level, we established linear and non-linear (exponential, hyperbolic, and polynomial) regressions between TROPOMI SIF and GPP data from 25 flux tower sites covering the eight biomes. The results indicate that non-linearity between SIFtotal and GPP exists among eight major biomes, and exponential regression is the best regression method to capture the non-linearity between SIF and GPP at the site level. We developed a simple variable, SIFnon-linear, to capture the non-linearity between SIFtotal and GPP. R2 values of the linear correlations between SIFnon-linear and GPP are equal or close to those of exponential correlations between SIFtotal and GPP. SIFnon-linear captures the non-linearity well, even though different biomes have different degrees of non-linearity. On the global scale, TROPOMI SIF was compared with GPP simulations of the BEPS model and the SMAP GPP product, showing that SIFnon-linear is a better proxy of GPP than SIFobs and SIFtotal. We find that the non-linearity between SIFtotal and GPP generally exists among eight major biomes regardless of the spatial or temporal scales. The non-linearity is driven by the seasonal variation of the absorbed photosynthetically active radiation (APAR). The degree of the non-linearity varies among different biomes, and it is controlled by three factors: the light saturation point of GPP, the variation of APAR during the growing season, and the ratio between the sunlit and shaded portion of the canopy. Temporal aggregation of SIFtotal reduces the non-linearity. Our results indicate that the simple variable SIFnon-linear can replace SIFobs or SIFtotal to improve global SIF-GPP related studies in the future.

Far-Red Chlorophyll Fluorescence Radiance Tracks Photosynthetic Carbon Assimilation Efficiency of Dark Reactions

Solar-induced chlorophyll fluorescence (SIF) observations from space have shown close relationships with terrestrial photosynthesis rates. SIF originates from the light reactions of photosynthesis, whereas carbon fixation takes place during the dark reactions of photosynthesis. Questions remain regarding whether SIF is able to track changes in the efficiency of the dark reactions in photosynthesis. Using concurrent measurements of leaf-scale gas exchange, pulse amplitude-modulated (PAM) fluorescence, and fluorescence spectral radiances, we found that both far-red fluorescence radiances and PAM fluorescence yields responded rapidly to changes in photosynthetic carbon assimilation due to changes in environmental factors or induced stomatal closure under constant light conditions. Uncertainties in outgoing and incoming irradiance mismatch for SIF measurements may very likely obscure the contributions of the dark reactions, thereby causing the inconsistent findings previously reported, which were no change in far-red SIF and PAM fluorescence yields after clear reductions in the photosynthetic carbon assimilation efficiency of dark reactions. Our results confirm that high-quality SIF measurements have the potential to provide insights into the dark reactions of photosynthesis. This study is particularly relevant for better interpreting satellite SIF observations that are obtained under roughly constant overpass times and relatively stable light intensities.

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

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 (fesc) 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 fesc 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 (SIF760canopy) to leaf level (SIF760leaf). For a homogeneous crop such as winter wheat, it seems to be sufficient to determine fesc once a day to reliably scale SIF760 from canopy to leaf level. In contrast, for more complex canopies such as fruit trees, calculating fesc 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 (εSIF) at leaf level, a parameter directly related to the diurnal variations of plant photosynthetic efficiency.

Evaluation of the FluorWPS Model and Study of the Parameter Sensitivity for Simulating Solar-Induced Chlorophyll Fluorescence

Solar-induced chlorophyll fluorescence (SIF) has been used as an indicator for the photosynthetic activity of vegetation at regional and global scales. Canopy structure affects the radiative transfer process of SIF within canopy and causes the angular-dependencies of SIF. A common solution for interpreting these effects is the use of physically-based radiative transfer models. As a first step, a comprehensive evaluation of the three-dimensional (3D) radiative transfers is needed using ground truth biological and hyperspectral remote sensing measurements. Due to the complexity of forest modeling, few studies have systematically investigated the effect of canopy structural factors and sun-target-viewing geometry on SIF. In this study, we evaluated the capability of the Fluorescence model with the Weighted Photon Spread method (FluorWPS) to simulate at-sensor radiance and SIF at the top of canopy, and identified the influence of the canopy structural factors and sun-target-viewing geometry on the magnitude and directional response of SIF in deciduous forests. To evaluate the model, a 3D forest scene was first constructed from Goddard’s LiDAR Hyperspectral and Thermal (G-LiHT) LiDAR data. The reliability of the reconstructed scene was confirmed by comparing the calculated leaf area index with the measured ones from the scene, which resulted in a relative error of 3.5%. Then, the performance of FluorWPS was evaluated by comparing the simulated at-sensor radiance spectra with the spectra measured from the DUAL and FLUO spectrometer of HyPlant. The radiance spectra simulated by FluorWPS agreed well with the measured spectra by the two high-performance imaging spectrometers, with a coefficient of determination (R2) of 0.998 and 0.926, respectively. SIF simulated by the FluorWPS model agreed well with the values of the DART model. Furthermore, a sensitivity analysis was conducted to assess the effect of the canopy structural parameters and sun-target-viewing geometry on SIF. The maximum difference of the total SIF can be as large as 45% and 47% at the wavelengths of 685 nm and 740 nm for different foliage area volume densities (FAVDs), and 48% and 46% for fractional vegetation covers (FVCs), respectively. Leaf angle distribution has a markedly influence on the magnitude of SIF, with a ratio of emission part to SIF range from 0.48 to 0.72. SIF from the grass layer under the tree contributed 10%+ more to the top of canopy SIF even for a dense forest canopy (FAVD = 3.5 m−1, FVC = 76%). The red SIF at the wavelength of 685 nm had a similar shape to the far-red SIF at a wavelength of 740 nm but with higher variability in varying illumination conditions. The integration of the FluorWPS model and LiDAR modeling can greatly improve the interpretation of SIF at different scales and angular configurations.

Sensitivity of Estimated Total Canopy SIF Emission to Remotely Sensed LAI and BRDF Products

Remote sensing of solar-induced chlorophyll fluorescence (SIF) provides new possibilities to estimate terrestrial gross primary production (GPP). To mitigate the angular and canopy structural effects on original SIF observed by sensors (SIF obs ), it is recommended to derive total canopy SIF emission (SIF total ) of leaves within a canopy using canopy interception ( i 0 ) and reflectance of vegetation ( R V ). However, the effects of the uncertainties in i 0 and R V on the estimation of SIF total have not been well understood. Here, we evaluated such effects on the estimation of GPP using the Soil-Canopy-Observation of Photosynthesis and the Energy balance (SCOPE) model. The SCOPE simulations showed that the R 2 between GPP and SIF total was clearly higher than that between GPP and SIF obs and the differences in R 2 ( Δ R 2 ) tend to decrease with the increasing levels of uncertainties in i 0 and R V . The resultant Δ R 2 decreased to zero when the uncertainty level in i 0 and R V was ~30% for red band SIF (RSIF, 683 nm) and ~20% for far-red band SIF (FRSIF, 740 nm). In addition, as compared to the TROPOspheric Monitoring Instrument (TROPOMI) SIF obs at both red and far-red bands, SIF total derived using any combination of i 0 (from MCD15, VNP15, and CGLS LAI products) and R V (from MCD34, MCD19, and VNP43 BRDF products) showed comparable improvements in estimating GPP. With this study, we suggest a way to advance our understanding in the estimation of a more physiological relevant SIF datasets (SIF total ) using current satellite products.

Simulation-Based Evaluation of the Estimation Methods of Far-Red Solar-Induced Chlorophyll Fluorescence Escape Probability in Discontinuous Forest Canopies

The escape probability of Solar-induced chlorophyll fluorescence (SIF) can be remotely estimated using reflectance measurements based on spectral invariants theory. This can then be used to correct the effects of canopy structure on canopy-leaving SIF. However, the feasibility of these estimation methods is untested in heterogeneous vegetation such as the discontinuous forest canopy layer under evaluation here. In this study, the Discrete Anisotropic Radiative Transfer (DART) model is used to simulate canopy-leaving SIF, canopy total emitted SIF, canopy interceptance, and the fraction of absorbed photosynthetically active radiation (fAPAR) in order to evaluate the estimation methods of SIF escape probability in discontinuous forest canopies. Our simulation results show that the normalized difference vegetation index (NDVI) can be used to partly eliminate the effects of background reflectance on the estimation of SIF escape probability in most cases, but fails to produce accurate estimations if the background is partly or totally covered by vegetation. We also found that SIF escape probabilities estimated at a high solar zenith angle have better estimation accuracy than those estimated at a lower solar zenith angle. Our results show that additional errors will be introduced to the estimation of SIF escape probability with the use of satellite products, especially when the product of leaf area index (LAI) and clumping index (CI) was underestimated. In other results, fAPAR has comparable estimation accuracy of SIF escape probability when compared to canopy interceptance. Additionally, fAPAR for the entire canopy has better estimation accuracy of SIF escape probability than fPAR for leaf only in sparse forest canopies. These results help us to better understand the current estimation results of SIF escape probability based on spectral invariants theory, and to improve its estimation accuracy in discontinuous forest canopies.