Linking canopy scattering of far-red sun-induced chlorophyll fluorescence with reflectance

Yang, Peiqi; Tol, Christiaan van der

doi:10.1016/j.rse.2018.02.029

Cited by 191 publications

(202 citation statements)

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“…At the leaf scale, we show that the spectrally integrated spectrum is only modulated by 8% through the reabsorption of emitted ChlF, but by up to about 20% in the red region (Figure c); notably, the spectral and sample weights would be significantly amplified for highly chlorotic leaves, which are not included in this study. However, at the canopy scale, we do observe an amplification of the red:far‐red SIF ratio during senescence (Figure d) because more red photons are prone to escape the canopy when leaves become highly chlorotic (Figures S9 and S10; Fournier et al, ; Goulas et al, ; Yang & van der Tol, ). Further, while we attribute PC2 at steady‐state changes primarily to chlorophyll concentration, confounding effects of leaf‐level radiative transfer should not be negated.…”

Section: Discussionmentioning

confidence: 74%

“…From this work, several findings have implications for remote sensing of the SIF spectral shape: (1) across a wide range of species, ~85% of the spectral variance in ChlF can be explained by changes in the magnitude of a mean spectral shape. This implies that the spectral shape is quite stable and that all wavelength ranges respond to changes in variable PSII fluorescence yield driven by PQ and NPQ; (2) far‐red SIF is more closely correlated to photosynthesis and APAR than red SIF (Figures ), because it is less prone to re‐absorption by Chl at both leaf and canopy scales (Fournier et al, ; Yang & van der Tol, ). At the canopy scale, the strong correlation between SIF and APAR (Figures and ) confirms previous results in crop canopies (Miao et al, ; Yang et al, ), which makes intuitive sense since APAR covaries with seasonal changes in leaf area in these systems, and is prone to less error than calculated GPP fluxes; and (3) Since wavelengths in the far‐red spectrum are less impacted by confounding factors such as chlorophyll or canopy structure (Figure S12), current satellite retrievals in this region may be better suited to assess stress induced downregulation of photosynthesis than changes in the ChlF spectral shape alone.…”

Section: Discussionmentioning

confidence: 99%

“…Research suggests that changes in the spectral shape might help, but we find that these changes impose high requirements on measurement accuracy from space and a sound quantitative understanding of radiative transfer. Rather, information on the red:far‐red SIF ratio, or vegetation reflectance data, could be used to decouple the dominant APAR impact on the SIF magnitude (Yang & van der Tol, ).…”

Section: Discussionmentioning

confidence: 99%

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Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence: Implications for Remote Sensing of Photosynthesis

et al. 2019

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Novel satellite measurements of solar‐induced chlorophyll fluorescence (SIF) can improve our understanding of global photosynthesis; however, little is known about how to interpret the controls on its spectral variability. To address this, we disentangle simultaneous drivers of fluorescence spectra by coupling active and passive fluorescence measurements with photosynthesis. We show empirical and mechanistic evidence for where, why, and to what extent leaf fluorescence spectra change. Three distinct components explain more than 95% of the variance in leaf fluorescence spectra under both steady‐state and changing illumination conditions. A single spectral shape of fluorescence explains 84% of the variance across a wide range of species. The magnitude of this shape responds to absorbed light and photosynthetic up/down regulation; meanwhile, chlorophyll concentration and nonphotochemical quenching control 9% and 3% of the remaining spectral variance, respectively. The spectral shape of fluorescence is remarkably stable where most current satellite retrievals occur (“far‐red,” >740nm), and dynamic downregulation of photosynthesis reduces fluorescence magnitude similarly across the 670‐ to 850‐nm range. We conduct an exploratory analysis of hourly red and far‐red canopy SIF in soybean, which shows a subtle change in red:far‐red fluorescence coincident with photosynthetic downregulation but is overshadowed by longer‐term changes in canopy chlorophyll and structure. Based on our leaf and canopy analysis, caution should be taken when attributing large changes in the spectral shape of remotely sensed SIF to plant stress, particularly if data acquisition is temporally sparse. Ultimately, changes in SIF magnitude at wavelengths greater than 740 nm alone may prove sufficient for tracking photosynthetic dynamics.

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Section: Discussionmentioning

confidence: 74%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence: Implications for Remote Sensing of Photosynthesis

et al. 2019

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“…Besides EVI data, satellite SIF signal is affected by bidirectional effects from the Sun‐surface‐sensor geometry (Guanter et al, ; Köhler et al, ; Yang & Van Der Tol, ; Zhang, Zhang, et al, ). Considering the impact, Köhler et al () reported that the normalized GOME‐2 SIF data have the peak around January/February (wet season) and at the end of the dry season (September) and the annual dip during the early dry season (May and June) in Amazon evergreen broadleaf forests, which is similar to the SIF seasonality in Figure a in this study that ignored the bidirectional effect on SIF.…”

Section: Discussionmentioning

confidence: 99%

Effects of Light Component and Water Stress on Photosynthesis of Amazon Rainforests During the 2015/2016 El Niño Drought

et al. 2019

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Whether enhanced sunshine increases photosynthesis in Amazon rainforests during drought is unclear. Here we used a light component‐based two‐leaf‐photosynthesis model, driven with climate data and satellite vegetation data, to inspect the controlling mechanisms among climate factors on gross primary production (GPP) during the 2015/2016 El Niño drought event. We found that simulated GPP and Moderate Resolution Imaging Spectroradiometer enhanced vegetation index indicated an Amazonian “browning” and not a “green up” during the 2015/2016 El Niño year relative to the 2011–2014 interval. The result shows that, along with intensified sunlight, diffuse sunlight and diffuse fraction as well as canopy light use efficiency decreased, which further produced a decreased potential GPP* (determined by light components and leaf area index of shaded and sunlit leaves). The decreased GPP* and drought‐induced water stress jointly reduced canopy photosynthesis of Amazon rainforests during the 2015/2016 drought. The light component variations caused a reduction in GPP but with a magnitude inferior to the GPP reduction from water stress. These findings suggest that intensified sunlight did not enhance photosynthesis of Amazon rainforests and highlight the important role of light components in interannual and seasonal variations of photosynthesis in Amazon rainforests.

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“…2013; Yang and van der Tol, 2018). Many previous studies have shown high correlation between SIF and vegetation indices (VIs), especially VIs related to the chlorophyll concentration (Frankenberg et al, 2011;Guanter et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

A global spatially Continuous Solar Induced Fluorescence (CSIF) dataset using neural networks

Zhang¹,

Joiner²,

Alemohammad³

et al. 2018

Preprint

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Abstract. Satellite-retrieved Solar Induced Chlorophyll Fluorescence (SIF) has shown great potential to monitor the photosynthetic activity of terrestrial ecosystems. However, several issues, including low spatial and temporal resolution of 10 the gridded datasets and high uncertainty of the individual retrievals, limit the applications of SIF. In addition, inconsistency in measurements footprints also hinder the direct comparison between gross primary production (GPP) from eddy covariance (EC) flux towers and satellite-retrieved SIF. In this study, by training a neural network (NN) with surface reflectance from the MODerate-resolution Imaging Spectroradiometer (MODIS) and SIF from Orbiting Carbon Observatory-2 (OCO-2), we generated two global spatially continuous SIF (CSIF) datasets at moderate spatio-temporal resolutions (0.05 degree 4-day) 15 during 2001-2016, one for clear-sky conditions and the other one in all-sky conditions. The clear-sky instantaneous CSIF (CSIFclear-inst) shows high accuracy against the clear-sky OCO-2 SIF and little bias across biome types. The all-sky daily average CSIF (CSIFall-daily) dataset exhibits strong spatial, seasonal and interannual dynamics that are consistent with daily SIF from OCO-2 and the Global Ozone Monitoring Experiment-2 (GOME-2). An increasing trend (0.39%) of annual average CSIFall-daily is also found, confirming the greening of Earth in most regions. Since the difference between satellite 20 observed SIF and CSIF is mostly caused by the environmental down-regulation on SIFyield, the ratio between OCO-2 SIF and CSIFclear-inst can be an effective indicator of drought stress that is more sensitive than normalized difference vegetation index and enhanced vegetation index. By comparing CSIFall-daily with gross primary production (GPP) estimates from 40 EC flux towers across the globe, we find a large cross-site variation (c.v. = 0.36) of GPP-SIF relationship with the highest regression slopes for evergreen needleleaf forest. However, the cross-biome variation is relatively limited (c.v. = 0.15). These two 25 continuous SIF datasets and the derived GPP-SIF relationship enable a better understanding of the spatial and temporal variations of the GPP across biomes and climate.

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Linking canopy scattering of far-red sun-induced chlorophyll fluorescence with reflectance

Cited by 191 publications

References 49 publications

Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence: Implications for Remote Sensing of Photosynthesis

Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence: Implications for Remote Sensing of Photosynthesis

Effects of Light Component and Water Stress on Photosynthesis of Amazon Rainforests During the 2015/2016 El Niño Drought

A global spatially Continuous Solar Induced Fluorescence (CSIF) dataset using neural networks

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