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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-387
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-387
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 26 Sep 2019

Submitted as: research article | 26 Sep 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

A double peak in the seasonality of California's photosynthesis as observed from space

Alexander J. Turner1,2,3, Philipp Köhler4, Troy S. Magney3,4,5, Christian Frankenberg3,4, Inez Fung1, and Ronald C. Cohen1,2 Alexander J. Turner et al.
  • 1Department of Earth and Planetary Sciences, University of California, Berkeley, CA, 94720, USA
  • 2College of Chemistry, University of California, Berkeley, CA, 94720, USA
  • 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, USA
  • 4Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, 91226, USA
  • 5Department of Plant Sciences, University of California, Davis, CA, 95616, USA

Abstract. Solar-Induced chlorophyll Fluorescence (SIF) has been shown to be a powerful proxy for photosynthesis and gross primary productivity (GPP). The recently launched TROPOspheric Monitoring Instrument (TROPOMI) features the required spectral resolution and signal-to-noise ratio to retrieve SIF from space. Here we present an oversampling and downscaling method to obtain 500-m spatial resolution SIF over California. We report daily values based on a 14-day window. TROPOMI SIF data show a strong correspondence with daily GPP estimates at AmeriFlux sites across multiple ecosystems in California. We find a linear relationship between SIF and GPP that is largely invariant across ecosystems with an intercept that is not significantly different from zero. Measurements of SIF from TROPOMI agree with MODIS vegetation indices (NDVI, EVI, and NIRv) at annual timescales but indicate different temporal dynamics at monthly and daily timescales. TROPOMI SIF data show a double peak in the seasonality of photosynthesis, a feature that is not present in the MODIS vegetation indices. The different seasonality in the vegetation indices may be due to a clear-sky bias in the vegetation indices, whereas SIF has a low sensitivity to clouds and can detect the downregulation of photosynthesis even when plants appear green. We further decompose the spatio-temporal patterns in the SIF data based on land cover. The double peak in the seasonality of California's photosynthesis is due to two processes that are out of phase: grasses, chaparral, and oak savanna ecosystems show an April maximum while evergreen forests peak in June. An empirical orthogonal function (EOF) analysis corroborates the phase offset and spatial patterns driving the double peak. The EOF analysis further indicates that two spatio-temporal patterns explain 84 % of the variability in the SIF data. Results shown here are promising for obtaining global GPP at sub-kilometer spatial scales and identifying the processes driving carbon uptake.

Alexander J. Turner et al.
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Status: final response (author comments only)
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Alexander J. Turner et al.
Alexander J. Turner et al.
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Short summary
The magnitude and variability of global carbon uptake via photosynthesis has proved elusive over the past 5 decades. We aim to address the question of What is driving the pulses of photosynthesis in California? Here we present the highest resolution SIF dataset from satellite measurements, providing previously unobservable phenomena related to plant photosynthesis. We observe a double peak in the seasonality of California's photosynthesis, not seen by traditional vegetation indices (ie MODIS).
The magnitude and variability of global carbon uptake via photosynthesis has proved elusive over...
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