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

Creators: Turner, Alexander J.; Köhler, Philipp; Magney, Troy S.; Frankenberg, Christian; Fung, Inez; Cohen, Ronald C.

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 a downscaling method to obtain 500 m spatial resolution SIF over California. We report daily values based on a 14 d 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 MODerate Resolution Imaging Spectroradiometer (MODIS) vegetation indices – the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and near-infrared reflectance of vegetation index (NIR_v) – 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 previous work has shown SIF to have a low sensitivity to clouds and to detect the downregulation of photosynthesis even when plants appear green. We further decompose the spatiotemporal 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 spatiotemporal 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.

Additional Information

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. Received: 24 September 2019 – Discussion started: 26 September 2019. Revised: 16 December 2019 – Accepted: 6 January 2020 – Published: 29 January 2020. Alexander J. Turner is supported as a Miller Fellow with the Miller Institute for Basic Research in Science at UC Berkeley. Ronald C. Cohen acknowledges support from the TEMPO project SV3-83019. Philipp Köhler and Christian Frankenberg are funded by the Earth Science US participating investigator (grant no. NNX15AH95G). This research used the Savio computational cluster resource provided by the Berkeley Research Computing program at the University of California, Berkeley (supported by the UC Berkeley Chancellor, Vice Chancellor for Research, and Chief Information Officer). This research also used resources from the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy under contract no. DE-AC02-05CH11231. TROPOMI SIF and MODIS NBAR data are publicly available at ftp://fluo.gps.caltech.edu/data/tropomi/ (last access: 2 January 2020) and https://e4ftl01.cr.usgs.gov/MOTA/MCD43A4.006/ (last access: 2 January 2020), respectively. Funding for AmeriFlux data resources was provided by the US Department of Energy's Office of Science. We would like to thank Dennis Baldocchi (UC Berkeley) for sharing the AmeriFlux data and for providing extensive feedback on the work. Finally, we are extremely grateful to the team that has realized the TROPOMI instrument, consisting of the partnership between Airbus Defence and Space, KNMI, SRON, and TNO, commissioned by NSO and ESA. This research has been supported by the Adolph C. and Mary Sprague Miller Institute for Basic Research in Science at the University of California Berkeley, the NASA TEMPO project (grant no. SV3-8019), and the NASA Earth Science participating investigator (grant no. NNX15AH95G). Author contributions: AJT designed the research and conceived the methods. PK performed the satellite retrievals. AJT, PK, TSM, CF, IF, and RCC analyzed data. AJT wrote the paper; PK, TSM, CF, IF, and RCC provided comments on the paper. The authors declare that they have no conflict of interest. Review statement: This paper was edited by Martin De Kauwe and reviewed by Luis Guanter and one anonymous referee.

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