12 Aug 2020

12 Aug 2020

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Sun-induced Fluorescence and Near Infrared Reflectance of vegetation track the seasonal dynamics of gross primary production over Africa

Anteneh Getachew Mengistu1,4, Gizaw Mengistu Tsidu1,2, Gerbrand Koren3, Maurits L. Kooreman4, K. Folkert Boersma3,4, Torbern Tagesson6,7, Jonas Ardö6, Yann Nouvellon8,9, and Wouter Peters3,5 Anteneh Getachew Mengistu et al.
  • 1Department of Physics, Addis Ababa University, Addis Ababa, Ethiopia
  • 2Department of Earth and Environment, Botswana International University of Science and Technology, Palapye, Botswana
  • 3Wageningen University, Meteorology and Air Quality Group, Wageningen, the Netherlands
  • 4Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 5University of Groningen, Centre for Isotope Research, Groningen, the Netherlands
  • 6Department of Physical Geography and Ecosystem Science, Lund University, Sweden
  • 7Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark
  • 8Eco&Sols, Univ Montpellier, CIRAD, INRA, IRD, Montpellier SupAgro, 34060 Montpellier, France
  • 9CIRAD, UMR Eco&Sols, 34060 Montpellier, France

Abstract. The carbon cycle of tropical terrestrial vegetation plays a vital role in the storage and exchange of atmospheric CO2. But large uncertainties surround the impacts of land-use change emissions, climate warming, the frequency of droughts, and CO2 fertilization. This culminates in poorly quantified carbon stocks and carbon fluxes even for the major ecosystems of Africa (savannas, and tropical evergreen forests). Contributors to this uncertainty are the sparsity of (micro-)meteorological observations across Africa's vast land area, a lack of sufficient ground-based observation networks and validation data for CO2, and incomplete representation of important processes in numerical models. In this study, we, therefore, turn to two remotely-sensed vegetation products that have shown to correlate highly with Gross Primary Production (GPP): Sun-Induced Fluorescence (SIF) and Near-Infrared Reflectance of vegetation (NIRv). The former is available from an updated product that we recently published (SIFTER v2), which specifically improves retrievals in tropical environments.

A comparison against flux tower observations of daytime-partitioned Net Ecosystem Exchange from six major biomes in Africa shows that SIF and NIRv reproduce the seasonal patterns of GPP well, resulting in correlation coefficients of > 0.9 (N = 12 months, 4 sites) over savannas in the northern and southern hemispheres. These coefficients are slightly higher than for the widely used MPI-BGC GPP products and Enhanced Vegetation Index (EVI). Similar to SIF signals in the neighboring Amazon, peak productivity occurs in the wet season coinciding with peak soil moisture, and is followed by an initial decline during the early dry season, that reverses when light availability peaks. This suggests similar leaf dynamics are at play. Spatially, SIF and NIRv show a strong linear relation (R > 0.9, N = 250 + pixels) with multi-year MPI-BGC GPP even within single biomes. Both MPI-BGC GPP and EVI show saturation relative to peak NIRv and SIF signals during high productivity months, which suggests that GPP in the most productive regions of Africa might be larger than suggested.

Anteneh Getachew Mengistu et al.

Anteneh Getachew Mengistu et al.

Anteneh Getachew Mengistu et al.


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Short summary
In this study, we assess the usefulness of Sun-Induced Fluorescence of Terrestrial Ecosystems Retrieval (SIF) data from the GOME-2A instrument and Near Infra-red reflectance of vegetation (NIRv) from MODIS to capture the seasonality and magnitudes of Gross Primary Production (GPP) derived from six eddy covariance flux towers from Africa in the overlap years between 2007–2014. We also test the robustness of SIF and NIRv to track the seasonality of GPP for the major biomes in comparison to others.