Articles | Volume 14, issue 5
Biogeosciences, 14, 1333–1348, 2017

Special issue: Ecosystem processes and functioning across current and future...

Biogeosciences, 14, 1333–1348, 2017

Research article 17 Mar 2017

Research article | 17 Mar 2017

Modelling spatial and temporal dynamics of gross primary production in the Sahel from earth-observation-based photosynthetic capacity and quantum efficiency

Torbern Tagesson1,2, Jonas Ardö2, Bernard Cappelaere3, Laurent Kergoat4, Abdulhakim Abdi2, Stéphanie Horion1, and Rasmus Fensholt1 Torbern Tagesson et al.
  • 1Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10,Ø 1350 Copenhagen, Denmark
  • 2Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
  • 3HydroSciences Montpellier, IRD, CNRS, Univ. Montpellier, Montpellier, France
  • 4Géosciences Environnement Toulouse, (CNRS/UPS/IRD), 14 av E Belin, 31400 Toulouse, France

Abstract. It has been shown that vegetation growth in semi-arid regions is important to the global terrestrial CO2 sink, which indicates the strong need for improved understanding and spatially explicit estimates of CO2 uptake (gross primary production; GPP) in semi-arid ecosystems. This study has three aims: (1) to evaluate the MOD17A2H GPP (collection 6) product against GPP based on eddy covariance (EC) for six sites across the Sahel; (2) to characterize relationships between spatial and temporal variability in EC-based photosynthetic capacity (Fopt) and quantum efficiency (α) and vegetation indices based on earth observation (EO) (normalized difference vegetation index (NDVI), renormalized difference vegetation index (RDVI), enhanced vegetation index (EVI) and shortwave infrared water stress index (SIWSI)); and (3) to study the applicability of EO upscaled Fopt and α for GPP modelling purposes. MOD17A2H GPP (collection 6) drastically underestimated GPP, most likely because maximum light use efficiency is set too low for semi-arid ecosystems in the MODIS algorithm. Intra-annual dynamics in Fopt were closely related to SIWSI being sensitive to equivalent water thickness, whereas α was closely related to RDVI being affected by chlorophyll abundance. Spatial and inter-annual dynamics in Fopt and α were closely coupled to NDVI and RDVI, respectively. Modelled GPP based on Fopt and α upscaled using EO-based indices reproduced in situ GPP well for all except a cropped site that was strongly impacted by anthropogenic land use. Upscaled GPP for the Sahel 2001–2014 was 736 ± 39 g C m−2 yr−1. This study indicates the strong applicability of EO as a tool for spatially explicit estimates of GPP, Fopt and α; incorporating EO-based Fopt and α in dynamic global vegetation models could improve estimates of vegetation production and simulations of ecosystem processes and hydro-biochemical cycles.

Short summary
Vegetation growth in semi-arid regions is an important sink for human-induced fossil fuel emissions of CO2 and this study addresses the strong need for improved understanding and spatially explicit estimates of CO2 uptake by semi-arid ecosystems. We show that a model incorporating photosynthetic parameters upscaled using satellite-based earth observation simulates CO2 uptake well for the Sahel, one of the largest semi-arid regions in the world.
Final-revised paper