Precipitation as driver of carbon fluxes in 11 African ecosystems

Lutz Merbold ,B A El-Tahir,A De Grandcourt,N Boulain,M Muchinda,C Bruemmer,M M Mukelabai,Nicolas Brüggemann ,Laurent Kergoat ,Jean-Marc Bonnefond ,Sally Archibald ,Jonas Ardö ,Olivier Roupsard ,Éric Ceschia ,Yann Nouvellon ,Almut Arneth ,Bernard Cappelaere ,Éric Mougin ,David Ramier ,Ulrike Falk ,Robert J Scholes ,Hatim Abdalla M Elkhidir ,E.m Veenendaal ,Jon Lloyd ,Valérie Le Dantec ,Franck Timouk ,Werner L Kutsch

doi:10.5194/bg-6-1027-2009

Abstract

Abstract. This study reports carbon and water fluxes between the land surface and atmosphere in eleven different ecosystems types in Sub-Saharan Africa, as measured using eddy covariance (EC) technology in the first two years of the CarboAfrica network operation. The ecosystems for which data were available ranged in mean annual rainfall from 320 mm (Sudan) to 1150 mm (Republic of Congo) and include a spectrum of vegetation types (or land cover) (open savannas, woodlands, croplands and grasslands). Given the shortness of the record, the EC data were analysed across the network rather than longitudinally at sites, in order to understand the driving factors for ecosystem respiration and carbon assimilation, and to reveal the different water use strategies in these highly seasonal environments. Values for maximum net carbon assimilation rates (photosynthesis) ranged from −12.5 μmol CO2 m−2 s−1 in a dry, open Millet cropland (C4-plants) up to −48 μmol CO2 m−2 s−1 for a tropical moist grassland. Maximum carbon assimilation rates were highly correlated with mean annual rainfall (r2=0.74). Maximum photosynthetic uptake rates (Fpmax) were positively related to satellite-derived fAPAR. Ecosystem respiration was dependent on temperature at all sites, and was additionally dependent on soil water content at sites receiving less than 1000 mm of rain per year. All included ecosystems dominated by C3-plants, showed a strong decrease in 30-min assimilation rates with increasing water vapour pressure deficit above 2.0 kPa.

Highlights

Information about Africa’s role in the global carbon cycle is sparse
We suggest that analysis of variation in ecosystematmosphere exchange of carbon dioxide and water measured by eddy covariance, can be a fourth approach to understanding the structure and function of African ecosystems and unravelling the role of Africa in the global carbon cycle
We investigate whether Net Ecosystem Exchange of carbon (NEE)-soil moisture relationships known from observations in drought-stressed temperate (Mediterranean) or boreal ecosystem that contain only C3 species are found in tropical ecosystems with their variable mix of C3 and C4 photosynthetic systems, and rainfall rather than temperaturecontrolled seasonality

Summary

Introduction

Information about Africa’s role in the global carbon cycle is sparse. It remains unknown whether Africa, as a whole, represents a net sink or source of atmospheric carbon, and how carbon exchange varies temporally and spatially at the continental scale (Williams et al, 2007). The first focused on the biocomplexity of savannas (Sankaran et al, 2005, 2008; Bucini and Hanan, 2007) and followed a long tradition of ground-based ecophysiological research trying to understand the role of water, nutrients, herbivory and fire in savanna dynamics, and in particular on tree-grass interactions (Walter, 1939, 1971; Scholes and Walker, 1993; Scholes and Archer, 1997; Jeltsch et al, 2000; Scholes et al, 2004; Bond and Keeley, 2005; Bond et al, 2005) This approach supports to interpret functional properties of savannas by understanding their structure

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Discussion

Conclusion