Photosynthesis and carbon balance of a Sahelian fallow savanna

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Abstract
Eddy‐covariance measurements of CO2 exchange above a Sahelian savanna consisting of small shrubs over a near‐continuous herb layer were made during the HAPEX‐Sahel experiment in Niger, West Africa. The measurements were made near‐continuously during an 8‐week period, covering the main part of the rainy season and three weeks at the beginning of the dry season. The measurements were corrected for in‐canopy storage of CO2 and the night‐time measurements used to derive respiration functions for the soil, roots and above‐ground plant material. Photosynthetic CO2 uptake was estimated and compared to simulations using a biochemical photosynthesis model in a simple, ‘big‐leaf’, implementation. The model satisfactorily reproduced the measurements (coefficient of determination 0.80) using parameters defined from the literature and based on soil nutrient concentrations. When the quantum yield (α) and rubisco capacity (Vmr) were fitted to the data with allowance for physiological changes through the season, an excellent agreement between model and measurements was obtained (coefficient of determination 0.93, RMS error 1.46 μmol m–2 s–1).The fitted photosynthesis and respiration model was used to estimate the carbon balance of the savanna site during the growing season of 1992 and for the complete calendar year. Harvest estimates of net plant biomass accumulation during the growing season and annual wood accumulation agreed well with modelled net photosynthesis and annual net carbon accumulation, respectively. Peak instantaneous ecosystem CO2 uptake was comparable to peak values observed in other biomes, but annual photosynthesis and carbon sequestration were considerably lower than observed elsewhere.