Abstract
Abstract. Tropical savannas and grasslands are estimated to contribute significantly to the total primary production of all terrestrial vegetation. Large parts of African savannas and grasslands are used for agriculture and cattle grazing, but the carbon flux data available from these areas are limited. This study explores carbon dioxide fluxes measured with the eddy covariance method for 3 years at a grazed savanna grassland in Welgegund, South Africa. The tree cover around the measurement site, grazed by cattle and sheep, was around 15 %. The night-time respiration was not significantly dependent on either soil moisture or soil temperature on a weekly temporal scale, whereas on an annual timescale higher respiration rates were observed when soil temperatures were higher. The carbon dioxide balances of the years 2010–2011, 2011–2012 and 2012–2013 were −85 ± 16, 67 ± 20 and 139 ± 13 gC m−2 yr−1, respectively. The yearly variation was largely determined by the changes in the early wet season fluxes (September to November) and in the mid-growing season fluxes (December to January). Early rainfall enhanced the respiratory capacity of the ecosystem throughout the year, whereas during the mid-growing season high rainfall resulted in high carbon uptake.
Highlights
Savannas are highly dynamic ecosystems which cover about 40 % of Africa and 20 % of the global land area (Scholes and Walker, 1993)
The highest inter-annual variation in CO2 flux occurred during the wet season, whereas during the dry season CO2 fluxes were rather similar in magnitude
The dry season (JJA) CO2 flux rates were an order of magnitude lower than the wet season rates and canopy assimilation was rarely saturated with respect to photosynthetically active radiation (PAR)
Summary
Savannas are highly dynamic ecosystems which cover about 40 % of Africa and 20 % of the global land area (Scholes and Walker, 1993). The savanna ecosystems are generally characterized by alternating wet and dry seasons, during the latter of which wildfires can occur. There are large differences between savannas in terms of their tree cover, species composition and soil type. Large parts of African savannas have been inhabited by humans throughout the evolution of our species and are modified by activities such as grazing and logging. The tropical savannas and grasslands are estimated to account for 30 % of the global primary production of all terrestrial vegetation (Grace et al, 2006). It has recently been shown that the inter-annual variability of the terrestrial carbon cycle is dominated by semi-arid ecosystems (Ahlström, 2015)
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