In South Africa, the Albany Thicket ecosystem has been widely promoted for carbon sequestration, yet large uncertainty remains regarding its actual carbon fluxes, particularly under semi-arid and water-limited conditions. Improved, observation-based estimates are, therefore, required to inform restoration investment decisions and the credibility of climate-change mitigation initiatives. In this study, we quantified carbon and water fluxes of a passively restored Portulacaria afra-dominated Great Fish Thicket using eddy covariance measurements over two relatively dry hydrological years (2015–2016 and 2016–2017). Flux observations were compared with outputs from a process-based biogeochemical model (BGC-MAN) and a satellite-derived product (MOD17A2H Version 6). Net ecosystem exchange indicated that the system functioned as a consistent but modest carbon sink, with annual sequestration of 152 g C m-2 yr-1 and 136 g C m-2 yr-1 in the two respective years. Carbon uptake persisted during hot, dry periods, including nocturnal net CO2 assimilation, providing ecosystem-scale evidence of facultative crassulacean acid metabolism (CAM) activity. The BGC-MAN model successfully reproduced daily and seasonal patterns of observed fluxes using independent meteorological data, whereas MOD17A2H substantially underestimated carbon exchange, likely due to the absence of a CAM functional representation. These results demonstrate that P. afra-dominated thicket can act as a carbon sink even under dry conditions, but at rates lower than previously assumed. The findings highlight the importance of incorporating CAM physiology into carbon models and adopting conservative, observation-driven estimates when evaluating the carbon sequestration potential of thicket restoration projects.
Gwate et al. (Fri,) studied this question.
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