Carbon fluxes over passively restored Great Fish Thicket, South Africa, reveal modest sequestration during drier rainfall years

Abstract

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.