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

In South Africa, the Albany Thicket ecosystem has been widely promoted for carbon sequestration, yet large uncertainty remainsregarding its actual carbon fluxes, particularly under semi-arid and water-limited conditions. Improved, observation-based estimatesare, therefore, required to inform restoration investment decisions and the credibility of climate-change mitigation initiatives. In thisstudy, we quantified carbon and water fluxes of a passively restored Portulacaria afra-dominated Great Fish Thicket using eddycovariance measurements over two relatively dry hydrological years (2015–2016 and 2016–2017). Flux observations were comparedwith outputs from a process-based biogeochemical model (BGC-MAN) and a satellite-derived product (MOD17A2H Version 6). Netecosystem exchange indicated that the system functioned as a consistent but modest carbon sink, with annual sequestrationof 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, includingnocturnal net CO2 assimilation, providing ecosystem-scale evidence of facultative crassulacean acid metabolism (CAM) activity. TheBGC-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 lowerthan previously assumed. The findings highlight the importance of incorporating CAM physiology into carbon models and adoptingconservative, observation-driven estimates when evaluating the carbon sequestration potential of thicket restoration projects.