Summary Intensively managed pine plantations in the south‐eastern United States can play an important role in global carbon sequestration both through accumulation of carbon in wood used in long‐lasting products as well as through increased soil carbon storage. Fertilization and understorey‐elimination are two commonly used intensive management practices in the south‐eastern United States that have the potential to increase carbon storage in vegetation and affect soil carbon. In this study, we assessed the effects of these practices on carbon accumulation in vegetation biomass and in the soil of 17‐year‐old slash pine Pinus elliottii plantations in the flatwoods of northern Florida, USA. Three treatments, fertilization, understorey‐elimination, and fertilization plus understorey‐elimination, were evaluated and compared with an untreated control. All three treatments increased above‐ground biomass accumulation compared with the untreated control; understorey‐elimination also increased biomass of the forest floor litter, with or without fertilization. Although understorey‐elimination increased above‐ground production, as a result of reduced below‐ground production total net primary production was decreased in plots from which the understorey was eliminated. Soil carbon storage was lower in plots where the understorey was eliminated, with or without fertilization. This appeared to be the result of reduced fine root growth and mortality but also may have reflected reduced litterfall inputs early in the rotation. Our results indicate that intensive management of pine plantations on sandy flatwoods soils can increase carbon sequestration, but these increases will be the result of increased carbon accumulation in biomass and its long‐term uses rather than through increased soil carbon.

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