We contribute to the study of the role of the earliest forests in the partial pressure of carbon dioxide during the Devonian by calculating the above-ground biomass of pseudosporochnalean trees. A three-dimensional model of Pseudosporochnus is generated, and the dynamic of its morphological changes during growth is visualized with the use of AmapSim software and its computer simulator. The carbon content of the tree and of its components is calculated at any time in its growth using the mean carbon density of two Carboniferous plants of comparable structure. The relative contributions of the trunk and branches to the total carbon content of the tree are compared at all growth stages, and the significance of the lateral branches is emphasized. The carbon content of the above-ground part of a 3-m-high Pseudosporochnus is estimated to be in the range of 837–1300 g C and that of a Pseudosporochnus forest to be 4.3–15.3 t C/ha. The low biomass productivity of these trees may result from their adaptation to constrained environments.