Variation in carbon content of tropical tree species from Ghana

Most research on the carbon content of trees has focused on temperate tree species with little information existing for tropical tree species. Questions remain regarding how much carbon can be sequestered by various tree species and in different forest climatic zones. This study was designed to investigate the variation in carbon content of selected tropical tree species and compare the carbon content of Khaya spp from two ecozones in Ghana. Two to three individuals of 18 tree species were randomly selected and harvested from 12-year-old and 7-year-old plantations in Ghana. The diameter at breast height (dbh at 1.3 m above ground) and length of the main stem were measured. A 2-cm thick wood disc was cut from the bottom, middle and top positions of the main stem of each tree, and used to estimate wood density and carbon concentration. Estimates of tree stem carbon were computed using tree stem volumes, density and carbon concentration. Allometric equations developed for mixed plantation stands for the wet evergreen forest verified the expected strong relationship between tree stem volumes and dbh (r>0.93) and between volumes and dbh×height (r>0.97). Carbon concentration, wood density and carbon content differed significantly among tree species. Carbon concentration of the tree species ranged from 46.3 to 48.9 %. Volume for the 12-year-old trees varied widely among species, from 0.01 m to 1.04 m. Wood densities differed among tree species and 17 the three stem positions. Differences in wood density at the three positions on the stem were independent of tree species. Wood density was highly variable among species, ranging from 0.27 g cm to 0.76 g cm. Species specific knowledge of wood density was much more important than knowledge of carbon concentration for ensuring accurate conversion of allometric volume estimates to tree carbon content. Significant differences in wood density did exist among Khaya spp from wet and moist semi-deciduous ecozones, suggesting climatic factors may also need to be considered. This study has provided baseline species-level information that will be useful for carbon accounting and development of carbon sequestration strategies in Ghana and other tropical African countries.