Carbon Content of Ecosystems on Land

Figure 1 gives estimates of the carbon content of several different major ecosystems on land. For each, the estimated land area is in hectares (1 hectare is 10,000 square meters or 2.47 acres) and carbon content in units of 10¹⁵ grams ( =1 petagram or 1 gigaton). From this table we can see that tropical seasonal forests and tropical evergreen forests account for nearly half of the plant carbon on the planet. The tropical rain forests, of course, have received considerable attention in the scientific literature and in the public press as being particularly valuable because they contain so much of the earth's plant carbon and also because they serve as hosts for perhaps even millions of biological species, many of which have not yet even been cataloged.

Boreal forests (high latitudes, mostly Canadian, Scandanavian, and Russia) have carbon approximately comparable with the tropical evergreen forests, and temperate forests add somewhat smaller but significant amounts. Grasslands and pasture in temperate zones, such as native prairie in the US Midwest, contribute a relatively small amount to the vegetation carbon total compared to the forested regions of the tropics or boreal areas. It is informative, however, to consider the amount of plant carbon per unit area in each land class (divide the values in the second column by the values in the first column to get values in the fourth column). Cultivated land in temperate zones supports only slightly less than half as much carbon per hectare as native temperate prairies. By breaking the prairie sod, the early settlers in the Midwest began the process of reducing plant carbon in the region by over 40%. This is, of course, in addition to the accompanying deforestation that took place at the prairie margins. It might be tempting to argue that a lush Iowa cornfield with 30,000 or more plants per acre would have more plant carbon than native prairie vegetation, but the data suggest otherwise.

NEXT: Carbon Content in Soils