Organic Carbon Densities of Soils and Vegetation of Tropical, Temperate and Boreal Forests

Abstract

Available estimates of the soil organic carbon (SOC) density in northern and tropical forests vary between 8.5 and 13.9 kgC·m⁻² for the top first meter. Values of SOC for boreal forests are higher when considering the carbon stored as peat and in the forest floor. Overall, current SOC estimates often underestimate the total soil carbon content of boreal and tropical forests, because in many cases sampling is often limited to the soil's first meter.

The estimates of organic carbon sequestered in the vegetation of Amazonian forests (15.2 to 23.3 kgC·m⁻²) are two to five times higher compared to boreal (4.0 to 6.4 kgC·m⁻²) and temperate forests (4.8 to 5.7 kgC·m⁻²). Apart from their greater productivity, the more stable natural conditions prevailing in tropical rainforests have contributed to the accumulation of large amounts of carbon as biomass. On the other hand, the frequent recurrence of forest fires and insect outbursts in northern forests greatly limit carbon storage in the biomass.

The distribution of the total organic carbon stock between soil and vegetation varies with latitude. In northern forests, 72% of the organic carbon is found in the soil, with the remainder (28%) in the plant biomass. In tropical forests, the distribution is reversed, with 38% of the organic carbon stored in the soil and 62% in the vegetation. This difference can be explained by slower decomposition rates and a shorter growing season in relatively cold and humid boreal regions.

The boreal peatland carbon pool (98 to 335 PgC) is comparable to that of the whole boreal forest (180 to 330 PgC), but its surface area is five times smaller with an organic carbon content two to 10 times greater (39 to 134 kgC·m⁻²).

Export rates of organic carbon from terrestrial to aquatic ecosystems are small compared to their total stock but could be significant in the global forest carbon balance.