Changes in terrestrial carbon storage in the United States. 1: The roles of agriculture and forestry

1 Changes in the areas of croplands and pastures, and rates of wood harvest in seven regions of the United States, including Alaska, were derived from historical statistics for the period 1700–1990. These rates of land-use change were used in a cohort model, together with equations defining the changes in live vegetation, slash, wood products and soil that follow a change in land use, to calculate the annual flux of carbon to the atmosphere from changes in land use. 2 The calculated flux increased from less than 10 TgC/yr in 1700 to a maximum of about 400 TgC/yr around 1880 and then decreased to approximately zero by 1950. The total flux for the 290-year period was a release of 32.6 PgC. The area of forests and woodlands declined by 42% (160 × 106 ha), releasing 29 PgC, or 90% of the total flux. Cultivation of soils accounted for about 25% of the carbon loss. Between 1950 and 1990 the annual flux of carbon was approximately zero, although eastern forests were accumulating carbon. 3 When the effects of fire and fire exclusion (reported in a companion paper) were added to this analysis of land-use change, the uptake of carbon calculated for forests was similar in magnitude to the uptake measured in forest inventories, suggesting that past harvests account for a significant fraction of the observed carbon sink in forests. 4 Changes in the management of croplands between 1965 and 1990 may have led to an additional accumulation of carbon, not included in the 32.6 PgC release, but even with this additional non-forest sink, the calculated accumulation of carbon in the United States was an order of magnitude smaller than the North American carbon sink inferred recently from atmospheric data and models.