Ra. Houghton et Jl. Hackler, Changes in terrestrial carbon storage in the United States. 1: The roles of agriculture and forestry, GLOBAL EC B, 9(2), 2000, pp. 125-144
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 ch
ange were used in a cohort model, together with equations defining the chan
ges 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 maxim
um 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 x 10(6) ha), releas
ing 29 PgC, or 90% of the total flux. Cultivation of soils accounted for ab
out 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 pape
r) were added to this analysis of land-use change, the uptake of carbon cal
culated for forests was similar in magnitude to the uptake measured in fore
st inventories, suggesting that past harvests account for a significant fra
ction 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 rele
ase, but even with this additional non-forest sink, the calculated accumula
tion 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.