Introduction - Mathematics and Statistics for the Environment - page 34

1. Determine the carbon stock change between years by subtracting carbon stocks in year t
from carbon stocks in year ( t + 1). (This includes carbon stocks in the above-ground bio-
mass, below-ground biomass, dead wood, litter, and soil organic carbon pools.)
2. Determine the annual net change in carbon stocks (i.e., sequestration) per area by dividing
the carbon stock change in U.S. forests from step 1 by the total area of U.S. forests remain-
ing in forests in year ( n + 1) (i.e., the area of land that did not change land-use categories
between the time periods).
Applying these calculations to data developed by the USDA Forest Service for the Inventory of U.S.
Greenhouse Gas Emissions and Sinks yields a result of 150 metric tons of carbon per hectare (or 61
metric tons of carbon per acre) * for the carbon stock density of U.S forests in 2010, with an annual
net change in carbon stock per area in 2010 of 0.82 metric tons of carbon sequestered per hectare
per year (or 0.33 metric tons of carbon sequestered per acre per year). These values include carbon
in the five forest pools of above-ground biomass, below-ground biomass, deadwood, litter, and soil
organic carbon, and they are based on state-level Forest Inventory and Analysis (FIA) data. Forest
carbon stocks and carbon stock change are based on the stock difference methodology and algo-
rithms described by Smith et al. (2010).
1.7.10.2 Conversion Factors for Carbon Sequestered
Annually by 1 Acre of Average U.S. Forest
Due to rounding, performing the calculations given in the equations below may not return the
exact results shown. In the following calculation, negative values indicate carbon sequestration.
(-0.33 metric ton C per acre/year) × (44 units CO 2 ÷ 12 units C) = -1.22 metric ton CO 2
sequestered annually by one acre of average U.S. forest.
Note that this is an estimate for “average” U.S. forests in 2010 (i.e., for U.S. forests as a whole in
2010). Significant geographical variations underlie the national estimates, and the values calculated
here might not be representative of individual regions of states. To estimate carbon sequestered for
additional acres in one year, simply multiply the number of acres by 1.22 metric tons CO 2 per acre/
year. From 2000 to 2010, the average annual sequestration per area was 0.73 metric tons C per hect-
are/year (or 0.30 metric tons C per acre/year) in the United States, with a minimum value of 0.36
metric tons C per hectare/year (or 0.15 metric tons C per acre/year) in 2000, and a maximum value
of 0.83 metric tons C per hectare/year (or 0.34 metric tons C per acre/year) in 2006.
1.7.11 a Cres oF u.s. F orest p reserved From C onversion to C ropland
The carbon stock density of U.S. forests in 2010 was 150 metric tons of carbon per hectare (or 61
metric tons of carbon per acre) (USEPA, 2013a). This estimate is composed of the five carbon pools
of above-ground biomass (52 metric tons C per hectare), below-ground biomass (10 metric tons C
per hectare), dead wood (9 metric tons C per hectare), litter (17 metric tons per C hectare), and soil
organic carbons (62 metric tons C per hectare).
The Inventory of U.S. Greenhouse Gas Emissions and Sinks estimates soil carbon stock
changes using U.S.-specific equations and data from the USDA Natural Resource Inventory and the
CENTURY biogeochemical model (USEPA, 2013a). When calculating carbon stock changes in bio-
mass due to conversion from forestland to cropland, the IPCC guidelines indicate that the average
carbon stock change is equal to the carbon stock change due to removal of biomass from the outgo-
ing land use (i.e., forestland) plus the carbon stocks from one year of growth in the incoming land
use (i.e., cropland), or the carbon in biomass immediately after the conversion minus the carbon in
biomass prior to the conversion plus the carbon stocks from one year of growth in the incoming
* 1 hectare = 10,000 m 2 ; 100 m by 100 m; 2.47 acres.