Forest ecosystems in the United States contain approximately 57.8 billion tons (52.5 billion metric tons) of carbon above and below the ground. This is about 4 percent of all the carbon stored in the world's forests (Ajtay and others 1979). The area of U.S. forests is 731 million acres, or 5 percent of the world's forest area.
The average forest in the United States contains 158 thousand pounds per acre (17.7 kg/m2) of organic carbon. Trees, including tree roots, account for 31 percent of all forest ecosystem carbon (fig. 2). Live and standing dead trees contain 17.7 billion tons (16.1 billion metric tons) of carbon, or an average of 49 thousand pounds per acre (5.5 kg/m2). Of this total, 51 percent is in live tree sections classified as growing stock volume, 24 percent is in other live solid wood above the ground, 17 percent is in the roots, 6 percent is in standing dead trees, and 3 percent is in the foliage.
The largest proportion of carbon in the average U.S. forest is found in the soil, which contains 59 percent of the carbon in the forest ecosystem, or approximately 93 thousand pounds per acre (10.4 kg/m2). About 9 percent of all carbon is found in litter, humus, and coarse woody debris on the forest floor, and about 1 percent is found in the understory vegetation. By adding carbon in tree roots to the carbon in the soil, the average proportion of carbon below the ground in the United States is estimated to be 64 percent.
Carbon Storage by Region
The quantity of carbon varies considerably between regions, with Pacific Coast
States containing 205 thousand pounds per acre (23.0 kg/m2) and South Central
States containing 117 thousand pounds per acre (13.1 kg/m2) in the average
forest (fig. 3). Of the total 57.8 billion tons (52.5 billion metric tons) of
carbon in U.S. forests, 22.6 billion tons (20.5 billion metric tons), or 39
percent, is found in Pacific Coast forests, far more than in any other region
(fig. 4). The Rocky Mountains and the Northeast each contain about 15 percent
of U.S. forest carbon. The Southeast, South Central, and North Central regions
each contain about 10 percent of U.S. forest carbon.
Pacific Coast States, including Alaska, contain the highest average carbon in
forest soils, 64 percent of the total. The lowest proportion of soil carbon is
found in the Rocky Mountain States, with 49 percent of the total. Soil carbon
is closely related to temperature and precipitation, with higher amounts of
soil carbon found in regions with cooler temperatures and higher precipitation.
The cooler temperatures slow the oxidation of soil carbon, while higher
rainfall tends to produce more vegetation and thus the fine roots and litter
that are the main sources of organic soil carbon.
Carbon in the forest floor varies by region in a way similar to carbon in the
soil. Western and Northern States contain the most carbon on the forest floor,
and Southern States contain the least.
There is a clear pattern of increasing forest carbon from Southern to Northern
States (fig. 5). The two main factors are climate and average age of the
forests. The cooler, wetter climates favor higher retention of carbon on the
forest floor and in the soil, and northern forests tend to be older and less
frequently disturbed than forests in the South.
Carbon Storage by Forest Type
There are significant differences in carbon storage among forest types. For
example, selected eastern softwood types show large differences in total carbon
storage and the relative storage by forest ecosystem component (fig. 6).
Loblolly pine plantations are younger on average, so there is less carbon in
the trees, and since they are mostly located in the South, the soil carbon is
lower. Spruce - fir, common in the Northeast, has higher total carbon as a
result of the large amount of carbon stored in the soil. Douglas - fir contains
the highest average carbon because of the large quantity stored in the trees.
Pinyon - juniper has the lowest amount of carbon because it occurs in dry
climates that support lower vegetation densities.
Changes in Carbon Storage
U.S. forests are constantly changing. The total area of forest land declined by
4 million acres between 1977 and 1987 (Waddell and others 1989). Most of the
loss was from forest clearing for urban and suburban development, highways, and
other rights-of-way. Many more million acres were cleared for agricultural use,
but this loss was roughly balanced by agricultural land that was planted with
trees or allowed to revert naturally to forest. In addition to land-use
changes, each year about 4 million acres of timberland are harvested for timber
products and regenerated to forests, 4 million acres are damaged by wildfire,
and 2.5 million acres are damaged by insects and diseases (estimates based on
various unpublished Forest Service data sources). And of course, all forest
lands change continually as trees and other vegetation germinate, grow, and
die.
Changes in carbon storage in the forest ecosystem are primarily related to
changes in carbon storage in live trees. The rate of accumulation of carbon in
live trees is greatest in the forest areas where trees typically have the
fastest volume growth, the Southeast and the Pacific Northwest (fig. 7). On
average, live trees are accumulating carbon at a rate of 1,252 pounds per acre
per year (0.14 kg/m2/yr), a rate of increase of 2.7 percent of the amount
stored in live trees.
The accumulation of carbon in live and dead trees totals 508 million tons (461
million metric tons) per year, while the total removal of tree carbon from U.S.
forests resulting from timber harvest, landclearing, and fuelwood use amounts
to 391 million tons (355 million metric tons, fig. 8). A comparison of
accumulation and removal suggests that U.S. forest trees are storing additional
carbon at a rate of 117 million tons (106 million metric tons) per year. This
is equivalent to about 9 percent of the annual U.S. emission of carbon to the
atmosphere (1.2 billion metric tons) per year (Boden and others 1 990).
Trees dying annually because of insects, diseases, fire, and weather contain
about 83 million tons (75 million metric tons) of carbon. Only a portion of
tree mortality was deducted from accumulation in the comparison of accumulation
and remov-al since much of the carbon remains in the forest ecosystem for some
time as standing dead trees, coarse woody debris on the forest floor, and
eventually other organic matter in the forest ecosystem.
There are significant regional differences in relative and total estimates of
carbon accumulation, removal, and mortality. For softwoods, Pacific coast
forests are accumulating the most carbon annually, followed by the Southeast,
South Central, and Rocky Mountain regions (fig. 9). Because softwood removal is
so low relative to growth in the Rocky Mountains, the increase in carbon
storage in softwood species is much greater there than elsewhere. Mortality is
the highest in the Rocky Mountains and on the Pacific coast. In the South
Central region, tree removal is causing a net loss of carbon storage in
softwood trees.
Most of the hardwood resource in located in the Eastern United States. The
Northeast has the largest excess of hardwood carbon accumulation over removal,
but there are also large increases in hardwood carbon storage occurring in the
Southeast and on the Pacific coast (fig. 10).
