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Carbon Dioxide Sinks
dioxide sink or CO2 sink is a carbon reservoir that is increasing in
size, and is the opposite of a carbon "source". The main natural
sinks are the oceans and growing vegetation. Both remove carbon from
the atmosphere by incorporating it into biomass such as plankton and
is the term describing processes that remove carbon from the
atmosphere. A variety of means of artificially capturing and storing
carbon, as well as of enhancing natural sequestration processes, are
being explored. This is intended to help mitigate global warming.
Preservation of natural sinks is extremely important to the future
overall health of our planet.
Enormous amounts of carbon are naturally stored in the forest by
trees and other plants, as well as in the forest soil. As part of
photosynthesis, plants absorb carbon dioxide from the atmosphere,
store the carbon as sugar, starch and cellulose, while oxygen is
released back into the atmosphere. A young forest, composed of
rapidly growing trees, absorbs carbon dioxide, but the sink effect
exists only when they grow in size. Furthermore, forests,
particularly new ones, may not be straightforward carbon sinks.
Although a forest is a net CO2 sink over time, the plantation of new
forests may also initially be a source of carbon dioxide emission
when carbon from the soil is released into the atmosphere. Mature forests, made up of a
mix of various aged trees as well as dead and decaying matter, may
be carbon neutral above ground. In the soil, however, the gradual
buildup of slowly decaying organic material will continue to
accumulate carbon, thereby acting as a sink.
Over the life of an individual tree or other forest plant, the
carbon capturing (sequestering) and releasing is neutral. As the
plant grows carbon is absorbed from the atmosphere and then released
back into the atmosphere as the plant matures, dies, and rots. Most
forests are a mix of old and new trees or plants, and carbon is
stored and released continuously depending on the plant and the
phase of its life at the time. Also, a severe forest fire will
quickly release absorbed carbon back into the atmosphere.
The dead trees, plants, and moss in peat bogs undergo slow
anaerobic decomposition below the surface of the bog. This process
is slow enough that in many cases the bog grows rapidly and fixes
more carbon from the atmosphere than is released. Over time, the
peat grows deeper. Peat bogs inter approximately one-quarter of the
carbon stored in land plants and soils .
Under some conditions, forests and peat bogs may become sources
of CO2. This can happen, for example, when a forest is flooded by
the construction of a hydroelectric dam. The rotting vegetation is a
source of CO2 and methane comparable in magnitude to the amount of
carbon released by a fossil-fuel powered plant of equivalent
Oceans are natural carbon dioxide sinks, and are the largest
active carbon sinks. As the level of carbon dioxide increases in the
atmosphere, the level in the oceans also increases, creating
potentially disastrous acidic oceans. Ocean water can hold a
variable amount of dissolved CO2 depending on temperature and
pressure. Phytoplankton in the oceans, like trees, use
photosynthesis to extract carbon from CO2. They are the starting
point of the marine food chain. Plankton and other marine organisms
extract CO2 from the ocean water and convert it to the mineral
calcite, CaCO3, to build their skeletons and shells. This removes
CO2 from the water, allowing more to dissolve in from the
atmosphere. These calcite skeletons and shells, along with the
organic carbon of the organisms, eventually fall to the bottom of
the ocean when the organisms die. The carbon or plankton cells have
to sink to the deep water in 2000 to 4000 meters to be sequestered
for ca. 1000 years.
The carbon-sequestration potential of soils (by increasing soil
organic matter) is substantial; below-ground organic carbon storage
is more than twice above-ground storage. Soils' organic carbon
levels in many agricultural areas have been severely depleted.
Improving the humus levels of these soils would both improve soil
quality and increase the amount of carbon sequestered in these
Grasslands contribute huge quantities of soil organic matter over
time, mostly in the form of roots, and much of this organic matter
can remain unoxidized for long periods. Since the 1850s, a large
proportion of the world's grasslands have been tilled and converted
to croplands, allowing the rapid oxidation of large quantities of
soil organic carbon. No-till agricultural systems can increase the
amount of carbon stored in soil, and conversion to pastureland,
particularly with good management of grazing, can sequester even
more carbon in the soil.
Carbon Sinks and the Kyoto
The protocols hold that, since growing vegetation absorbs carbon
dioxide, countries that have large areas of forest (or other
vegetation) can deduct a certain amount from their emissions, thus
making it easier for them to achieve the desired emission levels.
The effectiveness of these provisions is controversial.
Some countries want to be able to trade in emission rights in
carbon emission markets, to make it possible for one country to buy
the benefit of carbon dioxide sinks in another country. It is said
that such a market mechanism will help find cost-effective ways to
reduce greenhouse emissions. There is as yet no carbon audit regime
for all such markets globally, and none is specified in the Kyoto
Protocol. Each nation is on its own to verify actual carbon emission
reductions, and to account for carbon sequestration using some less
In the Clean Development Mechanism, only afforestation and
reforestation are eligible to produce CERs (Clean Emissions
Reductions) in the
first commitment period of the Kyoto Protocol (2008-2012). Forest
conservation activities or activities avoiding deforestation, which
would result in emission reduction through the conservation of
existing carbon stocks, are not eligible at this time. Also
agricultural carbon sequestration is not possible yet.
Carbon Storage in the United
Carbon dioxide in the atmosphere has been increasing steadily
since at least 1958 (Keeling 1984). Predictions of future climate
change as a consequence of increasing atmospheric carbon dioxide
vary widely. Under a scenario of equivalent doubling of atmospheric
carbon dioxide by the middle of the next century, most predictions
show an increase in average global temperature of between 2 and 5
degrees centigrade and an increase in average global precipitation
of between 7 and 15 percent (Schneider 1989). These prospective
changes have generated interest in strategies to reduce emissions of
carbon dioxide to the atmosphere, or to offset emissions by storing
additional carbon in forests.
Across the entire Earth, the total amount of carbon in the
atmosphere has been estimated at 720 billion metric tons, the total
amount of carbon in terrestrial biomass is about 560 billion metric
tons, and the total amount of carbon in terrestrial soils is about
1,500 billion metric tons (Solomon and others 1985). Although oceans
store a far greater amount of carbon than terrestrial ecosystems,
our ability to manage terrestrial ecosystems is greater and likely
to have a greater mitigation effect.
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. 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
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.
Forest ecosystems are capable of storing large quantities of
carbon in solid wood and other organic matter. Forests may add to
the pool of carbon dioxide in the atmosphere through burning of
forest lands, deforestation, or decomposition of wood products and
byproducts. Forests may also reduce the amount of carbon dioxide in
the atmosphere through increases in biomass and organic matter
accumulation. Young, growing forests take up carbon at high rates,
while carbon uptake in mature forests is balanced by carbon release
from decaying vegetation. The end use of timber harvested from
forests is an important factor in evaluating the contributions of
forestry to the global carbon cycle. If the end uses of forest
products are in long-term durable goods such as furniture or timber
bridges, the carbon is stored in those materials. If the end use is
for paper products that are rapidly used and discarded to decay,
then the carbon is released to the atmosphere. Carbon in waste from
the manufacturing process and discarded wood products may be
sequestered in landfills for long periods of time. Because of the
relation between forests and atmospheric carbon dioxide, there are
opportunities to manage forests in ways that would result in storage
of additional carbon and thus reduce atmospheric carbon dioxide.
Major forestry opportunities include increasing forest area,
increasing the productivity of existing forest lands, reducing
forest burning and deforestation, increasing biomass production and
utilization, planting trees in urban environments, and increasing
use of wood in durable products.
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 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
There is a clear pattern of increasing forest carbon from
Southern to Northern States . 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
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. 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
Changes in Carbon
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
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. 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
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).
Annual mortality--The volume of sound wood in tree died from
natural causes during a specific year.
Annual removals--The net volume of trees removed from the
inventory during a specified year by harvesting, cultural operations
such as timber stand improvement, or land clearing.
Cull tree--A live tree, 5.0 inches in diameter at breast height
(d.b.h.) or larger, that is unmerchantable for saw logs
prospectively because of rot, roughness, or species. (See
definitions for rotten and rough trees.)
Forest land--Land at least 10 percent stocked by trees of any
size, including land that formerly had such tree cover and that will
be naturally or artificially regenerated. Forest land includes
transition zones, such as areas between heavily forested and
nonforested lands that are at leas t 10 percent stocked with forest
trees and forest areas adjacent to urban and built-up lands. Also
included are pinyon-juniper and chaparral areas in the West and
afforested areas. The minimum area for classification of forest land
is 1 acre. Roadside, streamside, and shelterbelt strips of timber
must have a crown width of at least 120 feet to qualify as forest
land. Unimproved roads and trails, streams, and clearings in forest
areas are classified as forest if less than 120 feet wide.
Forest type--A classification of forest land based on the species
presently forming a plurality of the live-tree stocking.
Major eastern forest-type
White-red-jack-pine--Forests in which eastern white pine,
red pine, or jack pine, singly or in combination, make up a
plurality of the stocking. Common associates include hemlock, aspen,
birch, and maple.
Spruce-fir--Forests in which spruce or true firs, sir in
combination, make up a plurality of the stocking. Common associates
include white-cedar, tamarack, maple, birch, and hemlock.
Longleaf-slash pine--Forests in which longleaf or pine,
singly or in combination, make up a plurality of stocking. Common
associates include other southern pines, oak, and gum.
Loblolly-shortleaf pine--Forests in which loblolly
shortleaf pine, or southern yellow pines, except longleaf or slash
pine, singly or in combination, make up a plurality of the stocking.
Common associates include oak, hickory and gum.
Oak-pine--Forests in which hardwoods (usually upland oaks)
make up a plurality of the stocking, but in which pine or eastern
redcedar makes up 25-50 percent of the stocking. Common associates
include gum, hickory yellow-poplar.
Oak-hickory--Forests in which upland oaks or hickory,
singly or in combination, make up a plurality of the stocking except
where pines make up 25-50 percent, in which case the stand is
classified as oak-pine. Common associates include yellow-poplar,
elm, maple, and black walnut.
Oak-gum-cypress--Bottomland forests in which tupelo,
blackgum, sweetgum, oaks, or southern cypress, singly or in
combination, make up a plurality of the stocking except where pines
make up 25-50 percent, in which case the stand is classified as
oak-pine. Common associates include cottonwood, willow, ash, elm,
hackberry, and maple.
Elm-ash-cottonwood--Forests in which elm, ash, or
cottonwood, singly or in combination, make up a plurality of the
stocking. Common associates include willow, sycamore, beech, and
Maple-beech-birch--Forests in which maple, beech, or
yellow birch, singly or in combination, make up a plurality of the
stocking. Common associates include hemlock, elm, basswood, and
Aspen-birch--Forests in which aspen, balsam poplar, paper
birch, or gray birch, singly or in combination, make up a plurality
of the stocking. Common associates include maple and balsam fir.
Major western forest-type groups:
Douglas-fir--Forests in which Douglas-fir makes up
plurality of the stocking. Common associates include western
hemlock, western redcedar, the true firs, redwood, ponderosa pine,
Hemlock-Sitka spruce--Forests in which western hemlock or
Sitka spruce, or both, make up a plurality of the stocking. Common
associates include Douglas-fir, silver fir, and western
Redwood--Forests in which redwood makes up a plurality of
the stocking. Common associates include Douglas-fir, grand fir, and
Ponderosa pine--Forests in which ponderosa pine makes up a
plurality of the stocking. Common associates include Jeffrey pine,
sugar pine, limber pine, Arizona pine, Apache pine, Chihuahua pine,
Douglas-fir, incense-cedar, and white fir.
Western white pine--Forests in which western pine makes up
a plurality of the stocking. Common associates include western
redcedar, larch, white fir, Douglas-fir, lodgepole pine, and
Lodgepole pine--forests in which lodgepole pine makes up a
plurality of the stocking. Common associates include alpine fir,
western white pine, Engelmann spruce, aspen, and larch.
Larch--Forests in which western larch makes up a'
plurality of the stocking. Common associates include Douglas-fir,
grand fir, western redcedar, and wester pine.
Fir-spruce--Forests in which true firs, Engelmann or
Colorado blue spruce, singly or in combination, make up a plurality
of the stocking. Common associates include mountain hemlock and
Western hardwoods--Forests in which aspen, red or other
western hardwoods, singly or in combination make up a plurality of
Pinyonjuniper--Forests in which pinyon pine or juniper, or
both, make up a plurality of the stocking.
Growing stock--A classification of timber inventory that includes
live trees of commercial species meeting specified standards of
quality or vigor. Cull trees are excluded. When associated with
volume, includes only trees 5.0 inches and larger.
Hardwood--A dicotyledonous tree, usually broad-leaved and
Industrial wood--All commercial roundwood product except
Net annual growth--The net increase in the volume trees during a
specified year. Components include the increment in net volume of
trees at the beginning of the specific year surviving to its end,
plus the net volume reaching the minimum size class during the year,
minus the volume of trees that died during the year, and minus the
volume of trees that became cull trees during the year.
Net volume in cubic feet--The gross volume in cubic feet less
deductions for rot, roughness, and poor form. Volume is computed for
the central stem from a 1-foot-high stump to the point where the
diameter of the outside bark equals 4 inches, or to the point where
the central stem breaks into limbs.
Nonstocked area--Timberland less than 10 percent stocked with
growing stock trees.
Other forest land--Forest land other than timberland and reserved
timberland. It includes available and reserve unproductive forest
land that is incapable of producing. annually 20 cubic feet per acre
of industrial wood under natural conditions because of adverse site
conditions Such as sterile soils, dry climate, poor drainage, high
elevation, steepness, or rockiness.
Other removals--Unutilized wood volume from cut or otherwise
killed growing stock, from nongrowing stock sources on timberland
(for example, precommercial thinnings), or from timberland clearing.
Does not include volume removed from inventory through
reclassification of timberland to reserved timberland.
Other sources--Sources of roundwood products that are nongrowing
stock. These include salvable dead trees, rough and rotten trees,
trees of noncommercial species, trees less than 5.0 inches d.b.h.,
tops, and roundwood harvested from nonforest land (for example,
Productivity class--A classification of forest land in items of
potential annual cubic-foot volume growth per acre at culmination of
mean annual increment in fully stocked natural stands.
Reserved timberland--Forest land that would otherwise be
classified as timberland except that it is withdrawn from timber
utilization by statute or administrative regulation.
Rotten tree--A live tree of commercial species that does not
contain a saw log now or prospectively primarily because of rot
(that is, when rot accounts for more than 50 percent of the total
Rough tree- (a) A live tree of commercial species that does not
contain a saw log now or prospectively primarily because of
roughness (that is, when sound cull due to such factors as poor
form, splits, or cracks accounts for more than 50 percent of the
total cull volume) or (b) a live tree of noncommercial species.
Softwood--A coniferous tree, usually evergreen, having needles or
Stocking--The degree of occupancy of lands by trees, measured by
basal area or number of trees by size and spacing, or both, compared
to a stocking standard; that is, the basal area or number of trees,
or both, required to fully utilize the growth potential of the
Timberland--Forest land that is producing or is capable of
producing crops of industrial wood and not withdrawn from timber
utilization by statute or administrative regulation. (Note: Areas
qualifying as timberland are capable of producing in excess of 20
cubic feet per acre per year of industrial wood in natural stands.
Currently inaccessible and inoperable areas are included.)
Unreserved forest land--Forest land that is not withdrawn from
use by statute or administrative regulation.
Weight--The weight of wood and bark, oven-dry basis
(approximately 12 percent moisture content).
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