Carbon Storage and Accumulation in U.S. Forests

Carbon Storage in the United States

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.

Figure 2--Carbon storage in U.S. forest ecosystems by forest ecosystem component (in billion tons). Total storage in the United States is 57.8 billion tons--about 4 percent of all the carbon stored in the world's forests.

Figure 3--Average carbon storage in U.S. forest ecosystems by forest ecosystem component and region.

Figure 4--Total carbon storage in U.S. forest ecosystems by region. Total storage in the United States is 57.8 billion tons.

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.

Figure 5--Average carbon storage per acre of forest land in the United States.

Figure 6--Average carbon storage in the soil, forest floor, and trees for selected forest types.

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).

Figure 7 - Average carbon accumulation in live trees on forest land in the United States.

Figure 8 - Annual changes in carbon storage in live trees on all forest lands by softwoods and hardwoods.

Figure 9 - Annual changes in carbon storage in live softwood trees on all forest land by region.

Figure 10 - Annual changes in carbon storage in live hardwood trees on all forest land by region.

Table 2.1 - Area of forest land in the United States by region, State, and forest land class, 1987        
         
         
  Forest land class
         
Region and State All forest
land		 Unreserved
timberland		 Reserved
timberland		 Other forest
land
         
         
  ----------(1,000 acres)------------
Southeast:        
Florida 16,721 15,238 461 1,022
Georgia 23,906 23,383 505 18
North Carolina 18,892 18,359 490 43
South Carolina 12,257 12,179 78 0
Virginia 15,968 15,436 471 61
Total 87,744 84,595 2,005 1,144
         
         
South Central:        
Alabama 21,725 21,659 66 0
Arkansas 16,987 16,673 91 223
Louisiana 13,883 13,873 10 0
Mississippi 16,694 16,673 9 12
Oklahoma 7,284 4,748 23 2,513
Tennessee 13,258 12,839 395 24
Texas 13,656 12,414 120 1,122
Total 1,103,487 98,879 714 3,894
         
         
Northeast and Mid-Atlantic:        
Connecticut 1,815 1,777 21 17
Delaware 398 388 3 7
Kentucky 12,256 11,908 267 81
Maine 17,713 17,175 276 262
Maryland 2,632 2,461 153 18
Massachusetts 3,097 3,010 0 87
New Hampshire 5,021 4,803 70 148
New Jersey 1,985 1,914 41 30
New York 18,775 15,799 2,549 427
Ohio 7,310 7,141 120 49
Pennsylvania 16,996 16,186 532 278
Rhode Island 398 368 8 22
Vermont 4,479 4,424 25 30
West Virginia 11,942 11,799 116 27
Total 4,817 99,153 4,181 1,483
         
         
North Central and Central:        
Illinois 4,266 4,030 236 0
Indiana 4,439 4,296 143 0
lowa 1,562 1,459 76 27
Kansas 1,358 1,207 23 128
Michigan 18,221 17,364 623 234
Minnesota 16,583 13,571 1,178 1,834
Missouri 12,523 11,996 224 303
Nebraska 722 536 23 163
North Dakota 460 337 0 123
South Dakota 1,690 1,447 22 221
Wisconsin 15,319 14,727 261 331
Total 77,143 70,970 2,809 3,364
         
         
Rocky Mountain:        
Arizona 19,384 3,789 1,090 14,505
Colorado 21,337 11,739 1,714 7,884
Idaho 21,818 14,533 3,051 4,234
Montana 21,910 14,736 1,396 5,778
Nevada 8,927 221 1 8,705
New Mexico 18,527 5,181 1,399 11,947
Utah 16,233 3,078 346 12,809
Wyoming 9,966 4,332 2,943 2,691
Total 138,102 57,609 11,940 68,553
         
         
Pacific Coast:        
Alaska 129,045 15,763 5,292 107,990
California 39,381 16,712 2,940 19,729
Hawaii 1,748 700 113 935
Oregon 28,057 22,084 1,777 4,196
Washington 21,857 16,848 2,765 2,244
Total 220,088 72,107 12,887 135,094
         
U.S. total 731,381 483,313 34,536 213,532
Table 2.2 - Average storage of carbon in the United States by region,
State, and forest ecosystem component, 1987		          
           
           
           
  Forest ecosystem component
     
           
Region and State Total Trees Soil Forest floor Understory
           
  ----------Lbs/ac----------        
Southeast:          
Florida 96,393 33,337 54,753 5,679 2,624
Georgia 120,371 47,399 64,637 5,710 2,624
North Carolina 140,870 57,049 75,939 5,258 2,624
South Carolina 124,576 51,719 64,627 5,606 2,624
Virginia 138,744 59,018 72,284 4,818 2,624
Total 124,146 49,515 66,577 5,430 2,624
           
           
South Central:          
Alabama 111,016 44,005 58,406 5,677 2,928
Arkansas 122,847 50,085 64,441 5,393 2,928
Louisiana 124,151 57,869 57,794 5,560 2,928
Mississippi 122,179 51,395 62,226 5,630 2,928
Oklahoma 88,818 25,318 55,888 4,684 2,928
Tennessee 134,491 57,694 69,089 4,785 2,924
Texas 101,783 43,474 50,047 5,515 2,747
Total 116,748 48,423 60,019 5,402 2,904
           
           
Northeast and Mid-Atlantic:          
Connecticut 178,993 59,118 103,769 14,699 1,406
Delaware 160,940 63,476 83,007 13,051 1,406
Kentucky 151,615 56,804 81,978 11,427 1,406
Maine 198,899 44,071 136,455 17,208 1,165
Maryland 164,791 71,066 79,769 12,551 1,406
Massachusetts 182,673 56,550 108,637 16,079 1,406
New Hampshire 190,440 59,937 112,586 16,544 1,373
New Jersey 137,830 40,235 82,229 13,961 1,406
New York 159,823 46,016 97,309 15,102 1,396
Ohio 149,357 49,391 87,888 10,671 1,406
Pennsylvania 147,234 47,947 86,950 10,931 1,406
Rhode Island 164,521 47,662 100,638 14,815 1,406
Vermont 188,276 57,316 113,995 15,594 1,371
West Virginia 150,212 56,356 81,524 10,926 1,406
Total 165,021 50,955 99,120 13,585 1,360
           
           
North Central and Central:          
llinois 158,103 55,978 89,088 11,645 1,391
Indiana 168,576 59,215 95,870 12,100 1,391
lowa 152,392 50,835 88,442 11,724 1,391
Kansas 123,201 39,007 71,571 11,232 1,391
Michigan 179,724 46,107 115,262 17,238 1,117
Minnesota 178,618 37,470 123,825 16,206 1,117
Missouri 122,662 40,639 68,238 12,394 1,391
Nebraska 139,336 40,933 84,102 12,911 1,391
North Dakota 161,225 33,563 113,466 13,070 1,117
South Dakota 149,313 40,839 87,809 19,273 1,391
Wisconsin 165,950 41,327 106,537 16,695 1,391
Total 162,948 43,446 102,957 15,279 1,266
           
           
Rocky Mountain:          
Arizona 106,218 44,658 49,227 11,256 1,077
Colorado 124,993 44,405 62,536 16,975 1,077
Idaho 148,190 60,961 64,417 21,735 1,077
Montana 185,368 67,902 95,732 20,657 1,077
Nevada 83,099 42,658 32,608 6,755 1,077
New Mexico 90,610 30,643 45,790 13,100 1,077
Utah 107,586 38,459 58,225 9,824 1,077
Wyoming 150,012 47,034 81,892 20,009 1,077
Total 128,040 48,316 62,941 15,706 1,077
           
           
Pacific Coast:          
Alaska 238,185 39,075 171,994 23,682 3,434
California 127,372 55,672 53,224 15,042 3,434
Hawaii 96,733 8,066 75,253 9,980 3,434
Oregon 172,749 64,469 82,976 21,870 3,434
Washington 202,655 83,073 93,911 22,237 3,434
Total 205,363 49,405 130,871 21,653 3,434
           
U.S. total 158,225 48,667 92,811 14,456 2,291