GEB166.fm Page 125 Wednesday, May 10, 2000 2:02 PM
Global Ecology & Biogeography (2000) 9, 125 – 144
G C T E / L U C C R E S E A R C H A RT I C L E
Changes in terrestrial carbon storage in the United States.
1: The roles of agriculture and forestry
Blackwell Science, Ltd
R. A. HOUGHTON and J. L. HACKLER The Woods Hole Research Center, PO Box 296,
Woods Hole, Massachusetts 02543, U.S.A. [email protected]; [email protected]
ABSTRACT
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 change were
used in a cohort model, together with equations defining the changes 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 maximum 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 × 106 ha), releasing 29 PgC, or 90% of
the total flux. Cultivation of soils accounted
for about 25% of the carbon loss. Between 1950
and 1990 the annual flux of carbon was
INTRODUCTION
The conversion of forests to agricultural lands,
the increase in secondary forests as a result of
harvests and the accumulation of wood in manmade structures have changed the amount of
carbon stored on land. A number of studies
have calculated recent or contemporary changes
in carbon storage for specific geographical
regions, but few studies have considered changes
over the longer term of centuries. Estimating
the changes in storage over the long term helps
define temporal variations in the global carbon
approximately zero, although eastern forests
were accumulating carbon.
3 When the effects of fire and fire exclusion
(reported in a companion paper) were added to
this analysis of land-use change, the uptake of
carbon calculated for forests was similar in
magnitude to the uptake measured in forest inventories, suggesting that past harvests account for
a significant fraction 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 release, but even with this
additional non-forest sink, the calculated accumulation 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.
Key words Agriculture, carbon emissions, carbon
sink, carbon storage, forestry, land-use change,
soil carbon, terrestrial ecosystems, United States.
balance and thus may provide insight into the
factors responsible for terrestrial sources and
sinks of carbon not related to land-use change
(Dai & Fung, 1993; Houghton, 1995). Initial
efforts to define the long-term global emissions
of carbon from land-use change were carried
out almost 20 years ago (Moore et al., 1981;
Houghton et al., 1983; Richards et al., 1983;
Woodwell et al., 1983). Subsequent work has
focused on geographical regions that were less
well documented in these early analyses and
regions where the greatest changes have been
taking place. Here we present an analysis of
© 2000 Blackwell Science Ltd. http://www.blackwell-science.com/geb
125
GEB166.fm Page 126 Wednesday, May 10, 2000 2:02 PM
126
R. A. Houghton and J. L. Hackler
land-use change in the United States over the
period 1700 –1990. The approach was similar to
that used in earlier studies (Houghton et al., 1983,
1987), but the data used were more recent and
considerably more detailed. In the earlier analyses,
the United States was combined with Canada for
an analysis of North America that included five
types of natural ecosystems and two types of land
use. Here, we considered seven regions in the
United States, each region having 2–3 natural ecosystems in addition to croplands and pastures.
METHODS
General approach
The general approach was to obtain, from
historical statistics, rates of change in the major
uses of land (for example, expansion and abandonment of croplands and harvest of wood) and to
document the changes in terrestrial carbon that
follow each change in land use (for example,
rates of decay of plant material, wood products,
and soil carbon and rates of regrowth of live
vegetation). From these two types of information we calculated annual changes in terrestrial
carbon. Below we list the regions and ecosystems
and their initial (1700) areas, the data used to
define rates of land-use change (including harvests), the biomass and soil carbon of ecosystems
and the changes that these stocks of carbon
undergo following a change in land use. The
approach addresses those changes in terrestrial
carbon that result from deliberate human activity
(that is, land use). It ignores changes that may
have resulted from natural changes or from
indirect anthropogenic effects, such as increased
CO2, increased deposition of N or climatic change.
The analysis ignores all those lands not affected
by land-use change. Thus, the calculated flux does
not represent the total net flux of carbon from
land. It is net with respect to land-use change,
however, because it includes both the releases of
carbon from land-use disturbance and the uptake
of carbon by ecosystems recovering from use.
Initial areas
The United States was divided into seven
regions (Fig. 1), chosen to correspond to different climates and, hence, comprised of different
ecosystems (Table 1). The distribution of ecosystems and their areas in 1700 were initially
estimated from Bailey (1995). The initial areas
were subsequently modified, where necessary, so
that the areas at the end of the analysis (1990)
were consistent with observations (e.g. Birdsey,
1992). We assumed, in other words, that the
current areas and the changes over time were
better known than areas in 1700.
Croplands and pastures
Three primary sources of data were used to
construct changes in the area of croplands,
pastures, forests and other lands in each region.
For the period 1850 –1910, the area of forest
cleared by decade and by state was obtained
from Primack (1963; in Williams, 1989). For the
period 1850 –1970, the area in farms was
obtained from data in the US Bureau of the
Census (1977, Series K 1–16 and K 17 – 81).
Changes were reported by decade until 1920
and by 5-year intervals thereafter. Between 1945
and 1992 the absolute areas of forests, croplands
and pastures were obtained at approximately
5-year intervals from agricultural censuses
(USDA, 1996). These recent data on land cover
also enumerated the areas of parks, other land
and special land. Other land included ungrazed
grasslands, deserts, rocks and ice. Special land
included urban areas, military reservations, industrial areas, rural transportation and rural parks
and wildlands.
Land in farms is not equivalent to the sum of
land in crops and pastures; it may also include
forests (wood lots) (Hart, 1968). To estimate the
types of ecosystems cleared for expanding agricultural land, we used the following assumptions.
For the period 1850–1910, forest loss (from
Primack, 1963) was assumed to be equivalent
to the increase in agricultural land (croplands and
pastures combined). This assumption, together
with farmland area over the same period (US
Bureau of the Census, 1977), defined the fraction of farmlands that was derived from forest.
The fraction of farmland that did not come
from forest was assumed to come from grasslands or other non-forest ecosystem. The change
in forest area relative to the change in farmland for the period 1850 –1910 was then applied
to changes in farmlands between 1910 and 1945
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 127 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
127
Fig. 1. Regions and biomes considered in this study (modified from Bailey, 1995).
GEB166.fm Page 128 Wednesday, May 10, 2000 2:02 PM
128
R. A. Houghton and J. L. Hackler
Table 1 The biomes in each region of the United States, their areas in 1700 and 1990, and the carbon
held in live biomass in 1700 and 1990
Region
North-east
East north central
West north central
Rocky Mountains
Pacific
Southern plains
South-east
US total
Area (106 ha)
Biome
Broadleaved forest
Mixed forest
Grassland
Agricultural land
Broadleaved forest
Mixed forest
Grassland
Agricultural land
Broadleaved forest
Coniferous
Grassland
Agricultural land
Coniferous forest
Woodland
Prairie/sagebrush
Agricultural land
Coniferous forest
Chaparral
Grassland/sagebrush
Alaskan coniferous forest
Alaskan taiga
Alaskan tundra
Agricultural land
Woodland
Grassland/sagebrush
Agricultural land
Broadleaved forest
Mixed forest
Grassland
Agricultural land
Forest and woodland
Grassland /sagebrush
Agricultural land
Total
to determine changes in the area of both forests
and agricultural lands over this period. These
assumptions yielded areas in 1945 that were
consistent with the major land areas reported
by the USDA (1996). Only in the Mountain
region was the area of farmlands in 1945 less
than the area of croplands and pastures, presumably because of differences in the definition
or measurement of pastures, rangelands and
other grazing lands within or between the US
Bureau of the Census (1977) and USDA (1996).
Total carbon (PgC)
1700
1990
1700
1990
17.6
20.2
4.5
0.2
41.5
16.6
26.2
0.0
8.4
8.4
94.7
0.0
27.0
27.0
168.0
0.0
34.1
9.7
21.0
10.1
44.1
111.1
0.00
33.4
52.8
0.0
16.6
108.3
15.1
0.1
423
493
0
916
14.6
13.1
4.5
10.1
20.2
5.1
5.7
53.3
5.8
2.4
8.8
94.4
24.0
27.0
29.4
141.6
29.2
6.2
8.3
10.1
44.1
111.1
21.4
11.5
4.1
70.5
15.7
33.4
15.1
75.8
262
187
467
916
2.46
2.83
0.04
0.00
6.22
3.32
0.26
0.00
1.26
1.68
0.95
0.00
4.05
2.43
1.68
0.00
6.82
0.39
0.21
1.70
3.93
0.89
0.00
3.01
0.53
0.00
2.32
15.16
0.15
0.00
57.6
5.00
0.00
62.6
1.47
1.36
0.04
0.00
1.78
0.65
0.13
0.02
0.54
0.28
0.09
0.30
3.06
2.33
0.29
1.23
3.71
0.21
0.08
1.68
3.76
0.89
0.11
0.90
0.04
1.08
1.45
2.39
0.15
0.00
25.6
1.7
2.7
30.0
Before 1945, data on forest clearing (Primack,
1963) and data on the area of farms (US Bureau
of the Census, 1977) were used to provide
changes in the area of forests and agricultural
land. Only after 1945 were actual areas defined,
including the division of agricultural lands into
croplands and pastures (USDA, 1996). Because
areas were defined after 1945, and because
changes had been established for the period
1850 –1945, areas were also defined for all dates
back to 1850. In sum, changes in the area of
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 129 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
forest were determined in each region by the loss
of forests that had occurred by 1850 (Primack, 1963),
by the changes in farmlands between 1850 and
1945 (US Bureau of the Census, 1977) and directly with data from the USDA (1996) after 1945.
When more than one type of forest occurred in
a region, the different types were generally cleared
in proportion to their areas.
Before 1850, the areas of forests, croplands
and pastures were calculated from changes in
population in each region (McEvedy & Jones,
1978), using the 1850 per capita areas of cropland and pasture.
The approach determined only net changes in
the areas of ecosystems, and it is likely that
gross rates of clearing and abandonment were
somewhat higher (Hart, 1968). Most of the
gross changes in farmlands seem to have been
captured by dividing the United States into
seven regions, however. The abandonment of
agricultural lands in the east, for example, was
simultaneous with the clearing of new lands in
more western regions. Over the period 1850 –
1969, the net increase in United States farmland
was 312 × 106 ha. The gross rate of clearing
summed over the seven regions during this interval
was 420 × 106 ha (35% greater), and the gross rate
summed for individual states was 446 × 106 ha
(an additional 8%) (US Bureau of the Census,
1977). Thus, we probably underestimated gross
rates of clearing and abandonment by less
than 10%. The exception was in the south-east
where we increased rates of both clearing and
abandonment, so that by 1920 the area of primary
forest had been largely eliminated (Delcourt &
Harris, 1980; Pyne, 1982).
Harvest of wood
Two categories of wood harvest were considered
in this analysis: industrial wood (timber, lumber,
pulp) and non-industrial wood (largely fuelwood). Data concerning industrial wood harvest
were obtained from the US Bureau of the Census
(1977), FAO (1996), and the US Forest Service
(1998). Data from the US Forest Service,
including the years 1960 – 92, provided annual
production and consumption of total industrial
roundwood and each of its components: sawnwood,
wood-based panels, wood pulp, and paper and
paperboard. The data were not provided by region.
129
For the years 1900–1970, data on the production, net imports and apparent consumption
of industrial timber products (in m3 of roundwood equivalents) were obtained from the US
Bureau of the Census (1977, Series L 72–86).
The data included lumber, plywood and veneer
and pulp products. Lumber production for the
years 1869–1970 was provided by region (US
Bureau of the Census, 1977, Series L 113–121),
and we distributed total production in proportion
to lumber production. Prior to 1869, industrial
harvests were estimated on the basis of regional
population (McEvedy & Jones, 1978) and per
capita use of industrial wood in 1869. For industrial wood harvest the Pacific region was divided
into two subregions: Alaska and the three western
states of the conterminous United States. Rates of
industrial wood harvest are shown in Figure 2.
Industrial wood products were partitioned
into pools that decayed with time constants of
0.1 year –1 and 0.01 year –1. Pulpwood, paper and
paperboard were assigned to the pool with the
more rapid decay; sawnwood and wood panels
(lumber) were assigned to the more slowly
decaying pool. The proportion of total industrial
wood production assigned to these pools varied
through time (US Bureau of the Census, 1977;
FAO, 1996; US Forest Service, 1998). The proportion of industrial wood harvested for pulp
increased over time from 3% in 1900, before
pulp became significant, to 45% by 1980. The
change reduced the average lifetime of products.
The efficiency of industrial wood production
also increased through time (Wernick et al.,
1998). In the Pacific North-west, for example,
the amount of slash generated with logging
decreased by at least 25% over the last century
(Harmon et al., 1996). We assumed that efficiency increased in all regions, and we reduced
slash production (averaged for the country)
from 33% of the biomass in stands harvested in
1700 to 25% of the biomass by 1990. The absolute
reduction in the amount of slash generated per
ha was larger than these relative proportions
suggest because the biomass of forests harvested
also decreased over time. The rate of change in
efficiency was made proportional to the ratio of
pulpwood production to total industrial wood
production. Slash generated at harvest decayed
exponentially at rates that were ecosystem specific
(0.02 – 0.06 year–1) (Table 2).
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 130 Wednesday, May 10, 2000 2:02 PM
130
R. A. Houghton and J. L. Hackler
Fig. 2 Annual harvests of industrial wood and fuelwood from the United States. Rates (in m 3) were
obtained from several sources (see text) and converted to carbon assuming dry wood densities of 0.45 and
0.65 mg /m3 for softwood and hardwood, respectively, and assuming dry wood to be 50% carbon.
Primary, or undisturbed, forests were harvested
preferentially until only 15% of the initial area
remained. Thereafter, secondary forests (50 – 80%
of primary forest biomass) were harvested preferentially. The selection of 15% yielded a small
amount of primary forest at the end of the
analysis (1990), consistent with observation.
The opportunity to specify either primary or
secondary forests for harvest satisfied several
requirements. It enabled harvests to match more
closely forestry practices, which recognize that
the optimal age for harvest is considerably less
than the age at full biomass or recovery. Thus,
the time required for rotational harvest was
reduced. It also allowed areas of primary forest to
remain undisturbed if the data required it.
Historical rates of fuelwood harvest were
determined from three sources. For the period
1700– 1930, fuelwood use (in cords) was obtained
from Reynolds & Pierson (1942). Cords were
converted to m3 using a wood volume/cord volume
ratio of 0.47 (Earl, 1975). For the years 1952,
1962, 1970, 1976 and 1986, estimates of fuelwood consumption were obtained from Waddell
et al. (1989). Both Reynolds & Pierson (1942)
and Waddell et al. (1989) provided estimates for
individual regions and distinguished between
softwood and hardwood. The estimates from
Waddell et al. (1989) were consistent with estimates
from FAO (1996), and we used the latter from
1961 to 1992 because they were annual (Fig. 2).
Linear interpolations were used to fill gaps in
the data between 1930 and 1952 and between
1952 and 1961. Softwood and hardwood densities
were 0.45 and 0.65 Mg dry weight m–3, respectively. Carbon content was assumed to be 0.5.
Biomass and rates of growth
The current biomass of US forests is not well
known; the biomass of forests in 1700 or 1850
is even less well known. We began the analysis
with estimates of biomass based largely on global
summaries by Whittaker & Likens (1973), Ajtay
et al. (1979) and Olson et al. (1983), supplemented
with more geographically specific studies of oldgrowth forests (see Whitney, 1994) (Table 2). These
estimates are high for current biomass, but they
seem defensible for the forests of 1700. The effect
of native Americans on biomass is addressed in a
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
Biome
Carbon in
undisturbed
vegetation
(MgC/ha)
Carbon in
undisturbed
soil (MgC/ha)
Carbon in
cropland
soil (MgC/ha)
Time for abandoned
cropland to recover to
70% of initial biomass
and soil carbon (Yr)
Time for abandoned
cropland to recover to
100% of initial biomass
and soil carbon (Yr)
Exponential
decay constant
for slash
decay (Yr–1)
North-east
Broadleaved forest
Mixed forest
Broadleaved forest
Mixed forest
Broadleaved forest
Coniferous
Woodlands
Coniferous
Woodlands
Prairie/sagebrush
Coniferous forests
Chaparral
Grassland/sagebrush
Woodlands
Grasslands/sagebrush
Broadleaved
Mixed forest
140
140
150
200
150
200
150
160
150
160
150
160
112
120
112
120
112
120
50
50
50
50
50
80
150
150
150
150
150
230
0.05
0.035
0.06
0.035
0.06
0.035
150
90
10
200
40
10
90
10
140
140
100
90
80
160
80
80
90
80
150
160
75
68
60
120
60
60
68
60
112
120
50
50
15
75
50
50
50
15
50
50
100
80
16
235
51
51
80
16
150
150
0.03
0.03
0.035
0.02
0.02
0.03
0.03
0.04
0.06
0.04
East north central
West north central
Rocky Mountains
Pacific
Southern plains
South-east
GEB166.fm Page 131 Wednesday, May 10, 2000 2:02 PM
Region
Carbon storage in the U.S.A.: agriculture and forestry
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
Table 2 Parameters used to calculate annual changes in carbon storage following a change in land use in different biomes of the United States
131
GEB166.fm Page 132 Wednesday, May 10, 2000 2:02 PM
132
R. A. Houghton and J. L. Hackler
companion paper (Houghton et al., 2000). We
compared the 1990 biomass calculated as a
result of 290 years of land-use change with
estimates of current biomass obtained from recent
forest inventories (Birdsey, 1992; Schroeder et al.,
1997), and modified the starting biomass so that
modelled and observed estimates in 1990 were
similar.
The current (1987) biomass of US forests,
based on data from the Forest Inventory and
Analysis (FIA) of the US Forest Service
(USDA, 1989), has been estimated by Birdsey
(1992) for different forest types and regions.
Although the data obtained by the FIA represent
an adequate sampling of US forests, the measured
unit is volume of growing stock. Birdsey (1992)
converted these volumes to total live biomass
with factors held constant for each species and
region. A recent analysis of two types of hardwood forests in the north-east, however, found
that the conversion factor was not constant but
varied as a function of stand volume (Schroeder
et al., 1997). Their estimates of above-ground
biomass were 31– 42% (average 35%) greater than
calculated by Birdsey (1992). Although Schroeder
et al. (1997) considered only two types of forests,
we assumed current biomass to be 35% higher
than Birdsey’s (1992) estimates in all regions.
We adjusted the initial (1700) biomass of forests
in each region (Table 2) so as to end (in 1990)
with a total average biomass about 35% greater
than reported by Birdsey (1992).
The regrowth of biomass following logging
and the abandonment of agriculture was simulated
here with two constant rates, a rapid rate for
the first 50 years of recovery and a slower one
for the next 100 years, at which time the level
of original biomass was reached (Table 2).
Soil carbon
Estimates of the amount of soil organic carbon
to a depth of 1 m for each ecosystem were
obtained from Kern (1994). The variability of
soil carbon was high within most biomes. Some
of the variability resulted from differences among
the three approaches used by Kern (1994). We
chose values of soil carbon that were approximately weighted by the three approaches,
although they were not necessarily weighted by
the distribution of classes within each biome.
Cultivation was assumed to lead to a 25%
reduction in soil carbon in the first 15 years
of cultivation (Schlesinger, 1986; Davidson &
Ackerman, 1993) (Table 2). The rate of loss was
high for the first few years, and lower for the
next years until a new steady-state level was
reached. Following abandonment of croplands,
soils accumulated organic carbon rapidly for
the first 50 years and then more slowly for the
next 100 years until they reached the initial carbon
level. Soil carbon was not changed in pastures
and grazed lands; nor was it affected by harvests.
Although reductions in soil carbon have been
documented in pastures and following logging,
the results are highly variable and seem not to
support a consistent pattern of loss (Johnson,
1992). In addition, most pastures in this analysis
were found on natural grasslands, where the
replacement of natural grazers with domesticated
ones is unlikely to have involved significant changes
in carbon.
A model
The book-keeping model used to calculate
changes in terrestrial carbon storage following changes in land use was similar to models
described previously (Moore et al., 1981;
Houghton et al., 1983, 1987; Hall & Uhlig,
1991). The model included response curves that
defined the annual changes in live and dead
vegetation, and soil carbon that occur on a
hectare of land following a change in land use.
The response curves varied with type of ecosystem and type of land use. The fate of wood
removed from ecosystems was also tracked
through time, decaying at an exponential rate
dependent on end use.
Most of the live carbon on a site cleared for
cropland, for example, was either released to the
atmosphere in the year of clearing (burned) or
converted to slash (dead organic matter generated with disturbance) and wood products. Slash
(including roots, stumps, branches, twigs, and
leaves) decayed exponentially at rates that varied
with the region (climate) (Table 2). Wood products
were assigned to decay pools with time constants
of 0.1 year –1 (for pulp) and 0.01 year –1 (for lumber
and plywood). The response curves also defined
the annual loss of soil carbon following cultivation. With the abandonment of croplands, carbon
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 133 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
accumulated again in both live vegetation and
soil as the ecosystem returned to its initial state.
The number of hectares cleared, abandoned,
harvested or afforested in any year started those
hectares along the appropriate response curves in
the model. Clearing for croplands released carbon
to the atmosphere as vegetation burned or decayed
and as soils lost carbon through cultivation.
Abandonment of croplands reversed these emissions as both soil carbon and vegetation recovered
to their initial levels. The net flux of carbon from
a hectare harvested for wood varied through time,
as annual emissions from decay declined relative
to annual accumulations of carbon in regrowth.
For each region, the model kept track of the
area in each type of ecosystem (natural or managed)
and each age class (cohort) in a specific type of
land use. Thus, the model computed the average
age of forests. It also determined the amount of
carbon held in the biomass and soils of an ecosystem, as well as the carbon in slash or wood
products, again specific for each type of land
use, each type of ecosystem and each region.
Finally, the model calculated the annual losses
of carbon from burning and decay, the annual
uptake of carbon in growth of biomass and
development of soil and the net flux of carbon
as a result of changes in land use. The analysis
included only those changes in carbon that
resulted from deliberate human management or
disturbance of the land. Furthermore, the effect
of year-to-year or long-term variations in environmental factors on rates of growth or rates of
decay were not included. Rates of regrowth varied
only as a function of forest age and geographical
location. They were held constant over the 290 year period simulated here.
RESULTS
Changes in area
Cropland in the United States increased from
about 0.25 × 106 ha in 1700 to 236 × 106 ha in 1990
(Fig. 3). The most rapid expansion occurred
between 1800 and 1900, and since 1920 there
has been little net change in the area of US
croplands, although the areas in individual
regions have changed. Pastures increased from
0.01 × 106 to 231 × 106 ha, most of the increase
taking place between 1850 and 1950, and most
133
of it occurring in grasslands where carbon stocks
were not changed. The expansion of agriculture
moved westward with time. The mountain region
was settled approximately 100 years after the
start of the expansion of farmlands in the northand south-east.
The total area of forests and woodlands in
the United States declined by 160 × 106 ha (38%)
as a result of agricultural clearing (Fig. 4), but
this net change obscures the dynamics of forest
loss and recovery, especially in the eastern part
of the United States (Fig. 5). By 1920, 170 × 106
ha (40%) of the nation’s forests and woodlands
had been cleared. After 1920, however, forest
area increased by 14 × 106 ha nationwide as
farmlands were abandoned in the north-east,
south-east and, to a lesser extent, in north central
regions. Losses continued in the Mountain and
Pacific regions, and in the last 25 years another
4 × 106 ha of forest were lost in the south and
north central regions. Nationally, the recovery
of 10 × 106 ha offset only 6% of the net loss.
Harvest of industrial wood generally followed
the periods of major agricultural clearing in
each region. The increasing rates of harvest in
recent decades (Fig. 2) were largely in the
Pacific north-west and the south-east. Elsewhere
rates have declined or remained relatively constant
since 1930. Rates of fuelwood harvest peaked
between 1860 and 1880 and showed a recent
increase again after the mid-1970s.
Changes in carbon
The total amount of carbon in live vegetation
was 30 PgC in 1990, about 80% of which was
in forests and woodlands (Table 1). Soil carbon,
to a depth of 1 m, held about 86 PgC, 40% of
which was in forests. The total change in carbon
stocks over the 290-year period was a reduction
of 32.6 PgC (Fig. 6). About 32 PgC were lost
from live vegetation; about 7 PgC were lost from
soils. These losses were offset partially by the
accumulation of 6 PgC in wood products and
0.3 PgC in slash. The initial (1700) carbon in
live biomass was reduced by more than 50%
over the 290 years (Table 1). Changes in forests
(29 PgC) accounted for 90% of the carbon lost.
About 90% of the loss from forests resulted
from changes in the area of forests (deforestation
and reforestation); about 10% resulted from a
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 134 Wednesday, May 10, 2000 2:02 PM
134
R. A. Houghton and J. L. Hackler
Fig. 3 Areas of cropland in different regions of the United States. Data were obtained from agricultural
statistics (Primack, 1963; US Bureau of the Census, 1977; USDA, 1996) after 1850 and derived from
population growth before 1850.
Fig. 4 Major classes of land cover in the United States between 1700 and 1990. Changes in area were
determined by rates of agricultural expansion and abandonment in seven regions. The spatial distribution
of agricultural change determined the type of biome affected. The area of regrowing forests was calculated
from rates of harvest and agricultural abandonment.
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 135 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
135
Fig. 5 Changes in forest area in different regions of the United States between 1900 and 1990, showing the
increase in eastern forests beginning in 1910. Before 1945, changes in forest area were inferred from changes in the
area of farmlands (US Bureau of the Census, 1977); after 1945 they were obtained directly from USDA (1996).
Fig. 6 Stocks of carbon (Pg) in the United States
in 1990 (italics), changes in these stocks (TgC) over
the period 1700 –1990, and exchanges (TgC) between
the major carbon pools and the atmosphere (1700 –
1990) as a result of changes in land use. Carbon was
lost from living biomass and soils; it has accumulated
in wood products and slash (logging debris). Total
uptake and release of carbon may be obtained both
from summing the exchanges with the atmosphere
and from summing the changes in pools.
reduction of carbon stocks within forests (harvest
and regrowth). The release of 14 PgC from live
vegetation to the atmosphere (Fig. 6) resulted
from fires associated with agricultural clearing.
The uptake of 17 PgC represents regrowth
following harvests or agricultural abandonment.
Fuelwood accounted for about 30% of the 19 PgC
released from products.
Clearing natural ecosystems for agriculture,
with subsequent cultivation, accounted for more
than 83% of the long-term release of 32.6 PgC
(Fig. 7). The annual loss of carbon increased from
less than 50 TgC/yr before 1800 to a maximum
of 400 TgC/yr near 1880 before declining again.
Since 1950 the net flux has been close to zero,
varying between ± 50 TgC/yr. Emissions of carbon
from harvest of fuelwood peaked at about
60 TgC/yr around 1870. The reduced use of
fuelwood was responsible for the largest continued sink for carbon in regrowing forests.
Emissions of carbon associated with the harvest
of industrial wood, including the storage of
carbon in products and their oxidation, were
generally small, but since 1960 these emissions
have been the largest source of carbon to the
atmosphere.
Three regions, the north-east, the south-east
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 136 Wednesday, May 10, 2000 2:02 PM
136
R. A. Houghton and J. L. Hackler
Fig. 7 Annual flux of carbon between the United States and the atmosphere calculated for different types
of land-use change. Positive values indicate a source of carbon to the atmosphere. Clearing for croplands with
subsequent cultivation has been the major source of carbon to the atmosphere. Reduced rates of fuelwood
harvest after 1880 changed the annual flux of carbon from a source to a sink as a result of forest regrowth.
and the east-north-central, released the largest
amounts of carbon before 1920 and were also
the major sinks of carbon after 1940 (Fig. 8). The
annual sink has decreased since the late 1960s,
however, as rates of logging and clearing have
accelerated again in these regions (Figs 5 and 8)
(see also Irland, 1998; Miller et al., 1998).
DISCUSSION
Other management practices affecting
the storage of carbon
The loss (and gain) of soil organic carbon with
cultivation (and abandonment of cultivated land)
accounted for about 23% of the long-term
change in terrestrial carbon stocks calculated
here. This loss was calculated on the basis of
changes in the area of croplands, each ha losing
about 25% of its initial carbon to a depth of 1 m.
However, increases in crop productivity, conservation tillage and the Conservation Reserve Program have also affected the storage of soil carbon
over the last decades, and these activities were
not considered above. The use of fertilizers, for
example, increases not only crop production
but the amount of crop residue, some of which
remains in the soil after harvest and increases
carbon storage. Donigian et al. (1995), using
the CENTURY model over 87 × 106 ha of cropland in the central United States (60 –70% of US
cropland), calculated that soil organic carbon
had stabilized in the region by 1940 –50 and
had begun to increase after 1960 –70, largely as
a result of increased productivity. By 1990,
about 25% of the carbon initially lost through
cultivation had been recovered. The calculations
were based on changes in the top 20 cm of soil
(the plough layer), and the change to a depth of
1 m would have been only half as much relatively.
We estimate that about 2.7 PgC accumulated in
croplands from increased productivity (Table 3).
The rate of accumulation has averaged 107 TgC/yr
since the mid-1960s.
Carbon may also accumulate in soil when conventional tillage is replaced with reduced-till or
no-till practices. Smith et al. (1998), for example,
reported an accumulation rate of 0.73% year–1
for the top 25–30 cm of soil under no-till conditions. We assumed that a rate of 0.35% year–1
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 137 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
137
Fig. 8 Annual net flux of carbon between the atmosphere and different regions of the United States
calculated from changes in land use. Positive values indicate a release of carbon to the atmosphere.
Eastern regions became a net sink for carbon after about 1940.
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 138 Wednesday, May 10, 2000 2:02 PM
138
R. A. Houghton and J. L. Hackler
Table 3 Fluxes of carbon in the United States attributed to different forms of land management. Positive
values indicate a net release to the atmosphere; negative values indicate an accumulation on land
Changes in land use
Increased crop productivity
Conservation tillage
Conservation Reserve Program
Fire suppression
Total
1700 –1990 (PgC)
1950 –90 (TgC/yr)
32.6
–2.7
– 0.3
– 0.05
–24.6
5
–6
–1071
–222
–17 2
–2383
–390
1
These estimates refer to the shorter interval 1965 – 90. 2 This rate of accumulation applies only to the late
1980s. 3 113 TgC/yr in forests; 125 TgC/yr in non-forests as a result of woody encroachment (Houghton
et al., 2000).
would apply to a depth of 1 m and calculated
an increase in the rate of carbon storage from
essentially zero before 1965 to 22 TgC/yr in 1990
(Table 3). Conservation tillage was practised on
about 30% of US cropland by 1992 (Cannell &
Hawes, 1994).
The Conservation Reserve Program (CRP)
in the United States is also likely to have
increased the storage of carbon in soil. It was
introduced in the late 1980s to reduce erosion
by converting marginal croplands to perennial
grass cover. If the rate of carbon accumulation
observed after 5 years on CRP sites in Texas,
Kansas and Nebraska (~1 MgC ha–1 year–1)
(Gebhart et al., 1994) is applied to the 17 × 106 ha
in the CRP, the Program would account for
an additional carbon sink of about 17 TgC/yr
(Table 3). Adding all three of these agricultural
management processes together increased the
estimated carbon sink in non-forest soils by
146 TgC/yr over the last few years, largely in the
north central regions. Over the period 1700 –1990,
these practices reduced the net loss of carbon
by 3 PgC (less than 10%) (Table 3).
Comparison with earlier estimates
The net flux of carbon estimated here is different in timing and magnitude from the earlier
estimate for North America by Houghton &
Hackler (1995) (Fig. 9). The comparison is not
strictly valid because Canada is included in the
earlier estimate for North America and not in
the estimate reported here. Nevertheless, a comparison shows an increase in the long-term flux
(32.6 PgC here vs. 25.9 PgC for North America)
and a significant change in its historical pattern.
The revised flux is lower in the eighteenth century, higher in the nineteenth century, and little
changed after 1930. In comparison to the original estimate (12.7 PgC for the interval 1850 –
1990), the new estimate (22.6 PgC) is about 75%
higher. The reasons for the difference include a
greater spatial disaggregation (seven regions instead
of one, and 18 natural ecosystems instead of five),
better data on land-use change (including higher
temporal and spatial resolution), a greater use
of primary forests so that fewer remained in
1990 and generally lower estimates in this analysis of wood density, initial biomass and soil
carbon in grasslands. The greater spatial resolution allowed us to consider simultaneous clearing
and abandonment of agricultural land; for
example, abandonment of farms in the northeast and simultaneous clearing of new agricultural land in the north central region. The
higher emissions of carbon near the turn of the
century resulted from two changes: higher rates
of clearing and a greater proportion of the clearing
occurring in forests as opposed to grasslands.
Including Canada in a new North American
estimate would presumably increase the long-term
release of carbon reported here, although
Canadian forests appear to have been a small
sink for carbon between 1920 and 1970 (Kurz
& Apps, 1999).
The results for individual regions are consistent with earlier, independent analyses. For
example, the south-eastern United States was
found here to have been a source of carbon
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 139 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
139
Fig. 9 Estimates of the annual flux of carbon to or from the atmosphere from different analyses of landuse change. Compared to the earlier analysis of North America (Houghton & Hackler, 1995), the analysis
for the United States reported here calculated higher emissions of carbon during the nineteenth century
and, accounting for recent changes in management of agricultural soils, a higher sink for carbon after 1950.
until about 1940 and thereafter a sink, while
Delcourt & Harris (1980) found the transition
from source to sink to have occurred around
1950. The annual fluxes are comparable given
that the two analyses did not include exactly
the same states in the south-east. Huntington
(1995) also demonstrated a regional carbon sink
following abandonment of farmlands in Georgia
in the early 1900s. In contrast, the forests of
the Pacific north-west have been a net source of
carbon. The source calculated here is similar to
the source calculated for 10 × 106 ha of forested
land in western Oregon (Cohen et al., 1996).
Using Landsat data over the period 1972 – 91,
the authors found an average annual source of
1.13 MgC ha–1 year –1. The average source determined here for the same time period but for a
larger area of forests, including Oregon, Washington
and northern California, was 0.53 MgC ha–1 year –1.
Our results are also consistent with changes in
carbon storage associated with land-use change
in north-central regions: a net loss in soil carbon
in upper Michigan since European settlement
(Owens et al., 1999) and a net increase in the
carbon of both vegetation and soils of east-central
Minnesota since the early 1900s (Johnston
et al., 1996).
Independent estimates based on
different methods
It is important to recognize that the net flux
of carbon attributable to changes in land use
and forestry is not necessarily equivalent to
the total net flux from terrestrial ecosystems. One
of the primary reasons for calculating the landuse flux, despite its not being the total terrestrial flux, is that it enumerates one of the
mechanisms responsible for a carbon sink; that
is, the accumulation of carbon in forests recovering from past harvests or abandonment of agriculture. If the flux of carbon attributed to land-use
change and the total net terrestrial flux were
equivalent, there would be no need to invoke
mechanisms to explain an additional carbon sink.
If the sink determined from land-use change
were less than the total net sink, then other
mechanisms would be necessary. CO2 fertilization,
nitrogen deposition and variations in climate are
three mechanisms thought to be important.
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 140 Wednesday, May 10, 2000 2:02 PM
140
R. A. Houghton and J. L. Hackler
Distinguishing between them, however, is difficult. The advantage of addressing land-use change
is that its associated flux of carbon can be determined directly. The areas affected are generally
recorded, and the changes in carbon are usually
large and well-characterized.
Four independent methods have been used
for the estimation of the total net flux of carbon,
and the results of these methods can be compared
with the land-use estimate reported here (Table 4).
One type is based on data from forest inventories.
Estimates of a net sink range between 79 TgC/yr
for the 1980s (Turner et al., 1995) and 280 TgC/yr
averaged over the period 1952 – 92 (Birdsey &
Heath, 1995). For the years around 1992 the
estimate by Birdsey & Heath (1995) is somewhat
less, a sink of 211 TgC/yr. The budgeting was
not identical in these analyses. For example,
Turner et al. (1995) assumed that all of the
carbon in harvested wood was released to the
atmosphere in the year of harvest. If some of
it accumulated (in the ratio of accumulation/
release reported for Canada) (Apps & Kurz,
1994), the net sink becomes about 140 TgC/yr
(Houghton, 1998). Similarly, if the emissions of
carbon from wood products are added to the
forest budget of Birdsey & Heath (1995), the net
flux becomes 230 TgC/yr for the period 1952–92,
and 161 TgC/yr for the period around 1992.
Thus, the two estimates based on forest inventories
are in reasonable agreement (150 ± 100 TgC/yr).
The estimates are larger than, but overlap, the
1950– 90 average sink estimated here for all
managed ecosystems (6 ± 50 TgC/yr) and for
forests and woodlands alone (14 ± 50 TgC/yr).
Including the effects of fire and fire suppression
(Houghton et al., 2000) yields an estimated
carbon accumulation in US forests (67–119 TgC/yr)
that is within the range of estimates based on
forest inventories. The similarity suggests that
regrowth of forests from previous land-use change,
including fire suppression, explains a major part
of the observed sink in US forests.
In contrast to these low estimates of carbon
accumulation from changes in land use and from
forest inventories, the recent estimate for North
America based on atmospheric and oceanic data
and models (Fan et al., 1998) was 1.7 PgC/yr
for the period 1988–92. Although this estimate
includes all of North America, the greatest uptake
of carbon was for lands south of 51°, which
include a portion of Canada. The difference
between the estimates (0.15 ± 0.1 vs. 1.7 ± 0.5 PgC/yr)
is difficult to explain, but there are at least
three possibilities. The relatively short interval
1988 – 92 covered in the analysis by Fan et al.
(1998) includes a year and a half following the
eruption of Mt Pinatubo, when cooler temperatures may have reduced heterotrophic respiration
relative to net production, allowing a large, but
short term, accumulation of carbon that has not
appeared in the longer-term measurements of
forest inventories. The calculated effect of soils
may have been underestimated in the analysis
reported here and in analyses based on forest
inventories. Finally, ecosystems other than forests may have been important in storing carbon.
The assumptions used above to calculate the
current sink for carbon in agricultural soils
and the effects of fire suppression in non-forests
(Houghton et al., 2000) suggest that an upper
limit of 300 – 400 TgC/yr may be accumulated as a result of direct human activity in
both forests and non-forests (Table 3). These
liberal assumptions yield estimates that are less
than 25% of the sink found by Fan et al.
(1998). It is difficult to reconcile the large sink
with these other analyses unless it was short
term.
Measurements of CO2 flux above individual
forest stands offer a third independent measurement of carbon flux, although they are not easily
extrapolated to regions. Recent results from the
Harvard Forest (Massachusetts) (Goulden et al.,
1996) and Howland, Maine (Hollinger et al.,
1999), suggest annual net accumulations of about
2 MgC ha–1 year –1. These measured fluxes are
somewhat higher than the average for US forests
based on inventory data (0.32–1.15 MgC ha–1 year –1)
and for north-eastern forests based on changes
in land use (0.89 MgC ha –1 year–1) (Table 4).
Decay of wood products has been omitted from
these latter estimates to make them comparable
to the CO 2 flux observable in a forest. The
estimates are still lower than the uptake measured
at two forest sites, but that is to be expected
because large regions include recently logged stands
that are releasing carbon. The difference highlights
the importance of land-use history in determining
forest age and hence rate of forest growth. Knowing the spatial distribution of past and current
land use change is critical for extrapolating the
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
Reference
Area
Time period
Net uptake of carbon
Total
TgC/yr
Land-use change
This study
Land-use change
Houghton et al., 2000
Forest inventory
Birdsey & Heath, 1995
Birdsey & Heath, 1995
Turner et al., 1995
Fan et al., 1998
Atmospheric and
oceanic data and
models
CO2 flux
Ecosystem model
Goulden et al., 1996
Hollinger et al., 2000
Tian et al., 1999
Per ha of forest1
MgC ha−1 year−1
US2
US forests2
US forests2
(products excluded)
north-eastern
US forests
north-eastern US forests
(products excluded)
US forests2
(with fire suppression)
US
US
Conterminous US
North America
south of 51°N
1950 – 90
1950 – 90
1950 – 90
6
14
110
0.025
0.058
0.45
1950 – 90
1950 – 90
16
25
0.53
0.89
1950 – 90
67–119
0.41
Forest stand
Forest stand
Conterminous US
1991– 95
1996
1980s
1952– 92
~1992
1980s
1988 – 92
280
1603
79–1404
17005
78
1.15
0.66
0.32– 0.58
4.255
1.4–2.8
2.1
0.32
160 × 106 of Canadian forests south of 51°N; 243 × 106 ha of US forests (excluding Alaska and Hawaii); and 28 × 106 ha of forests in north-east US.
For comparison with other estimates, Alaska was excluded from this estimate. 3 Includes a 50 TgC/yr release from oxidation of wood products. 4 The
higher estimate assumes that some of the harvested wood is stored in long-term products rather than released to the atmosphere (Houghton, 1998).
5
Includes 160 × 106 ha of Canadian forests south of 51°N.
1
2
GEB166.fm Page 141 Wednesday, May 10, 2000 2:02 PM
Method
Carbon storage in the U.S.A.: agriculture and forestry
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
Table 4 Estimates of carbon uptake for different regions of the United States according to independent methods
141
GEB166.fm Page 142 Wednesday, May 10, 2000 2:02 PM
142
R. A. Houghton and J. L. Hackler
results of site-specific CO2 flux measurements
to larger regions.
Finally, a recent analysis based on an ecosystem
model that considered the effects of CO2 and
climatic variations on carbon storage found a
net average sink of 78 TgC/yr for the conterminous United States during the 1980s (Tian et al.,
1999) (Table 4). The uptake in forests was estimated
to be 43 TgC/yr. This estimate and the estimate
based on land-use change complement each
other because each includes processes ignored
by the other. Together, the estimates for forests (43 + (67–119) TgC/yr) (Table 4) are very close
to the estimates of carbon storage calculated
from forest inventories (150 ± 100 TgC/yr). Nonforests might account for an additional sink of
(35 + 271 =) 306 TgC/yr (Tian et al., 1999; this
study, respectively). The fact that four out of
five independent approaches are in general
agreement casts doubt on the existence of a large
North American sink.
ACKNOWLEDGMENTS
We thank Linda Heath of the US Forest Service
for bringing to our attention estimates of fuelwood use, Jim Jamski of the US Bureau of Census
for directing us to data on lumber and mill
production and Anatoly Shvidenko and Hanqin
Tian for thoughtful and constructive reviews of
an earlier version of the manuscript. Kira
Lawrence assisted with many aspects of the
work, and we gratefully acknowledge her help.
The research was supported through the Joint
Program on Terrestrial Ecology and Global
Change (TECO), grant number NAGW-4748 from
the Terrestrial Ecology Program in NASA’s
Office of Earth Science.
REFERENCES
Ajtay, G.L., Ketner, P. & Duvigneaud, P. (1979)
Terrestrial primary production and phytomass.
The Global Carbon Cycle (ed. by B. Bolin,
E.T. Degens, S. Kempe & P. Ketner), pp. 129 –182.
John Wiley and Sons, New York.
Apps, M.J. & Kurz, W.A. (1994) The role of Canadian
forests in the global carbon budget. Carbon Balance
of World’s Forested Ecosystems: Towards a Global
Assessment (ed. by M. Kanninen), pp. 14–39.
Publications of the Academy of Finland 3/93,
Helsinki.
Bailey, R.G. (1995) Description of the Ecoregions of
the United States, 2nd edn. Miscellaneous Publication
no. 1391, 108 pp. US Department of Agriculture,
Forest Service, Washington, DC.
Birdsey, R.A. (1992) Carbon storage and accumulation in the United States forest ecosystems. Forest
Service General Technical Report WO-59, 51 pp.
US Department of Agriculture, Washington, DC.
Birdsey, R.A. & Heath, L.S. (1995) Carbon changes
in U.S. forests. Productivity of America’s Forest
and Climatic Change (ed. by L.A. Joyce), pp. 56–
70. General Technical Report RM-GTR-271. US
Department of Agriculture, Forest Service, Rocky
Mountain Forest and Range Experiment Station,
Fort Collins, Colorado.
Cannell, R.Q. & Hawes, J.D. (1994) Trends in tillage
practices in relation to sustainable crop production
with special reference to temperate climates. Soil
and Tillage Research, 30, 245–282.
Cohen, W.B., Harmon, M.E., Wallin, D.O. &
Fiorella, M. (1996) Two decades of carbon flux
from forests of the Pacific northwest. Bioscience,
46, 836–844.
Dai, A. & Fung, I.Y. (1993) Can climate variability
contribute to the ‘missing’ CO 2 sink? Global
Biogeochemical Cycles, 7, 599–609.
Davidson, E.A. & Ackerman, I.L. (1993) Changes
in soil carbon inventories following cultivation
of previously untilled soils. Biogeochemistry 2,
161–193.
Delcourt, H.R. & Harris, W.F. (1980) Carbon
budget of the southeastern U.S. biota: analysis
of historical change in trend from source to sink.
Science, 210, 321–323.
Donigian, A.S., Patwardhan, A.S., Jackson, R.B.,
Barnwell, T.O., Weinrich, K.B. & Rowell, A.L.
(1995) Modeling the impacts of agricultural
management practices on soil carbon in the central
U.S. Advances in Soil Science: Soil Management
and Greenhouse Effect (ed. by R. Lal, J. Kimble,
E. Levine & B.A. Stewart), pp. 121–135. CRC
Press, Boca Raton, Florida.
Earl, D.E. (1975) Forest Energy and Economic
Development, 128 pp. Clarendon Press, Oxford.
Fan, S., Gloor, M., Mahlman, J. et al. (1998) A
large terrestrial carbon sink in North America
implied by atmospheric and oceanic CO2 data and
models. Science, 282, 442–446.
FAO (1996) FAOSTAT. Forestry Data. http://www.fao.org/
forestry
Gebhart, D.L., Johnson, H.B., Mayeux, H.S. &
Polley, H.W. (1994) The CRP increases soil organic
carbon. Journal of Soil and Water Conservation,
49, 488–492.
Goulden, M.L., Munger, J.W., Fan, S.-M., Daube, B.C.
& Wofsy, S.C. (1996) Exchange of carbon dioxide
by a deciduous forest: response to interannual
climate variability. Science, 271, 1576–1578.
Hall, C.A.S. & Uhlig, J. (1991) Refining estimates
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 143 Wednesday, May 10, 2000 2:02 PM
Carbon storage in the U.S.A.: agriculture and forestry
of carbon released from tropical land-use change.
Canadian Journal of Forest Research, 21, 118–131.
Harmon, M.E., Garman, S.L. & Ferrell, W.K.
(1996) Modeling historical patterns of tree utilization in the Pacific northwest: carbon sequestration
implications. Ecological Applications, 6, 641–652.
Hart, J.F. (1968) Loss and abandonment of cleared
farm land in the eastern United States. Annals of
the Association of American Geographers, 58, 417–
440.
Hollinger, D.Y., Goltz, S.M., Davidson, E.A., Lee,
J.T., Tu, K. & Valentine, H. (1999) Seasonal patterns and environmental control of carbon dioxide
and water vapor exchange in an ecotonal boreal
forest. Global Change Biology, 5, 891–902.
Houghton, R.A. (1995) Effects of land-use change,
surface temperature, and CO2 concentration on
terrestrial stores of carbon. Biotic Feedbacks in
the Global Climatic System: Will the Warming
Feed the Warming? (ed. by G.M. Woodwell &
F.T. Mackenzie), pp. 333 –350. Oxford University
Press, New York.
Houghton, R.A. (1998) Historic role of forests in the
global carbon cycle. Carbon Dioxide Mitigation in
Forestry and Wood Industry (ed. by G.H. Kohlmaier,
M. Weber & R.A. Houghton), pp. 1– 24. SpringerVerlag, Berlin.
Houghton, R.A. & Hackler, J.L. (1995) Continental
Scale Estimates of the Biotic Carbon Flux from
Land Cover Change: 1850–1980, ORNL/CDIAC-79,
NDP-050, 144 pp. Oak Ridge National Laboratory,
Oak Ridge, Tennessee.
Houghton, R.A., Hackler, J.L. & Lawrence, K.T.
(2000) Changes in terrestrial carbon storage in the
United States. 2. The role of fire and fire management. Global Ecology and Biogeography, 9, 145–
170.
Houghton, R.A., Hobbie, J.E., Melillo, J.M., Moore,
B., Peterson, B.J., Shaver, G.R. & Woodwell, G.M.
(1983) Changes in the carbon content of terrestrial
biota and soils between 1860 and 1980: a net release
of CO2 to the atmosphere. Ecological Monographs, 53, 235 –262.
Houghton, R.A., Boone, R.D., Fruci, J.R., Hobbie,
J.E., Melillo, J.M., Palm, C.A., Peterson, B.J.,
Shaver, G.R., Woodwell, G.M., Moore, B., Skole,
D.L. & Myers, N. (1987) The flux of carbon from
terrestrial ecosystems to the atmosphere in 1980 due
to changes in land use: geographic distribution of
the global flux. Tellus, 39B, 122–139.
Huntington, T.G. (1995) Carbon sequestration in
an aggrading forest ecosystem in the southeastern
USA. Soil Science Society of America Journal,
59, 1459 –1467.
Irland, L.C. (1998) Maine lumber production. 1839–
1997: a statistical overview. Maine History, 38, 36 – 49.
Johnson, D.W. (1992) Effects of forest management
on soil carbon storage. Water, Air and Soil Pollution, 64, 83 –120.
143
Johnston, M.H., Homann, P.S., Engstrom, J.K. &
Grigal, D.F. (1996) Changes in ecosystem carbon
storage over 40 years on an old-field/forest landscape in east-central Minnesota. Forest Ecology and
Management, 83, 17–26.
Kern, J.S. (1994) Spatial patterns of soil organic carbon
in the contiguous United States. Soil Science Society
of America Journal, 58, 439–455.
Kurz, W.A. & Apps, M.J. (1999) A 70-year retrospective analysis of carbon fluxes in the Canadian
forest sector. Ecological Applications, 9, 526–547.
McEvedy, C. & Jones, R. (1978). Atlas of World
Population History, 368 pp. Penguin Books, Middlesex, England.
Miller, A.B., Bryant, E.S. & Birnie, R.W. (1998) An
analysis of land cover changes in the northern forest
of New England using multitemporal Landsat
MSS data. International Journal of Remote Sensing,
19, 245–265.
Moore, B., Boone, R.D., Hobbie, J.E., Houghton,
R.A., Melillo, J.M., Peterson, B.J., Shaver, G.R.,
Vorosmarty, C.J. & Woodwell, G.M. (1981) A
simple model for analysis of the role of terrestrial
ecosystems in the global carbon budget. Carbon
Cycle Modelling (ed. by B. Bolin), pp. 365–385.
John Wiley and Sons, New York.
Olson, J.S., Watts, J.A. & Allison, L.J. (1983) Carbon in
live vegetation of major world ecosystems, TR004,
152 pp. US Department of Energy, Washington, DC.
Owens, K.E., Reed, D.D., Londo, A.J., Maclean,
A.L. & Mroz, G.D. (1999) A landscape level comparison of pre-European settlement and current
soil carbon content of a forested landscape in upper
Michigan. Forest Ecology and Management, 113,
179–189.
Primack, M.L. (1963) Farm formed capital in American
agriculture, 1850 –1910. PhD Dissertation, University
of North Carolina, Chapel Hill.
Pyne, S.J. (1982) Fire in America. A Cultural History
of Wildland and Rural Fire, 654 pp. Princeton
University Press, Princeton, New Jersey (republished
in 1997 by the University of Washington Press,
Seattle).
Reynolds, R.V. & Pierson, A.H. (1942) Fuelwood
used in the United States 1630–1930. Circular 641,
20 pp. US Department of Agriculture, Washington,
DC.
Richards, J.F., Olson, J.S. & Rotty, R.M. (1983)
Development of a data base for carbon dioxide
releases resulting from conversion of land to agricultural uses, ORNL/TM-8801. Oak Ridge
National Laboratory, Oak Ridge, TN.
Schlesinger, W.H. (1986) Changes in soil carbon storage and associated properties with disturbance and
recovery. The Changing Carbon Cycle: a Global
Analysis (ed. by J.R. Trabalka & D.E. Reichle),
pp. 194 – 220. Springer-Verlag, New York.
Schroeder, P., Brown, S., Mo, J., Birdsey, R. &
Cieszewski, C. (1997) Biomass estimate for
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144
GEB166.fm Page 144 Wednesday, May 10, 2000 2:02 PM
144
R. A. Houghton and J. L. Hackler
temperate broadleaf forests of the United States
using inventory data. Forest Science, 43, 424–434.
Smith, P., Powlson, D.S., Glendining, M.J. & Smith, J.U.
(1998) Preliminary estimates of the potential for
carbon mitigation in European soils through no-till
farming. Global Change Biology, 4, 679–685.
Tian, H., Melillo, J.M., Kicklighter, D.W.,
McGuire, A.D. & Helfrich, J.V.K. (1999) The
sensitivity of terrestrial carbon storage to historical
climate variability and atmospheric CO2 in the
United States. Tellus, 51B, 414 – 452.
Turner, D.P., Koerper, G.J., Harmon, M.E. & Lee, J.J.
(1995) A carbon budget for forests of the conterminous United States. Ecological Applications,
5, 421– 436.
US Bureau of the Census (1977) Historical Statistics
of the United States from Colonial Times to 1970.
US Bureau of the Census, Washington, DC.
US Forest Service (1998) Supply and consumption
of wood and wood products. http://www.fs.fed.us/
land/sustain_dev/sd / criter6.htm#PC31.
USDA (1989) RPA Assessment of the Forest and
Rangeland Situation in the United States, 1989.
Forest Resources Report 26. US Department of
Agriculture, Forest Service, Washington D.C.
USDA (1996) Major Land Uses (1945 – 92), Report
no. 89003. Economic Research Service, USDA,
Washington, DC. http://usda.mannlib.cornell.edu/.
Waddell, K.L., Oswald, D.D. & Powell, D.S. (1989)
Forest statistics of the United States, 1987,
Resource Bulletin PNW-RB-168, 106 pp. US
Department of Agriculture, Forest Service, Pacific
Northwest Research Station, Portland, OR.
Wernick, I.K., Waggoner, P.E. & Ausubel, J.H.
(1998) Searching for leverage to conserve forests.
The industrial ecology of wood products in the
United States. Journal of Industrial Ecology, 1,
125–145.
Whitney, G.G. (1994) From coastal wilderness to
fruited plain. A History of Environmental Change
in Temperate North America from 1500 to the
Present, 451 pp. Cambridge University Press,
Cambridge, UK.
Whittaker, R.H. & Likens, G.E. (1973) Carbon
in the biota. Carbon and the Biosphere (ed. by
G.M. Woodwell & E.V. Pecan), pp. 281–302.
Symposium Series 30. US Atomic Energy Commission, National Technical Information Service,
Springfield, Virginia.
Williams, M. (1989) Americans and Their Forests,
599 pp. Cambridge University Press, New York.
Woodwell, G.M., Hobbie, J.E., Houghton, R.A.,
Melillo, J.M., Moore, B., Peterson, B.J. & Shaver,
G.R. (1983) Global deforestation: contribution to
atmospheric carbon dioxide. Science, 222, 1081–
1086.
© 2000 Blackwell Science Ltd, Global Ecology & Biogeography, 9, 125 – 144