EPA/600/A-97/016
FORESTS AND CLIMATE CHANGE: ROLE OF FOREST
LANDS AS CARBON SINKS
Sandra Brown
US Environmental Protection Agency
National Health and Environmental Effects Research Laboratory
Western Ecology Division
200 SW 35th Street
Corvallis, OR 97333, USA
Summary
Forests potentially contribute to global climate change through their influence on the global
carbon (C) cycle. They store large quantities of C in vegetation and soil, exchange C with the
atmosphere through photosynthesis and respiration, are sources of atmospheric C when they are
disturbed, become atmospheric C sinks during abandonment and regrowth after disturbance, and
can be managed to alter their role in the C cycle. The world's forest contain about 830 Pg C
(10I5g) in their vegetation and soil, with about 1.5 times as much in soil as in vegetation.
During the 1980s, analysis of C budgets show that forest of the temperate and boreal countries
were a net sink of atmospheric C of about 0.7 Pg yr"1, but the tropics were a net source of about
1.6 Pg yr"1.However, accounting for the imbalance in the global C cycle suggests that forest are
not significantly contributing to the net increase in atmospheric C02 and thus not contributing to
global climate change. However, this may not continue into the future as temperate and boreal
forests reach maturity and become a smaller C sink, and if rates of tropical deforestation and
degradation continue to accelerate. Recent studies suggest that there is the potential to manage
forests to conserve and sequester C to mitigate emissions of carbon dioxide by an amount
equivalent to 11-15% of the fossil fuel emissions over the same time period. Aggressive
adoption of these forest management options are necessary to prevent forests becoming a
significant net source of C02 to the atmosphere in the future and contributing to climate change.
Keywords: carbon budgets, climate change, forests, global carbon cycle, mitigation
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INTRODUCTION
The global C cycle is recognized as one of the major biogeochemical cycles because of its role in
regulating the concentration of C02, an important greenhouse gas (GHG), in the atmosphere.
Increasing concentrations of C02 in the atmosphere are a major contributor to climate change
(Schimel et al, 1995), Forests play an important role in the global C cycle because they store
large quantities of C in vegetation and soil, exchange C with the atmosphere through
photosynthesis and respiration, are sources of atmospheric C when they are disturbed by human
or natural causes (e.g., wildfires, use of poor harvesting procedures, cleared and burned for
conversion to non-forest uses), and become atmospheric C sinks (i.e., net transfer of CO2 from
the atmosphere to the land) during land abandonment and regrowth after disturbance. Humans
have the potential through forest management to alter forest C pools and flux, and thus alter their
role in the C cycle and their potential to change climate.
Forests have the potential to influence climate change in other ways too, particularly
when they are disturbed by humans. For example, conversion of forests to other land cover types
can affect climate through changes in albedo or reflectivity of the land. Furthermore, the
destruction of forest biomass by burning releases GHGs in addition to C02 which are by-
products of incomplete combustion, namely, methane (CI14), carbon monoxide (CO), nitrous
oxide (N20), and NOx among others. Whereas complex accounting models and forest
inventories are needed to estimate the losses and gains of C02 over different time scales, the
emissions of these other gases from biomass burning are instantaneous and absolute transfers
from the biosphere to the atmosphere. Globally, biomass burning contributes about 10% of the
total annual CFI4 emissions, 10-20% of total annual N20 emissions, and about half of the CO
emissions, and so has a significant effect on atmospheric chemistry, especially on tropospheric
ozone levels (Houghton et al., 1992). Biomass burning also transfers a fraction (up to 10%) of
the C to an inert form (charcoal) with a turnover time that is practically infinite.
Many forests, in both boreal and tropical latitudes, grow on peat or organic soils that
contain large amounts of C. Undisturbed anaerobic, peatlands are sinks for C02 and sources of
CH4. Drainage of these soils to improve forest productivity virtually stops CH4 emissions, but
initiates rapid C02 emissions by aerobic decomposition. Draining peat soils for forest
establishment can produce a C loss from these soils that exceeds that stored in the forest if 20-30
cm of peat decompose as a result of the drainage (Cannell et al. 1993).
The purpose of this paper is to present the current state of knowledge about the role of
forest in the global C cycle and thus their influence on climate change. The paper focuses on the
magnitude of the present C pools and flux (sources and sinks) for the world's forests, and the
potential role of forest lands to mitigate C02 emissions through management. Much of the
material presented here is drawn from a recent review of these topics for the Intergovernmental
Panel on Climate Change (IPCC) Second Assessment Report for 1995 (Brown et al. 1996).
CURRENT ROLE OF FORESTS IN THE GLOBAL C CYCLE
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Native forests cover about 3.4 Gha (Gha = 10^ or billion ha) (Food and Agriculture
Organization [FAO] 1995). Most of the forests are in the low latitudes (0-25° N&S) or tropical
zone (52%), followed by the high latitudes (50-75° N&S) or boreal zone (30%), and mid
latitudes (25-50° N&S) or temperate zone (18%). Globally there is about an additional 1.7 Gha
of other wooded lands, with some forestry characteristics, including open woodland and scrub,
shrub and brushland. These lands are probably technically suitable for forests, but they are
presently degraded or otherwise under-producing because of environmental factors or human
misuse. Furthermore, in the tropics, there are 31 Mha (Mha = 10^ or million ha) of plantations,
and an additional 37.6 Mha in mid-latitude developing countries, most of which are in China
(85%) (FAO 1995). The total area of plantations in developed countries is not available, but
25.4 Mha were reported to have been established during the decade of the 1980s in most
European countries, Canada, former Soviet Union, and Japan (ECE/FAO 1992).
Forests are influenced by natural and human causes, including harvesting, over-
harvesting and degradation, large-scale occurrence of wildfire, fire control, pest and disease
outbreaks, and conversion to non-forest use, particularly agriculture and pastures. These
disturbances often cause forests to become sources of C02 because the rate of net primary
productivity is exceeded by total respiration or oxidation of plants, soil, and dead organic matter
(net ecosystem produc tion [NEPJ < 0). At the same time, however, some areas of harvested and
degraded forests or agricultural and pasture lands are abandoned and revert naturally to forests or
are converted to plantations, thus becoming C sinks, i.e., the rate of respiration from plants, soil
and dead organic matter is exceeded by net primary productivity (NEP > 0).
The current role of forests in the global C cycle is not only a function of present forest
land use, but also of past use and disturbance. Prior to this century C02 emissions from changes
in forest land use, mainly caused by agricultural expansion in mid- and high latitude countries,
were higher than emissions from the combustion of fossil fuels (Houghton and Skole, 1990).
From the turn of the century until about the 1930s, global CO2 emissions from changes in forest
land use were similar in magnitude to those from fossil fuel combustion. After about the 1940s,
CO2 emissions from the changes in forest land use in the tropics dominated the flux from the
biota to the atmosphere. Since then, world-wide fossil fuel use has soared, biotic emissions from
the mid- and high- latitude regions has declined greatly as forests expanded onto abandoned
agricultural lands and as logged stands regrew, and deforestation in the tropics has accelerated
(Houghton et al., 1987). The past and present patterns of land use are responsible for the current
situation in regard to the C pools and flux of the world's forests.
High-Latitude Forests
The total C pool in high-latitude forests is about 278 Pg(l Pg = 1015 g or billion metric
tons) (Table 1), however this does not include the Nordic countries which are included in Europe
in Table 1 (the Nordic countries could add about another 10 Pg C). The soil C pool dominates
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the total C pool in forests of the boreal zone (71% of the total pool) as expected. Forest of the
Former Soviet Union (FSU) contains most of the C in this zone (63%).
The high-latitude zone is currently estimated to be a C sink of 0.48 ± 0.2 Pg yr"1 (Table
1), with practically all of this occurring in the FSU. The magnitude of the C sink in forests of
Canada has been declining since the early 1980s as a result of increases in disturbance by
harvesting, insect outbreaks, and fires (Kurz and Apps 1996). The effect of increased
disturbance is to increase the dead organic matter pool resulting in higher C emissions due to
subsequent decomposition of this material.
Mid-Latitude Forests
Forests of the mid-latitudes contain 120 Pg of C in vegetation and soil, with soil
accounting for about 58% of the total (Table 1). Estimates of C budgets for some mid-latitude
countries, such as the non-tropical parts of South America, Africa, and Asia/Oceania are not
available and thus the C pool in all mid-latitude forests is underestimated. In vegetation alone
this could account for about an additional 12 Pg (based on biomass data in FAO [1995],
corrected for other aboveground biomass components and belowground biomass).
Like the high-latitude forests, mid-latitude forests are also estimated to be a C sink of
0.26 ± 0.1 Pg yr"1. However, as mentioned above C flux estimates have not been made for all
mid-latitude forests. About 230,000 ha of forests are being lost per yr for all the countries for
which no C flux estimates are available (FAO 1995). Although forests are being lost is does not
necessarily mean that they are a C source. For example, forests of the US are estimated to be a
C sink, but there is a net loss of forest area of about 300,000 ha per yr. Apparently the regrowth
of the remaining forests is more than enough to offset the C source form the deforestation. China
on the other hand has a large reforestation program, establishing more than 1.1 Mha of new
forests per yr, but at the same time native forests are being deforested (FAO 1995), with the net
effect that China is a small C source (Table 1).
Mid-latitude forests, as well as those in the high-latitude zone are for the most part C
sinks because: (1) they are, on average, composed of relatively young classes with higher rates of
net production as they recover from past human and natural disturbances; (2) a larger proportion
of these forests are actively managed, i.e., established, tended, and protected; and (3) some areas
may be responding to increased levels of atmospheric C02 and nitrogen (fertilization effect)
(Brown et. al. 1996). However, there is a finite time period over which this C sink can occur.
For example, the current C sink in European forests may disappear within 50 to 100 yr (Kauppi
et al. 1992), although others suggest that it may take forests up to several centuries to millennia
to reach a C steady state in all components, including coarse woody debris and soil (Lugo and
Brown 1986).
Low-Latitude Forests
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Tropical or low-latitude forests contain about 428 Pg C or 52% of the C pool of all the
World's forests (Table 1). The C is about equally divided between the vegetation and soil.
Forest in tropical America contain the most —about 53% of the total tropical pool, and tropical
Africa contains the least — about 27%. These proportions reflect the differences in area of
humid forests in the two regions; humid tropical forests contain high biomass C.
Tropical forests are estimated to be a relatively large net C source of 1.6 ± 0.4 Pg per yr
in 1990 (Table 1) caused by deforestation, harvesting, and gradual degradation of the growing
stock. The C flux to the atmosphere from the forests of Asia is about equal to that from the
forests of America, and both account for almost 80% of the tropical C source in about 1990. The
total tropical source is equivalent to almost 28% of the 1990 fossil fuel emissions.
The estimated large C source from the tropics is due mostly to the high rates of
deforestation in this region, currently estimated to be about 15.4 Mha/yr during 1980-90, but
with large uncertainties (FAO 1993). Much of the deforested area is converted to agricultural
land, pastures, or shifting cultivation which have considerably lower biomass C than forests. In
addition to deforestation, large areas of forests are harvested. For example, about 5.9 Mha/yr of
tropical forests were logged during 1986-90, mostly from mature forests (83%) rather than
secondary forests (FAO 1993). These harvested forests can regenerate and accumulate C if they
are not severely damaged during harvesting operations, are protected, or are relatively
inaccessible to human populations, but many of them become degraded (e.g.. Brown et al.
1993a,b). Forest degradation, resulting in a loss of C in the vegetation and/or soil, occurs
through activities such as damage to residual trees and soil from poor logging practices, log
poaching, fuel wood collection, overgrazing, and anthropogenic fire (Goldammer 1990; Brown et
al. 1993b: FAO 1993; Flint and Richards 1994).
Although the flux of 1.6 ± 0.4 Pg C yr"1 is the best estimate available in the literature,
there are many reasons to believe that the mean is smaller than and the uncertainty range is larger
than shown (Lugo and Brown 1992). Unlike the temperate and boreal forests where estimated C
fluxes are, for the most part, based on data from periodic national inventories (i.e., field
measurements), the estimated C flux for tropical forests is based on a model (Houghton et al.
1987). The model tracks forests that are cleared or harvested with regrowth, and C is allowed to
accumulate in regrowing forests in the models for up to about 50-100 years. Furthermore, the
model assumes that all other forests not reported to be directly affected by humans during the
period of model simulation (about 1850-1990) are in C steady state and that none of the regrowth
is influenced by increased levels of atmospheric €02 and nitrogen. Recent work questions the
steady state assumption because humans have brought about changes in forest cover over the
centuries to the present (Lugo and Brown 1986, Brown et al. 1992). This implies that the net
tropical C flux could be higher or lower than that reported here depending upon the relative
contribution of forest lands that are still gaining C through recovery from past human
disturbances or are losing C through continued human use (Lugo and Brown 1992; Brown et al.
1993a) (see next section).
Are the World's Forests a C Sink or Source?
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The estimated net C flux from the world's forests is a source of 0.9 ± 0.5 Pg/yr, or about
16% of the amount produced by burning fossil fuels and cement manufacture. The error terms
associated with the C flux estimate is basically derived from the range of values resulting from
the use of different assumptions in the C budgets for a given country or region. They do not
represent errors derived from statistical procedures. Error enters the flux estimation procedure
through uncertainties and biases in the primary data and these compound as the data are
combined to draw inferences (Robinson 1989). Many estimates for components of the forest
sector C budget are probably known no better than ± 30% of their mean and others may be
known no better than >+ 50% of their mean (Robinson 1989). These errors are compounded in
making global estimates of C flux, perhaps to large proportions, but to what extent is presently
unknown.
The average annual global C budget for the 1980s is estimated as follows (Schimel et al.
1995):
Pg C yr"1
Emissions from fossil fuel and cement production
5.5 ±0.5
Emissions from change in tropical land use
1.6± 1.0
Total emissions
7.1 ± 1.1
Increase in storage in atmosphere
3.310.2
Ocean uptake
2.0 ±0.8
Uptake by Northern Hemisphere forest growth
0.5 ±0.5
Total sinks
5.8+1.0
Difference (emissions-sinks)
1.3 ± 1.5
The imbalance between emissions and sinks of 1.3 ± 1.5 given above is often referred to as the
"missing sink", or that amount "needed" to balance the C budget. Schimel et al. (1995)
attributed this imbalance to enhanced forest growth due to C02 fertilization, increased N
deposition, and a positive response to climatic anomalies.
Substitution of the net C sink for high and mid-latitude forests reported here (Table 1)
into the global C budget (instead of the 0.5 ± 0.5) results in :
Pg yr'1
Total sink 6.0 ± 0.9
New difference or "imbalance" 1.1± 1.4
As the primary data for the biomass component of forest C budgets for Northern
Hemisphere countries originate from national forest inventories, any increased growth of forests
due to increased atmospheric C02 concentrations, increased N deposition, and climatic effects is
already included in the net flux estimates. In other words, the reported C sink for temperate and
boreal forests due to forest growth (Table 1) most likely includes all these factors already
because the data, for the most part, come from repeated forest inventories. In contrast, the
tropical forest C flux is based on a simulation model and not on repeated forest inventories
because there is no network of permanent forest inventory plots in most tropical countries.
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Furthermore, the model of tropical land-use change does not include algorithms to model the
effects of C02 and N fertilization and climate as discussed above. This leads one to conclude
that a large part of the imbalance in the global C budget must be due to a C uptake in tropical
latitudes. In other words, I propose that the C balance for tropical latitudes is;
The reduction in the tropical source could be due to a combination of stimulated regrowth from
CO, fertilization and N deposition and climate as well as more extensive forest regrowth and
continued C uptake by mature forests (Lugo and Brown 1992; Taylor and Lloyd 1992, Grace ct
al. 1995). It is clear that to resolve this issue, repeated national forest inventories, with
permanent plots, are needed in tropical latitudes.
The present state of understanding as given above suggests that the world's forest are
contributing little to the net increase in atmospheric C02 and thus contributing little to global
warming. However, this may not continue into the future as temperate and boreal forests reach
maturity and become a smaller C sink, and if rates of tropical deforestation and degradation
continue to accelerate. One solution for ensuring that forests do not become a larger C source is
through management of existing forests and increased establishment of forests on non-forested
lands, a "win-win" situation for sustainable development.
Forests have the potential to be managed to reduce atmospheric concentrations of C02
and thus mitigate climate change. Major objectives for managing forest lands generally include:
industrial wood and fuel production, traditional forest uses, protection of natural resources (e.g.,
biodiversity, water, and soil), recreation, rehabilitation of damaged lands, and the like. Forest
management practices that meet the objectives given above can be grouped into three categories
based on how they are viewed to curb the rate of increase in atmospheric C02; management for
C conservation, C storage, or C substitution (Brown et al. 1996). However, assessments of
forestry practices for mitigation of C02 emissions are often criticized. Critics generally assume
that such assessments view the sole purpose of forests for sequestering or conserving C. This is
usually not the case; the amount of C sequestered or conserved is an added benefit to more
traditional uses of forests.
The goal of conservation manafiement is to prevent C emissions to the atmosphere by
conserving existing C pools in forests as much as possible through options such as controlling
deforestation, protecting forest in reserves, changing harvesting regimes, and controlling other
anthropogenic disturbances such as fire and pest outbreaks. The most significant C conservation
practice clearly would occur in the tropics where deforestation and degradation is currently
estimated to emit about 1.6 Pg C yr"1 (Table 1). However, as much of the deforestation and
Emissions from changes in tropical land use
C uptake in tropics ("imbalance")
Net tropical source
Pfi vr'1
1.6 ± 1.0
1.1 ± 1.4
0.5
INCREASING C SINKS THROUGH FOREST MANAGEMENT
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forest degradation is caused by the expansion and degradation of arable and grazing lands and
subsistence and commodity demand for wood products, programs to reduce deforestation and
degradation must be accompanied by measures that increase agricultural productivity and
sustainability.
In recent years, there has been significant expansion of "protected areas" into areas of
both mature and secondary forests for conservation of biodiversity and sustainable timber
production. Carbon pools should remain the same or increase in size in these areas depending on
their present age-class distribution. It is also likely that the trend towards management of forests
for sustainable timber production will increase in the future. Using forests this way, including
extending rotation cycles, reducing damage to remaining trees, reducing logging waste,
implementing soil conservation practices, and using wood in a more C-efficient way, ensures
that a large fraction of their C is conserved.
The goal of storage management is to increase the amount of C in vegetation and soil of
forests by increasing the area and/or biomass C of natural and plantation forests, and to increase
storage in durable wood products. Increasing the C pool in vegetation and soil can be
accomplished by protecting secondary forests and other degraded forests whose biomass and soil
C densities are less than their maximum value and allowing them to sequester C by natural or
artificial regeneration and soil enrichment. Another approach is to establish plantations on non-
forested lands, promote natural or assisted regeneration in secondary forests followed by
protection, or increase tree cover on agricultural or pasture lands through agrofbrestry. The C
pool in durable wood products can be increased by expanding demand for wood products at a
faster rate than decay of wood and by extending the lifetime of wood products. Sequestering C
by storage management is only a short-term option, producing a finite C sequestration potential
beyond which little additional C can be accumulated. The process may take place over a time
period of the order of several decades to a century or more depending upon present age-class of
forests, the attainable maximum C density, forest type, species selection, wood products
produced, and latitudinal zone.
Substitution management aims at increasing the transfer of forest biomass C into products
(e.g., construction materials and biofuels) rather than using fossil-fuel-based energy and products
and cement-based products. Substitution management has the greatest mitigation potential in the
long term (>50 years) (Marland and Marland 1992). This approach involves expanding the use
of forests for wood products and fuels obtained either from establishing new forests or
plantations, or increasing the growth of existing forests through silvicultural treatments (Brown
et al. 1996). In the case of forests established on non-forested lands for energy products such as
fuelwood, there is not only an increase in the amount of C stored on the land but if the wood
burned as fuel displaces fossil fuel usage, it creates an effective rate of C sequestration in
unbumed fossil fuels (Sampson et al. 1993). Over long time periods, the displacement of fossil
fuels either directly, or through production of low-energy-intensive wood products, is likely to be
more effective in reducing C emissions than physical storage of C in forests or forest products.
Estimates of the Amount of C Conserved and Sequestered
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Two recent studies (Nilsson and Schopfhauser 1995; Trexler and Haugen 1995) were
combined to arrive at a global estimate of the potential amount of C that could be conserved and
sequestered bv different forested regions of the Earth between 1995 and 2050. These studies
were chosen because they were the only ones that they were global in nature, had done an
extensive literature review of the land availability issue, and included feasible rates of
establishment of management options. Both studies assumed aggressive, but unspecified, policy
and financial interventions in the forestry sectors, with no future change in climate that might
interfere with the proposed strategies.
The aforementioned studies estimated the potential for C sequestration and conservation
through a feasible global forestation program (afforestation and reforestation with plantations
and agroforestry), slowing tropical deforestation, and a program of natural or assisted
regeneration of tropical forests. For the global forestation program, estimates were made of the
amount of land likely available for countries and regions, feasible annual planting rates, likely
growth rates, and rotation lengths. A growth model was used to estimate the quantity of C fixed
in aboveground and belowground biomass, litter, and soil organic matter for forests harvested at
their designated rotation lengths for the period 1995 to 2100. No assumptions about the life
expectancy of the wood produced were made. For the tropical analyses, estimates were made of
current and projected future deforestation rates, the potential reduction in deforestation based on
feasible implementation of alternative land uses, and the area presently available for natural or
assisted native forest regeneration and the likely rates of implementation. Country-level
estimates were made for each decade from 1990 to 2050 for 52 tropical countries accounting for
virtually all of the tropical forests. Further details are given in the original sources and in Brown
et al. (1996).
Together, the studies suggest that globally 700 Mha of land might be available for C
conservation and sequestration programs (Table 2). This amount of land could conserve and
sequester 60 to 87 Pg C by 2050. Globally, forestation and agroforestry account for 50% of the
total (38 Pg C), with about 20% of this accumulating in soils, litter, and below-ground biomass
(Nilsson and Schopfhauser, 1995). The amount of C that could be conserved and sequestered by
forest sector practices by 2050 under baseline conditions is equivalent to about 11 to 15% of the
total fossil fuel emissions over the same time period (the lS92a scenario from Houghton et al.
1992).
The tropics have the potential to conserve and sequester by far the largest quantity of C
(80%), followed by the temperate zone (17%), and the boreal zone (3% only) (Table 2). More
than half of the tropical quantity would be from natural and assisted regeneration followed by
forest protection and slowing deforestation. Forestation and agroforestry would contribute less
than half of the tropical amount, but without them regeneration and slowing deforestation would
be highly unlikely (Trexler and Haugen, 1995).
Annual rates of C conservation and sequestration from all the practices would increase
over time and reach about 2.2 Pg/yr by 2050 (Fig. 1), with most C accumulating in the tropical
zone and the least in the boreal zone. Carbon savings from slowed deforestation and
regeneration would initially be the highest, but from 2025 onwards, when plantations would
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reach their maximum C accretion rate, they would sequester practically identical amounts as
slowed deforestation and regeneration (Fig. lb). During this period, tropical deforestation would
likely continue and the tropics would remain a net C source, albeit gradually diminishing. By
about 2030 the tropics would become a C sink (Trexler and Haugen, 1995).
The contribution of forestry to mitigation of CO2 emissions would be considerably higher
if the wood produced was used as a substitute for fossil fuels (Sampson et al., 1993). For
example, for the forestation program described here (Table 2), the quantity of biomass that could
potentially be produced over the 55 yr period was 147 billion m3 which is equivalent to about 39
billion tons coal (W. Schopfhauser, pers. comm.). If the wood was substituted for coal over the
same time period, the C emissions avoided would be about 29 Pg, or about 77% of the C
sequestered in the forest at ion/agroforestry program of 37.6 Pg (Table 2).
Impacts of Future Climate, Atmospheric Composition and Human
Demography on C Conservation and Sequestration
The mitigation potential of forests described above does not consider the effects of
changes in increased concentration of C02 and other atmospheric pollutants, the effects of a
changing climate, nor the effects of future changes in land use caused by increases in human
population density. Each of the promising forest management options for mitigation of C
emissions is likely to be affected differently under a changed climate and atmospheric
composition, and changed land use.
For the natural forest regeneration and slowing deforestation options in the tropics,
demand by an increasing human population for more land for agriculture and wood products
(e.g., for industrial and energy use) at the expense of native forest cover is likely to have a major
effect on land availability for sequestration projects and the feasibility of slowing deforestation;
the direct and indirect effects of climate change on land-use potentials may be less important in
comparison (Brown et al. 1993a; Solomon et al. 1996). In countries of the mid and high
latitudes, where changes in land use are relatively small at present, the effects of a change in
climate and atmospheric composition are likely to be more important (Kirschbaum et al. 1996).
For forestation options, the key factors are how a changed climate and atmosphere will affect
suitability and availability of lands for plantation and agroforestrv establishment as well as the
effects on species selection, rates of tree growth, and other pathways of sequestering C in, for
example, soil, litter, dead wood, and roots. However, because plantations are generally highly
managed, adaptations to changes in climate and atmospheric composition are feasible, including
species substitutions and shortening rotations.
CONCLUSION
To balance the global C budget, evidence suggests that net C02 emissions from the
worlds forests must be close to zero. That is, although forests are an important component of the
global C cycle through their regulation of C fluxes and pools, at present they are likely to be
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contributing little to global warming. This could change in the future for many reasons,
including continued increasing clearing and degradation of tropical forests, maturing of mid- and
high-latitudes forests, and increased mortality and wildfires of mid- and high-latitude forests as
they succumb to climate change. However, through the implementation of forest management
options that are compatible with traditional objectives of forestry, there is a potential to conserve
and sequester significant amounts of C over the next 50 yr or so. Aggressive adoption of forest
management options that conserve and sequester C arc not only necessary for sustainable
development but also for preventing forests from becoming a significant net source of CO2 to the
atmosphere in the future and contributing to climate change.
ACKNOWLEDGMENTS
Much of the material in this paper is drawn from Brown et al. (1996), a chapter in the IPCC
Second Assessment Report; I thank my co-authors and many of the contributors to that chapter
for ideas presented here. The information in this document has been funded wholly by the U. S.
Environmental Protection Agency. It has been subjected to the Agency's peer and administrative
review, and it has been approved for publication as an EPA document. Mention of trade names
or commercial products does not constitute endorsement or recommendation for use
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budget. Water, Air, and Soil Pollution 70:71-94.
Brown, S., L. R. Iverson, A. Prasad, and D. Liu, 1993b: Geographical distribution of carbon
in biomass and soils of tropical Asian forests. Geocarto International 8:45-60..
Brown, S., A. E. Lugo, and J. Wisniewski, 1992: Missing carbon dioxide. Science 257:11.
Brown, S., Sathaye, J., Cannell, M., Kauppi, P. E., 1996: Management of forests for
mitigation of greenhouse gas emissions. In: R. T Watson, M. C. Zinyowera, and R.
H. Moss (eds.), Climate change 1995: impacts, adaptations and mitigation of climate
change: scientific analyses. Contribution of working group II to the second
assessment report of the Intergovernmental Panel on Climate Change. Cambridge
University Press, Cambridge, pp. 773-798.
Cannell, M.G.R, R.C. Dewar and P.G. Pyatt, 1993: Conifer plantations on drained peatlands
in Britain: a net gain or loss of carbon? Forestry 66:353-368.
Dixon, R. K., S. Brown, R. A. Houghton, A. M. Solomon, M. C. Trexler, and J. Wisniewski,
1994a: Carbon pools and flux of global forest ecosystems. Science 263:185-190.
ECE/FAO, 1992: The forest resources of the temperate zones. The UN-ECE/FAO 1990
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Table 1. Estimated C pools and flux in forest vegetation (above and below-ground living and dead
mass; including woody debris) and soils (O horizon and mineral soil to 1 m depth) in forests of
the world. The date of the estimate varies by country and region, but covers the decade of the
1980s. The estimates are based on complete C budgets in all latitudes using data from original
source and/or from adjustments for completeness (revised version from Brown et. al. 1996)
Region C pools (Pg) C flux (Pg yr"*)
country Vegetation Soils
High latitude or boreal zone
FSU1 63
Canada2 15
Alaska _2
Subtotal 80
Mid latitude or temperate zone
USA 15
Europe3 10
China 17
Australia _9
Subtotal 51
Low latitide or tropical zone
Asia 41-54
Africa 52
America 119
Subtotal 212
Total 343
111 +0.3 to +0.5
76 +0.08
11 *
198 +0.48 ± 0.2
21 +0.08 to+0.25
18 +0,09 to+0.12
16 -0.02
14 trace
69 +0.26 ± 0.1
43 -0.50 to -0.90
63 -0.25 to -0.45
110 -0.50 to -0.70
216 -1.65 ±0.40
483 -0.9 + 0.5
"Included with USA
1 Soil pool excludes peat.
2 Vegetation includes estimates for roots (Kurz et al 1996); soil pool excludes co-located peat.
3 Includes Nordic countries. Total live biomass carbon was assumed to be the product of growing
stock in 1990, converted to carbon units, and the mid-point of the expansion factors given in
Kauppi et al.(1992); an additional 40% of live biomass was added to account for litter and dead.
Soil pool is the product of forest area and a soil C density of 9 kg nr2 (Dixon et al. 1994).
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Table 2. Global estimates of the potential amount of C that could be sequestered and
conserved by forest management practices between 1995 to 2050 (from Brown et al.
1996).
Latitudinal
Practice
Area
C sequestered
belt
(Mha)
& conserved (Pg)
High
Forestation
95.21
2.4
Mid
Forestation
113
11.8
Agroforestry
6.5
0.7
Low
Forestation
66.9
16,4
Agroforestry
63.2
6.3
Regeneration2
217
11.5-28.7
Slow deforestation1
138
10.8-20.8
Total
700
60-87
'includes not satisfactorily restocked forest lands in Canada.
^Includes an additional 25% of aboveground C to account for C bclowground in roots,
litter, and soil (based on data in Nilsson and Schopfliauser, 1995 and Brown et al.,
1993b); the range in values is based on the use of low and high estimates of biomass C
density resulting from the uncertainty in these estimates.
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~ Tropica! Asia
~ Tropical Africa
~ Tropical America
D Temperate
¦ Boreal
2.0- ~ Agroforestry
~ Forestation
0 Regeneration
1 Slow deforestation
1.5-
Figure 1. Average annual rates of C conservation and sequestration per decade through
implementation of forest management options given in Table 2 by (a) geographical
region and (b) forest management option (based on Brown et al. 1996).
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NHEERL-COR-2116A
TECHNICAL REPORT DATA
(Please read instructions on the reverse before comr
1, REPORT NO,
EPA/600/A-97/016
2.
3.
4. TITLE AND SUBTITLE
Forest arid climatge change: role of forest lands as carbon sinks.
5. REPORT DATE
6. PERFORMING ORGANIZATION
CODE
7. AUTHOR(S) Sandra Brown
8. PERFORMING ORGANIZATION REPORT
NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
National Health and Environmental Effects Laboratory
Western Ecology Division
200 Sw 35th Street
Corvallis, Oregon 97333
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
US EPA ENVIRONMENTAL RESEARCH LABORATORY
200 SW 35th Street
Corvallis, OR 97333
13. TYPE OF REPORT AND PERIOD
COVERED
14. SPONSORING AGENCY CODE
EPA/600/02
15. SUPPLEMENTARY NOTES:
16. ABSTRACT
Forests potentially contribute to global climate change through their influence on the global carbon ® cycle. They store large
quantities of C in vegetation and soil, exchange C with the atmosphere through photosynthesis and respiration, are sources of
atmospheric c when they are disturbed, become atmospheric C sinks during abandonment add regrowth after disturbance, and can
be managed to alter their role in the C cycle. The world's forest contains about 830 Pg C (1015g) in their vegetation and soil, with
about 1.5 times as much in soil as in vegetation. During the 1980's analysis of c budgets show that forest of temperate and borea!
countries were a net sink of atmospheric C of about 0.7 Pg yrbut the tropics were a net source of about 1.6 Pg yr'\ However,
accounting for the imbalance in the global C cycle suggests that forests are not significantly contributing to the net increase in
atmospheric C03 and thus not contributing to global climate change. However.this may not continue into the future as temperature
and boreal forests reach maturity and become a smaller C sink, and if rates of tropical deforestation and degradation continue to
accelerate. Recent studies suggest that there is the potential to manage forests to conserve and sequester C to mitigate emissions
of carbon dioxide by an amount equivalent to 11-15% of the fossil fuel emissions over the same time period. Aggressive adoption of
these forest management options are necessary to prevent forests becoming a significant net source of C02to the atmosphere in the
future and contributing to climate change.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Carbon budgets, climate change, forests,
gloval carbon cycle, mitigation.
18, DISTRIBUTION STATEMENT
19. SECURITY CLASS (This Report}
21. NO. OF PAGES 15
20. SECURITY CLASS {This page)
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE
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