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RESPONSES AND FEEDBACKS OF
GLOBAL FORESTS TO CLIMATE
CHANGE
Robert K. Dixon
by terrestrial ecosystems, especially forests (Dickinson
1989; Sedjo and Solomon 1989).
ABSTRACT
The accumulation ofgreenhouse gases in the atmosphere
over the past century is projected to cause a warming of
the Earth. Climate change predictions vary by region and
terrestrial biosphere response and feedbacks will be ecosystem specific. Forests playa major role in the Earth's carbon cycle through assimilation of CO2 , storage of carbon,
and emission ofgreenhouse gases. Simulation models
have been employed to examine the possible responses to
climate change of global forest ecosystems. Major shifts
in forest species distribution and composition are predicted
in response to projected climate change within the next
50-80 years. The range of some species is expected to shift
dramatically in biomes worldwide. Savanna-type vegetation could replace some forests under the more extreme
climate change predictions in temperate latitudes. The
ultimate response and feedbacks of forests will be influenced by the direction and magnitude of climate change,
site quality, and other stress agents. Establishment of new
forests and implementation of management practices could
potentially be used to sequester significant amounts of atmospheric CO . Preliminary evidence suggests the terrestrial biospher; could be managed to reduce accumulation
of greenhouse gases in the atmosphere and mitigate negative impacts of climate change.
THE GLOBAL CARBON CYCLE
The accumulation of CO2 in the atmosphere in recent
decades has increased interest in the global carbon cycle
(Tans and others 1990). The net increase in atmospheric
CO2 is the result of greater carbon release than that being
removed by the terrestrial biosphere and marine systems
(fig. 1). Two sources of CO2 are especially significant, the
combustion of fossil fuels and global deforestation. The
flux of carbon through the terrestrial biosphere (for example, by plant photosynthesis, respiration, and decomposition) is approximately 100 Gt annually. Oceans are large
pools of global carbon, but annual net flux with the atmosphere is relatively low.
FORESTS AND THE GLOBAL
CARBON CYCLE
Terrestrial ecosystems, especially forests, playa major
role in the Earth's carbon cycle through assimilation of
CO2 , storage of carbon, and emission of carbon gases to
the atmosphere (fig. 1; table 1). Forests have high rates
of ecosystem productivity (the amount of carbon photosynthesized less that respired) compared to most other
ecosystems. The world's forests hold approximately 90
percent (about 740 Gt) of all aboveground terrestrial carbon, and 40 percent (about 570 Gt) of all belowground
terrestrial carbon (Waring and Schlesinger 1985).
The cumulative global net release of carbon to the atmosphere due to forest clearing, from 1860 to 1980, is estimated to range from 135 to 228 Gt (Woodwell and others
1983). Between 1.8 and 4.7 Gt of carbon were released from
biotic sources in 1980 alone, of which 80 percent was due
to deforestation (Detwiler and Hall 1988). Carbon release
from forest burning in 1980 has been estimated to be 50
percent of the annual atmospheric increase of 3 Gt. The
clearing of forest land for agriculture, especially within
the tropical latitudes, is now the largest source of carbon
released to the atmosphere from the biota and soils globally. However, actual estimates differ by a factor of 2-3,
due to differences in estimated rates of clearing tropical
forests (Detwiler and Hall 1988; Woodwell and others 1983).
If deforestation increases in proportion to population
growth, the biotic release of carbon will reach about 9 Gtlyr
before forests are exhausted in the next century (two times
the current fossil fuel emissions) (Postel and Heise 1988).
Detwiler and Hall (1988) imply that of the 5.1-7.5 Gt
INTRODUCTION
Greenhouse gases (for example, CO2, CH 4 ) produced
from anthropogenic and biogenic emissions are accumulating in the atmosphere. The concentration of atmospheric
CO is projected to double from preindustrial levels by late
in the 21st century. Infrared radiation trapped by greenhouse gases in the troposphere is expected to influence
global climate. General Circulation Models (GCM's) of
climate change project the average temperature of the
Earth's surface will increase 1.5 to 4.5 °C and influence
regional frequency and distribution of precipitation
(Schneider 1989a,b). Considerable uncertainty exists
regarding the magnitude of global climate change, particularly projections of regional responses and feedbacks
Paper presented at the Symposium on Management and Productivity
of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Robert K. Dixon is Leader, Global Effects Team, Environmental Research Laboratory, U.S. Environmental Protection Agency, 200 SW 35th
Street, Corvallis, OR 97333.
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.
189
Atmosphere
740 Gt (in 1988)
+3 Gt per year
Photosynthesis
5 Gt
Fossil Fuel Use
1-2 Gt
Deforestation
I
t
Veg
93Gt
t
Biological
&
I
Chemical
Processes
90Gt
Biological
I
&
I
Chemical
560830 Gt
Fossil Fuels
5,000-10,000 Gt
Soli, Litter, Peat
1,170-1,740 Gt
Figure 1-The global carbon cycle including pool size and flux for terrestrial and marine systems
(adapted from Schneider 1989a).
Table 1-Anthropogenic and biogenic emissions (1980
estimates) of carbon to the atmosphere and
potential sequestration of carbon dioxide (C02)
by establishment and intensive management of
world forests (Schroeder and Ladd 1990; Tans
and others 1990; Woodwell and others 1983)
FOREST RESPONSE TO GLOBAL
CHANGE
Large uncertainties exist regarding forest response to
climate change (Jarvis and others 1989; Sedjo and Solomon
1989). As atmospheric concentration of CO2 increases and
climate change events unfold, forests could become either
a net source, or sink, of carbon. Climate change will almost
certainly cause some forest species in selected regions to
decline and migrate (Urban and Shugart 1989; Woodman
and Furiness 1989). Ecosystem shifts in species distribution and composition are projected to occur within the next
50-80 years. For example, the range of loblolly pine in the
southern United States is predicted to shift north several
hundred miles. A long-term decline in productivity of some
forest types could occur and timber production, biotic habitat, yield of water, site quality, and recreation opportunities may be altered.
Based on GCM estimates of climate change associated
with a doubling of atmospheric CO2 by the year 2050, and
the subsequent redistribution of vegetation, the world's forests could experience a substantial change in distribution
and composition (an areal increase or decrease). Estimates
of global forest redistribution vary widely between GCM's
used to estimate climate changes (10 percent decrease in
areal coverage with the GFDL model, 60 percent increase
with the GISS model) (Emanuel and others 1985; Prentice
and Fung 1990). Consequently, the resulting biosphere
Range of estimates
Gtlyr
Carbon source
Fossil fuel
4.8 - 6.6
Deforestation and
other biotic sources
1.8-4.7
Carbon sink
Forest establishment-500 Mha
3.5 - 4.0
Forest management-300 Mha
0.5 - 1.5
released annually by fossil fuel and deforestation, somewhere
between 0.3 to 2.8 Gt is assimilated in the terrestrial biosphere. Variation in the annual atmospheric concentration
of CO2 is greater in the northern hemisphere in part due
to a greater land mass and vegetation (forest) cover (Tans
and others 1990). Thus, the terrestrial biosphere plays a
major role in the global carbon cycle.
190
feedbacks to climate could be negative or positive. The
major negative feedback to climate change is CO2 enrichment of vegetation, while positive feedbacks include biogenic emissions of greenhouse gases (for example, CH4 ,
NMHC, H 2 0). Boreal, temperate, and tropical forests will
respond to-climate change differently, and must be managed differently to adapt to a changing environment
(Smith and Tirpak 1989).
Large areas of world forests (especially temperate and
boreal) could experience water stress due to warming and
drying, which could lead to widespread forest decline thus
producing a potential major source of atmospheric CO2 •
Timing of changes in forest condition is unclear, but based
on past climate-related events (as with extended droughts)
alterations could be manifested in the first half of the next
century. Alternatively, atmospheric CO2 enrichment could
increase forest productivity and water-use efficiency (Mooney
and others 1990). These positive ecophysiological effects
could help forests adapt to global warming. The ecophysiological response of broadleaf and conifer seedlings to CO2
enrichment is different in short-term studies (Mooney and
others 1990). Differences in seedling biomass, leaf area, rootshoot ratios, water-use efficiency, and nutrient-use efficiency
have been ascribed to short-term CO2 enrichment. Longterm responses to CO2 exposure are largely unknown.
Limitations of nutrient and water resources may not
preclude plant growth responses to CO2 enrichment
(Norby and O'Neill 1989).
accumulation of greenhouse gases (for example, CO2) in
the atmosphere through forest management appear promising. However, many biologic, socio-economic, and political barriers exist and global management of the carbon
cycle is probably decades in the future. Anthropogenic and
biogenic emissions are predicted to increase dramatically
in the next century. A combination of efforts to slow accumulation of greenhouse gases in the atmosphere including
a reduction in fossil fuel combustion, slowing deforestation,
and establishment of new forests may be practical and
complementary alternatives for developed and developing
nations.
CONCLUSIONS
Forests playa central role in the global carbon cycle.
Although estimates of global carbon pool size and flux
vary, forest distribution and productivity influence concentration of greenhouse gases (for example, CO2 , CH4) in the
atmosphere. Variation in annual atmospheric concentration of CO2 is greater in the northern latitudes in part due
to a greater land mass and vegetation (forest) cover. Substantial scientific uncertainty exists regarding the role of
forests in global change and the carbon cycle. Preliminary
evidence suggests forests could be managed worldwide to
reduce the accumulation of greenhouse gases in the atmosphere and possibly mitigate the negative impacts of global
change. Considerable research is required to reduce the
large uncertainties regarding the global carbon cycle and
projected climate change events.
MANAGEMENT OF FORESTS TO
MITIGATE GLOBAL CLIMATE
CHANGE
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Forest ecosystems can be managed to increase CO2 assimilation via photosynthesis and temporarily store large
amounts of carbon (table 1; Schroeder and Ladd 1990).
Establishment of 500 million ha (area approximately
the size of Australia) of new forests worldwide could fix
2.5 Gt of carbon/yr aboveground and another 1-1.5 Gtlyr
belowground (Trexler 1990; Wood and others 1984). Estimates of aboveground carbon assimilation rates range from
about 1 Gt/yr in boreal forests to about 8 Gtlyr in tropical
forests. It would take about 25 years to plant 500 million
ha assuming 20 million ha/yr could be planted. Of course,
many assumptions are included in these estimates, such
as: (1) no large-scale forest decline due to global change,
(2) level population growth and no land-use changes,
(3) reforestation is not offset by deforestation, and
(4) effects of CO 2 enrichment are negligible.
Intensifying silvicultural practices on existing land
in boreal, temperate, and tropical forests could result
in an additional 0.5 to 1.5 Gt/ha/yr of carbon being fixed
(Schroeder and Ladd 1990). The area of forest land where
silviculture could be intensified is about 300 Mba, globally
(10 percent of the world's closed forests) (Wood and others
1984). Thus, the carbon-sequestering rate could be stimulated by over 10 percent. Logging debris and soil organic
matter could also be managed to maintain or sequester
significant amounts of carbon.
Given the summary of estimated global carbon sources
and sinks shown in table 1, the prospects for reducing the
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Speakers answered questions from the audience after
their presentations. Following are the questions and answers on this topic:
Q.-Will fossil fuel burning continue beyond the year
2010 (or will the supply of fossil fuel be totally consumed)?
A.-Global fossil fuel reserves are predicted to be available beyond year 2010. Large reserves are available on
several continents, especially Asia.
Q.-A recent report from NASA stated that no measurable atmospheric temperature change was observed by
satellite in the last 10 years. Is the Earth warming?
A.-Short-term measurements (for example, 10 years) of
climate patterns are highly variable and can be misleading.
The long-term atmospheric temperature trends (previous
120 years) reveal the Earth has warmed 0.5-0.8 °C according to the Intergovernmental Panel on Climate Change
(IPCC).
Q.-Is the increase in atmospheric CO2 the sole responsibility of man?
A.-No, biogenic and anthropogenic sources of gases both
contribute to atmospheric chemistry.
Q.-Could global climate change be attributable to natural phenomena (for example, volcanism) rather than the
infl uence of man?
A.-Major catastrophic events in the terrestrial biosphere significantly contribute to changes in atmospheric
chemistry. The source-sink relationships of the terrestrial
biosphere remain a significant research question.
192