Download The Role of Forests in Carbon Cycles, Sequestration, and Storage

The Role of Forests in Carbon
Cycles, Sequestration, and Storage
Issue 1: Forests and the Global Carbon Cycle: Sources and Sinks
K.E. PERCY1, R. JANDL2, J.P. HALL3
AND
M. LAVIGNE1
1 Natural
Resources Canada, Canadian Forest Service,
Atlantic Forestry Centre, P.O. Box 4000, Fredericton, New Brunswick, Canada E3B 5P7
2 Institute of Forest Ecology, Austrian Federal Office and Research Centre for Forests,
Seckendorff- Gudent-Weg 8, A-1131 Vienna, Austria
3 Natural Resources Canada, Canadian Forest Service,
Science Branch, 580 Booth Street, Ottawa, Ontario, Canada K1A OE4
http://iufro.boku.ac.at/iufro/taskforce/hptfcs.htm
Contents
Foreword .................................... ......1
1.1 Executive summary ........... ......2
1.2 IUFRO and the
changing climate ............... ......2
1.3 Forests: a global resource .......2
1.4 Forests and the global
carbon cycle ....................... ......3
1.5 A role for forest
management ...................... ......4
1.6 Future prospects ................ ......4
1.7 Glossary.............................. ......4
1.8 Further reading .................. ......4
Acknowledgements ................... ......5
Edited by Kevin Percy and Robert Jandl
Technical editing: Johanna Kohl, Federal
Office and Research Centre for Forests,
Vienna, Austria
Requests for reproduction of articles
should be addressed to the editors.
([email protected],
[email protected])
Newsletters can be downloaded from
http://iufro.boku.ac.at/taskforce/hptfcs.htm
Newsletter
No.1 - 2003
Foreword
Climate change, land use change and the world’s forests are inextricably
linked. Man-made emissions of the greenhouse gas, carbon dioxide, into
the earth’s atmosphere continue to escalate. Forests cover more than 4
billion hectares of the Earth’s land surface area and contain huge reservoirs of carbon in their vegetation and soils. Understanding the role of
forests in carbon cycles and predicting whether they will be carbon sinks
or sources in the future are important to ongoing international dialogue on
the subject of climate change.
IUFRO is a non-profit, non-governmental international network of forest
scientists whose objectives are to promote international cooperation in
forestry and forest products research. Recognizing the duality of importance of forests in global carbon cycling and the uncertainty which exists
around it, IUFRO in 2001 established a Task Force on the Role of Forests
in Carbon Cycles, Sequestration and Storage. Its mandate is to report on
the issues with a view towards improved decision making.
IUFRO is pleased to introduce the first of eight Task Force e-NOTES which
together will provide a suite of timely, readily accessible, concise, and
informative state of science summaries. to be issued quarterly. This first
TF e-NOTE is intended to provide you, the reader, with a primer Following
issues, to be released quarterly, will focus on a broad range of topics associated with the role of forests in adaptation to, and mitigation of increasing
atmospheric carbon dioxide.
Dr. Kevin E. Percy
Canadian Forest Service
Task Force Coordinator
1.1 Executive summary
Forests play a significant role in the global carbon cycle,
having absorbed approximately one third of recent
anthropogenic emissions of carbon dioxide (CO2) to the
atmosphere. However, our activities in the forest have
also been a source of carbon emitted to the atmosphere, with deforestation (primarily in the tropics)
contributing about one fifth of the annual anthropogenic
emissions. Of great concern is the uncertainty over
whether forests will be sinks for carbon in the future. In
addition, as the world mobilizes to address the issue of
climate change, some are proposing to use forest
management to increase sequestration of carbon in the
biosphere in the short to medium term. This attention
on forests and forestry has increased the demand for
detailed knowledge of forest functioning and accurate
information about the state of the world’s forests.
IUFRO is active in addressing the need for better understanding and predictive capacity.
1.2 IUFRO and the changing
climate
IUFRO’s Vision is that of promoting “science-based
sustainable management of the world’s forest
resources for economic, environmental and social
benefits.”1 IUFRO believes that public policy decisions supported by sound science produce better
deciscions, thus engendering greater public support
and more benefits to the society.
Forests play a major role in the natural global carbon
cycle by capturing carbon (C) from the atmosphere
through photosynthesis, converting that photosynthate to forest biomass, and emitting C back into the
atmosphere during plant respiration and decomposiDistribution of world forests in 2000 ( source: FAO, 2001)3
tion. Globally, these exchanges of C between forests
and the atmosphere are influenced by human-caused
and natural disturbances.2 This forest atmosphere
interaction leads to the view that controlling landuse
change practices involving forests might prevent to
some degree the increase in atmospheric greenhouse
gases and, additionally, that some forest management
activities might effectively reduce the rate of CO2
accumulation in the atmosphere.
The United Nations, through its Framework Convention on Climate Change and the Kyoto Protocol, is
working to achieve international agreement on incorporating forestry activities in the international response
to this major environmental challenge. Ultimately it will
be forest managers who will be responsible for putting
forestry-related components of international agreements on climate change into effect on the ground.
These managers will require a sound scientific basis to
be successful, so IUFRO is mobilizing to help them
meet the challenge.
1.3 Forests: a global resource
In 2000, approximately 30% (3,869 Mha) of the earth’s
land area was covered by forest. Almost half of
forested lands are in the tropics, a third are in the
boreal region, and approximately 10% are in each of
the sub-tropical and temperate forest regions.
The proportion of the world’s terrestrial surface that is
forested has been changing as a result of anthropogenic activities. For instance, during the decade
1990 to 2000, the forested area within the tropics
decreased by 14.2 Mha per year, primarily because of
deforestation. Coincidentally, the forested area
increased by 1.7 Mha per year in non-tropical forest,
mainly from natural expansion.3
1.4 Forests and the global
carbon cycle
Forested ecosystems function through interaction
among the C, nutrient and hydrological cycles. In turn,
these cycles vary as a result of natural environmental
shifts that operate on a scale of centuries to millenia.
In historical times, human influences have imposed
another set of factors in the C balance of forests
through deforestation and forest management. More
recently, this is occurring due to human modification of
atmospheric chemistry and climate change.
The net terrestrial flow of C is the difference between
uptake (sinks) and sources. During the 1990s, the
terrestrial biosphere (of which forests are a major part)
was absorbing C at a rate of approximately 1.4 Gt per
year.4 Forest trees and soils have been estimated to
contain 1,146 Gt of C. Approximately 37% of this C is
held in low-latitude forests; 14% in mid-latitude
forests; and, the remaining 49% at high-latitudes.
Carbon density (mass of C per unit forest area) is
greatest in high-latitude forests owing to large stocks
of soil carbon (Table 1). Carbon density is lowest in
mid-latitude forests.5
Table 1. Estimated area-weighted carbon densities5
Latitudinal belt
High
Mid
Low
Russia
Canada
Alaska
Continental U.S.A.
Europe
China
Australia
Asia
Africa
Americas
Carbon densities
(106 g per hectare)
Vegetation
Soil
83
281
28
484
39
212
62
108
32
90
114
136
45
83
132-174
139
99
120
130
120
climate change. At present, it is not possible to
estimate the net effects of these changes in forest area
on C sequestration in the biosphere. However, there is
substantial concern that permafrost regions are
becoming suitable for forests and will be sources of C
as the previously frozen peat decomposes.
The present atmospheric CO2 concentration of about
370 ppm is substantially above the pre-industrial level
of about 280 ppm. The current rate of increase (0.4%
per year) is unprecedented in the past 20,000 years.6
The year-to-year variability of the annual growth of
atmospheric CO2 concentrations is high.4 Of importance to forestry is the observation that this variability
in short-term concentrations is caused by the effect of
climate on C pools having short lifetimes (foliage, plant
litter, soil microbes) through variations in photosynthesis, respiration, nutrient cycling and common fire.
In the absence of human intervention, the CO2 balance
of forests (uptake versus emission) is in dynamic equilibrium with long-term environmental changes and
cycles. Harvesting in excess of growth reduces landscape-level C. Conversion of forests for agriculture
results in a rapid net loss of C to the atmosphere. On
the other hand, when agricultural land reverts back to
forest, C pools can be increased. Retention time of C
in forests varies from days for fast decomposing
tissues to centuries or millennia for the slower C pools.
Retention times vary by forest type and climatic zone
and are affected by natural and non-natural disturbances.
Terrestrial ecosystems are performing as a sink at
present because they are aggrading outside of the
tropics, and because global environmental changes
have benefited photosynthesis more than respiration/decomposition. However, there is no certainty
that forests will continue to be sinks for carbon in the
future. Moreover, the component of ecosystems
where C tends to accumulate differs among forest
types, meaning that changes in sink activity of each
forest type may difffer.
Changes that make the climate more arid are reducing
growth, or rendering land unsuitable for forests in some
regions of the globe. Severity and extent of moisture
stress will increase as climate change continues.
However, some areas that were inhospitable for
forests in the past are becoming suitable because of
There may be unforeseen effects of changing climate
on forests. For example, recent evidence suggests
that increased atmospheric CO2 concentrations may
make trees more susceptible to defoliating insects and
that plants receiving additional nitrogen from atmospheric deposition do not harden off normally in the
autumn and are more susceptible to frost damage.
These indirect effects of changing climate will reduce
productivity, and consequently C sequestration. Moreover, increased levels of the pervasive air pollutant
ground-level ozone (main component of smog),
reduces forest productivity, and offsets gains from
rising atmospheric CO2 concentrations.7 Climate
change is also predicted to increase the levels of
natural forest disturbance by insects and fire, and
there is some evidence that this is happening. Forest
stand composition may change as the climate acts in a
less predictable manner, thus resulting in conversion
of forests from C sinks to C sources.8
1.5 A role for forest
management
Commercial forestry activities may have potential for
increasing the amount of C sequestered because longlived forest products are effective for storing C, and
harvesting often converts a stand having little or no
current net growth to an actively growing stand.
However, these benefits must be evaluated against
total ecosystem C. Loss of slash, tops, branches,
needles, etc. after harvesting is usually substantial
during the recovery of photosynthetic capacity. Silvicultural practices, such as fertilization and tree improvement, that increase stand growth might also increase
C sequestration. Moreover, protecting forests from
fire, insect, and disease damage, sustains C pools that
otherwise would have returned to the atmosphere by
acts of nature and, therefore, helps in the effort to limit
greenhouse gases entering the atmosphere.
tion from different points of view. Each of the seven
succeeding publications will be based on thorough
discussion amongst Task Force members and invited
experts. Effective global communication, of course,
requires the use of a widely accepted terminology, for
which IUFRO (http://iufro.boku.ac.at/iufro/) provides
the continuously evolving on-line terminology service
SilvaVoc.
1.7 Glossary
decomposition: degradation of organic matter by biological
and non-biological processes
FACE: Free-Air Carbon Dioxide Enrichment; open-air CO2
exposure experiments in North America and Europe
forest: land with tree crown cover (or equivalent stocking
level) of more than 10% and area of more than 0.5
hectares (ha)
Gt: Gigatonnes of carbon (1 Gt = 1,000,000,000 tonnes)
IPCC:
Intergovernmental
http://www.ipcc.ch/
Panel
on
Change
M: one million (2 Mha = two million hectares)
photosynthesis: process by which green plants use solar
light energy and CO2 to produce carbohydrates and
oxygen.
pool: a reservoir that can accumulate or release carbon
sequestration: removal and storage; carbon dioxide taken
from the atmosphere into plants by photosynthesis
sink: a process or mechanism which removes carbon from
the atmosphere
source: opposite of a sink
1.6 Future prospects
1.8 Further reading
Large uncertainties exist in our ability to project future
forest distribution, composition, and productivity.5
Through state-of-knowledge reporting such as the
2001 book on CO2 and forest ecosystems.9 IUFRO is
enhancing awareness within the forestry community
and is engaged in providing a balanced view on C
sequestration and storage. Scientists of IUFRO
member organizations are active at the science-policy
interface, reporting through the IPCC, and leading
large multinational research projects such as CARBOEurope, FluxnetCanada, AmeriFlux and FACE.
1. IUFRO. 2002. http://iufro.boku.ac.at
The theme ‘carbon sequestration’ reaches into central
paradigms of forestry such as sustainability and
multiple-use. It is not exclusive to this Task Force, and
is a key theme of several IUFRO Working Parties
within a number of IUFRO Technical Divisions. In this
series of Task Force e-NOTES, we embark upon a
presentation of selected aspects of carbon sequestra-
Climate
2. IPCC. 2000. IPCC Special Report: Land Use, Land-Use
Change and Forestry. Geneva.
http://www.ipcc.ch/pub/srlulucf-e.pdf
3. FAO. 2001. State of the World’s Forests. Part II Key
Issues in the Forest Sector Today. Rome.
http://www.fao.org/docrep/003/y0900e/y0900e05.
htm#P4_44
4. D.S. Schimel, J.I. House, K.A. Hibbard, P. Bousquet, P.
Cias, P. Peylin, B.H. Braswell, M.J. Apps, D. Baker, A.
Bondeau, J. Canadell, G. Churkina, W. Cramer, A.S.
Denning, C.B. Field, P. Friedlingstein, C. Goodale, M.
Heimann, R.A. Houghton, J.M. Melillo, B. Moore III, D.
Murdiyarso, I. Noble, S.W. Pacala, I.C. Prentice, M.R.
Raupach, P.J. Rayner, R.J. Scholes, W.L. Steffen and C.
Wirth. 2001. Recent patterns and mechanisms of carbon
exchange by terrestrial ecosystems. Nature 414: 169-172.
5. R.K. Dixon, S. Brown, R.A. Houghton, A.M. Solomon,
M.C. Trexler and J. Wisniewski. 1994. Carbon pools and
flux of global forest ecosystems. Science 263: 185-190.
8. W.A. Kurz and M.J. Apps. 1999. A 70-year retrospective
analysis of carbon fluxes in the Canadian forest sector.
Ecological Applications 9: 526-547.
6. Houghton et al (Eds) 2001. Climate Change 2001: The
Scientific Basis. Contribution of Working Group II of the
Intergovernmental Panel on Climate Change. Cambridge
University Press, Cambridge. Also
http://www.ipcc.ch/pub/wg2SPMfinal.pdf
9. D.F. Karnosky, R. Ceulemans, G.E. Scarascia-Mugnozza
and J.L. Innes, (Eds.). 2001. The Impact of Carbon
Dioxide and Other Greenhouse Gases on Forest Ecosystems. IUFRO Research Series Volume 8. CABI
Publishing in Association with IUFRO, Wallingford, UK.
7. Kevin E. Percy, Caroline S. Awmack, Richard L. Lindroth,
Mark E. Kubiske, Brian J. Kopper, J.G. Isebrands, Kurt S.
Pregitzer, George R. Hendrey, Richard E. Dickson, Donald
R. Zak, Elina Oksanen, Jaak Sober, Richard Harrington and
David F. Karnosky. 2002. Altered performance of forest
pests under CO2- and O3-enriched atmospheres. Nature
420: 403-407.
Acknowledgements
The authors thank Task Force members for their contributions. We are grateful to those who provided external
reviews of the manuscript, especially to Dr Pierre Bernier,
Natural Resource Canada, Canadian Forest Service Laurentian Forestry Centre.
Further publications in this series
Issue 2:
Influences of Natural/Non-Natural Disturbances on Forest Carbon Sequestration and Storage
Issue 3:
Increasing CO2, Forest Ecosystem Productivity and Mitigation Capacities
Issue 4:
Increasing CO2, Forest Composition, Structure and Adaptive Ability
Issue 5:
Operational Strategies to Enhance Adaptation and Mitigation
Issue 6:
Product Strategies to Enhance Mitigation
Issue 7:
Approaches to Forest Carbon Accounting
Issue 8:
A Summary Report
All Newsletters will be made available free of charge at
http://iufro.boku.ac.at/taskforce/hptfcs.htm “Newsletters”