conservation and sequestration of carbon

Conservation and sequestration of carbon

Forests play a major role in Earth’s carbon cycle through asslmllatlon, storage, and emission of COz. Estab- Ilshment and management of boreal, temperate, and tropical forest and agroforest systems could potentially enhance sequestration of carbon In the terrestrial biosphere. A blokgkal and economic analysis of forest establlsh- ment and management optlons from %I nations revealed that forestatlon, agroforestry, and sllvkulture could be employed to conserve and sequester one Petagmm (Pg) of carbon annually over a %-year period. The marglnal cost of lmplementlng these options to sequester !55 Pg of carbon would be approximately SlWMg.

Robert K. Dixon is with US EPA, US EPA Environmental Research Laboratory; Jack K. Winjum is with the National Council for Air and Stream Improvement, US EPA Environmental Research Laboratory; and Paul E. Schroeder is with ManTech Environmental Technology, Inc. US EPA Environmental Research Laboratory. The authors may be contacted at US Environ- mental Pro&tion Agency, Environmental Research Laboratory, 209 SW 35th Street, Corvallis, OR 97333, USA.

The information in this paper has been funded by the US Environmental Protec- tion 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

continued on page 160

The potential of forest and agroforest management practices

Robert K. Dixon, Jack K. Winjum and Paul E. Schroeder

The accumulation of greenhouse gases in the atmosphere due to deforestation, fossil fuel combustion, and other human activities may have begun to change the global climate.’ Given our current under- standing of global carbon sources and sinks, the prospect for managing the terrestrial biosphere to alter the carbon cycle and reduce the accumulation of greenhouse gases appears promising.2

Forest and agroforest systems play a prominent role in the global carbon cycle.3 Forests contain over 60% of the terrestrial above-ground carbon and about 45% of the terrestrial soil carbon. In addition, forests worldwide account for approximately 90%) or 90 Petagrams (Pg), of the annual carbon flux between the atmosphere and terrestrial ecosystems. Based on preliminary estimates, the application of forest management and agroforestry practices to stimulate biomass productivity on a global scale could potentially sequester or conserve several Petagrams of carbon annually.4

Agricultural systems also play a significant role in the global carbon cycle. They contain about 12% of the world’s terrestrial soil carbon, and conservation of this pool is essential to sustained crop productivity and decreasing CO2 emissions. ’ Many agricultural practices have been shown to increase soil carbon content by increasing carbon sequestration and/or reducing the loss of carbon. Practices such as reduced tillage, crop residue incorporation, field application of manure and sludge, and rotations using cover crops or leguminous crops store more carbon than conventional technology.6

Recognizing the prominent role of forest biomes in global ecology and the global carbon cycle, non-binding Global Forest Principles were promulgated at the UN Conference on Environment and Development in Brazil, June 1992.’ The Principles have several purposes:

0 Slow deforestation. 0 Protect biodiversity.

9959-379W93!02g159-15 0 1993 Butternorth-Heinemann Ltd 159

Conservation and sequestrarlon of carbon

continued from page 159 0 constitute endorsement or recommenda-

Stimulate sustained forest management and productivity.

tion for use. . Address threats to the world’s forests.

The authors gratefully acknowledge the Of primary concern in shaping these objectives were several proposals

effective contributions to this Paper bv during the past year for an international convention, charter, protocol, P.M. Bradley (Ability Netwoik); G.A. Baumgardner (METI); and S.G. McCannell

or other agreement to maintain, manage, or protect boreal, temperate,

(Word Design). and tropical forests.’

In 1989, delegates at the Noordwijk Ministerial Conference recog- ‘J.T. Houghton, G.J. Jenkins and J.J. Ephraum, eds, Climate Change, The IPCC

nized the role of forests in transnational environmental issues, including

Scientific Assessment, Cambridge Uni- global climate change, and stimulated interest in accelerated forestation

verslty Press, Cambridge, 1990; J.T. and sustainable ecosystem management options.’ The conference rec- Houghton, B.A. Callander and SK. Var- ney, eds, Climate Change 7992: The Sup-

ognized the significance of the observed increases in atmospheric carbon

plementary Report to the IPCC Scientific dioxide and established a provisional net world-forestation goal of 12

Assessment. Cambridoe Universitv Press. million ha per year, which is to be reached by the year 2000. Cambridge, i992. - *R.K. Dixon and D.P. Turner, ‘The global

The potential role of forest establishment in increasing carbon sinks

carbon cycle and climate change: Re- and stimulate sequestering of atmospheric carbon has been considered

soonses and feedbacks from below- giound systems’, Environmental Pollution,

by a number of authors.” These analyses have emphasized the major

Vol 73, 1991, pp 24!5-262; J.K. Win&m, forest regions on a continental basis, especially within tropical latitudes.

R.K. Dixon and P.E. Schroeder, ‘Estimat- Though preliminary, they have shown that forest and agroforest estab-

ing the global potential of forest and agro- lishment and management appear to have significant promise for forest management practices to conserve and sequester carbon’, Water, Air and Soil

contributing to global carbon sequestration and conservation. At the

Pollution, Vol64, 1992, pp 213-227. same time, implementation of these practices has the potential to

3P.P. Tans, I.Y. Fung, and T. Takahashi, provide a continuous flow of forest-based goods and services. ‘Observational constraints on the global atmospheric COz budget’, Science, Vol

Given the scope of science policy needs regarding global forests, the

247, 1990, pp 1431-1438. global carbon cycle, and climate change, this article has two primary

4Dixon and Turner, op tit, Ref 2; Winjum et objectives: I1 1 to identify promising technologies and practices that ( ) al, op tit, Ref 2. 5A.F. Bouwman, ‘Exchange of greenhouse

could be utilized at technically suitable sites to manage forest and

gases between terrestrial ecosystems and agroforest systems to stimulate biomass productivity and sequester and

the atmosphere’, in A.F. Bouwman. ed, conserve atmospheric carbon; and (2) to assess economic potential, Soils and the Greenhouse Effect: Proceed- ings of the fntemational Conference on

specifically costs at the site level, of establishing promising forest and

Soils and the Greenhouse Effect, Wiley, agroforest management practices.

New York, 1990, pp 62-127. 6R.K. Dixon, J.K. Winjum, K.J. Andrasko, J.J. Lee and P.E. Schroeder, ‘Integrated land-use systems: assessment of promising agroforest and alternative land-use practices to enhance carbon conservation and sequestration’, Climatic Change, in press; M.G. Johnson and J.S. Kern, Se- questering Carbon in Soils: A Workshop to Explore the Potential for Mitigating Global Climate Change (EPAl600/3-91/031), US Environmental Protection Agency, En- vironmental Research Laboratory, Corval- lis, OR, 1991. ‘L.W. Hill. H. Heiner and R. Pardo, ‘Fashioning global forestry guidelines’, Journal of Forestry, Vol90, 1992, p 9; J.S. Maini, ‘Towards an international instru- ment on forests’, in D. Hewlett and C. Sargent eds, Technical Workshop to Ex- plore Options for Global Forestry Manage- ment, International Institute for Enwron- ment and Development, London, 1991, pp 278-285. %laini, op tit, Ref 7. ‘Noordwiik Ministerial Conference. The Noordwijk Declaration on Climate Change, Noordwijk, The Netherlands, 1989. ‘OJ.K. Winjum, R.K. Dixon and P.E. Schroeder, ‘Estimating the global potential

continued on page 16 1

Materials and methods

Data collection

The assessment of forest management options to conserve and sequester carbon was based on a global database of biological and economic information on forest establishment and management options. Informa- tion regarding promising practices and initial establishment costs at the site level within forested nations representing boreal, temperate, and tropical regions on six continents were collected, using methods de- scribed by Moulton and Richards. ** Financial and biological data were collected in three major categories:

l Forest growth or conservation, as measured by biomass accretion, resulting from forest establishment and management practices.

0 Implementation costs for each management practice. 0 Area of land potentially suitable for each practice.

Database development, methodology and scope are thoroughly de- scribed by Winjum et al. I3 Biological and economic data were collected from the refereed technical literature, disparate databases from leading international forestry research organizations (eg the UN Food and Agriculture Organization, FAO), and from field assessments for 94 nations of the world. A relational database (RECITAL) was developed,

160 GLOBAL ENVIRONMENTAL CHANGE June 1993

continued from page 160 of forest and agroforest management practices to conserve and sequester carbon’, Water, Air and Soil Pollution, Vol64,1992, pp 213-227; G. Marland, The Prospect of Solving the CO, Problem through Global Reforestation, DOENBB-OtI82, US De- partment of Energy, Office of Energy Re- search, Washington, DC, 1988; K. Andras- ko, K. Heaton, and S. Winnett, ‘Evaluating the costs and efficiency of options to manage global forests’, in D. Howlett and C. Sargent, eds, Technical Workshop to Ex- plore Options for Global Forestry Manage- ment, International Institute for Environ- ment and Development, London, 1991, pp 21&233; A. Grainger, ‘Constraints on increasing tropical forest area to combat global climate change’, in D. Howlett and C. Sargent, eds, Technical Workshop to Explore Options for Global Forestry Man- agement, International Institute for En- vironment and Development, London, 1991, pp 196-208; R.A. Houghton, J. Un- ruh, and P.A. Lefebvre, ‘Current land use in the tropics and its potential for sequestering carbon’, in D. Hewlett and C. Sargent, eds, Technical Workshop to Ex- plore Options for Global forestry Manage- ment, International Institute for Environ- ment and Development, London, 1991, pp 297-310; R.A. Sedjo and A.M. Solomon, ‘Climate and forests’, in N.S. Rosenberg, W.E. Easterling, P.R. Crosson, and J. Dormstadter, eds, Greenhouse Warming: Abatement and Adaptation, Resources for the Future, Washington, DC, 1989, pp 105-119. “R.K. Dixon, J.K. Winjum, and O.N. Krankina, ‘Afforestation and forest management options and their costs at the site level’, in D. Howlett and C. Sargent, eds, Technical Workshop to Explore Options for Global Forestry Management, Internation- al Institute for Environment and Develop- ment, London, 1991, pp 319-328. 12R.J. Moulton, and K.R. Richards, Costs of Sequestering Carbon through Tree Planting and Forest Management in the United States, General Technical Report WO-58, US Department of Agriculture Forest Service, Washington, DC, 1990. ‘3J.K. Winjum, R.K. Dixon and P.E. Schroeder, ‘Estimating the global potential of forest and agroforest management practices to conserve and sequester carbon’, Water, Air and Soi/ Pollution, Vol64,1992, pp 213-227. “Marland, op tit, Ref 10; R.A. Sedjo, ‘Forests to offset the greenhouse effect’, Journal of Forestry, Vol87, No 7, 1989, pp 12-l 5; P.E. Schroeder and L. Ladd, ‘Slow- ing the increase of atmospheric carbon dioxide: biological approach’, CIimatic Change, Vol 19, 1991,283-299 15H.A. Mooney, B.G. Drake, R.J. Lux- moore, W.C. Oechel and L.F. Pitelka, ‘Pre- dieting ecosystem response to elevated CO;! concentrations’, Bioscience, Vol 41, 1991, pp 96-102. “R.L. Graham, R.D. Perlack, A.M.G. Pra-

continued on page 162


which catalogued biological and financial values for the forest management practices as well as land availability. This database provided the basis for carbon conservation and sequestration calculations and statistical analysis. In a global assessment involving large amounts of technical data from many sources, data quality will vary. Referenced technical data from this assessment were considered the best available. When data on land suitability, biological growth potential, or initial management costs were encountered that were clearly outside reported ranges and were not adequately explained, they were not used in the assessment.

Forest growth and carbon storage

Growth and yield of forests are normally expressed in terms of volume of stem wood. Stem wood volume was multiplied by the density (ie specific gravity) of wood for each species to yield stem wood biomass. Although the relationship varies between species, age of stands and site quality, it was assumed that a unit of stem wood biomass is associated with 1.6 units of whole-tree biomass, which includes bole, branches and leaves.14 Finally, it was assumed the carbon content of whole-tree biomass was 50%. Although below-ground carbon accretion is significant in forest systems, only above-ground accretion was considered in this assessment. Enrichment of forest stands with CO2 can influence carbon accretion rates positively or negatively and, given this uncertainty, this factor was omitted.15

Graham et al and Schroeder asserted that the relevant parameter in terms of carbon cycle calculations is the average amount of carbon on site over an indefinite number of rotations.16 If it is assumed that the system is sustainable and there is no yield reduction in later rotations, the result is the same as the average amount of carbon on-site over one full rotation. Because any number of biological, climatic, or social events could contribute to some level of yield reduction that cannot be predicted, the approach presented here may represent an upper bound.17 Carbon accretion and storage was calculated by summing the carbon standing crop for every year in the rotation and dividing by the rotation length. This approach assumes that, at or shortly after harvest, all stored carbon returns to the atmosphere.‘*

Costs of management practices

The relative costs of promising management options used in this assessment were estimates of direct costs at the site level for labour, materials, transportation, and the initial infrastructure (for up to three years) to employ the options.” Scaling of costs (between small and large projects) was not considered because previous analyses suggest this approach may be invalid.*’ The cost of land was not included in the analysis, because: land cost varies widely around the world; land values are difficult to establish where land is held in common by communities or virtually all land is government-owned; and no land market values exist.*’

Financial data are reported in 1990 US dollars. Costs for any reference year were adjusted based on the inflation and exchange rates for individual nations according to the International Financial Statistics (IFS) Tables published by the International Monetary Fund.** A nation’s inflation rate for the reference year, as measured by the Consumer Price Index, was extracted from the IFS Tables. The

GLOBAL ENVIRONMENTAL CHANGE June 1993


continued from page 161 sad, J.W. Ranney, and D.B. Waddle, Greenhouse Gas Emissions in Sub- Saharan Africa, ORNL-6640, US Depart- ment of Energy Oak Ridge National Laboratory, Oak Ridge, TN, 1990; P.E. Schroeder, ‘Carbon storage potential of short rotation tropical tree plantations’, Forest Ecology and Management, Vol 50, pp 31-41. “D M Smith, The Pfactice of Silviculture, Wif&y,‘New York, 1962; K.F. Wenger, ed, Fores&y ffand~k, ~iey-lnte~cience, New York, 1984. ‘*Moulton and Richards, op cii, Ref 12. ‘QDixon el al, op tit, Ref 11; Moulton and Richards, op tit, Ref 12. 2oC. Row, ‘Economics of track size in timber growing’, Journal of Forestry, Vol ;& ly pp 576482.

* . Trexler, ‘Estimating tropical biomass futures: A tentative scenario’, in 0. Hewlett and C. Sargent, eds, Technical Workshop to Explore Options for Global Forestry Management, International Insti- tute for Environment and Development, London, 1993, pp 311-318. ~fn~arnationa~ Financial Sratistics Year- book, Vol XLIIt, International Monetary Fund, Washington, DC, 1990; lntemational Financial Statistics. Vol XLIV, No 6. Inter- national Monetary Fund, Washington, DC, 1991. 23Dixon et al, op tit, Ref 11. 24Smith, op ci?, Ref 17. “L.S. Davis and K.M. Johnson, Foresf Management, McGraw-Hill, New York, 1987. =H. Gregersen, S. Draper, and D. Elt, PeoDfe and Trees: The Role of Social For&y in Sustainable Development, EDI Seminar Series, World Bank, Washington, DC, 1989. “R.G. Bailey, ‘Explanatory supplement to ecoragions map of the continents’, En- vironmental Conservation, Vol 16, No 4, 1989, pp 307-309. *@A. Grainger, ‘Estimating areas of degraded tropical lands requinng replenish- ment of forest cover’, lnternat~nai Tree Crops ~oumal, Vol5, 1988, pp 31-61. =R.A. Houghton, D.S.Lefkowitz and D.L. Skole, ‘Changes in the landscape of Latin America between 1850 and 1985’, Forest Ecokigy and Management, Vol 38, 1991, pp 143-172; R.A. Houghton, J. Unruh and P.A. Lefebvre, ‘Current land-use In the tropics and its potentral for sequestering carbon’, in 0. Hewlett and C. Sargent, ads, Technicat Workshop to fxptore Opiions for Global Forestry Management, international Institute for Environment and Develop ment, London, 1991; pp 297-310; O.N. Krankina and R.K. Dixon, ‘Forest management in Russia: Challenges and opportunities in the era of perestroika’, Journal of fores&v. Vol 90. DD 24-34. =M.C. %exler anb’C.M. Haugen, Keeping it Green: Evaluating Tropical Forestty Stratagies to Slow Global Warming, World Resources Institute, Washtngton, DC, in press.

Results

Management options

A wide range of promising forest and agroforest management practices and technologies were identified to promote conservation and seques-

reference year cost was then converted to a 1990 value and converted back to US dollars at the 1990 exchange rate.23

Because forests are periodically harvested and replanted,2” the costs of initiating forest management or establishing plantations are recurring costs. In estimating costs, it is important to account for these additional investments that will occur at more or less periodic intervals in the future.25 The net interest rate used was 5%, and the present value of future costs over a 50-year rotation period was computed for each practice or management option. Cost per Mg of carbon was calculated as the present value of all establishment costs over a 50-year period divided by mean carbon storage. Costs computed in this manner do not account for any financial benefits resulting from the initial investment.26

Land area technically suitable

The carbon accretion and storage values were based on a per unit area basis (eg Mg carbon per ha). The technically suitable land area for each management practice is required to estimate a total amount of atmospheric carbon removal and storage. Both land area and carbon storage for different management practices were classified within nations by ecoregion, following the system devised by Bailey.27 The broadest level of Bailey’s classification was employed. This level, the domain level, contains ecoregion subdivisions: boreal, temperate, and tropical. A coarse distinction was recognized within each ecoregion between low- land and upland zones (eg site quality).

Estimates of land available on which to implement forest establishment and management options are based on earlier assessments by Grainger,28 ~oughton et alYz9 and Trexler.“” Available land is ecologically suitable and can be used for tree crops from a socioeconomic perspective. Within tropical latitudes, the assessment of technically suitable land relies primarily on Advanced Very High Resolution Radiometer (AVHRR) Greeness Vegetation Index (GVI) analyses of land-use patterns. Land availability in the tropics was based on an intensive analysis of socioeconomic factors.“’ For the temperate zones, national inventories of land-use practice and patterns were consulted (eg Dixon et al and Moulton and Richardg2). Forest establishment and management practices were qualitatively assigned to ecoregions for the purpose of tallying carbon sequestration potential.

Non-parametric statistical analysis techniques (eg median and inter- quartile range) were employed to analyse biological and economic data collected for various forest management practices.‘” The Wilcoxon 2-Sample Non-Parametric Test was used to test significant differences for each of the three comparisons among the boreal, tropical, and temperate medians. Growth and yield curves were developed following regression and correlation analyses. Because of large differences in the growth and carbons sequestration of boreal, temperate and tropical forests, a range of vertical scales were employed in Figures 1 and 2.


Source: T. Allan and J.P. Laniy, Oversew 01 Status and Trends of War/d foresrs. UN Food and Agriculture Organization, Rome, 1991; N. Myers, Ceforestation Rates in Tropica/ Forests and Their Climale Implications, Friends of the Earth, London, 1989; World beources Institute, WoM aesoums IQQCLQI. Oxford University, Press, Oxford. 1990.

3’Trexler and Haugen, op tit, Ref 30. 32Dixon ef al, op tit, Ref 6; Moulton and Richards, op tit, Ref 12. =J. Devore and R. Peck, Statistics, The Exploration and Analysis of Data, West Publishing Company, %I Paul, MN, 1966. %T. Allan and J.P. Lanlv. ‘Overview of status and trends of worl% forests’, in D. Hewlett and C. Sargent, eds, Technical Workshoo to Exolore Ootions for Global Forestry ‘Mana&ment, International Insti- tute for Environment and Development, London, 1991, pp 17-39. 350ffice of Technology Assessment, Tech- nologies to Sustain Tropical Forest Re- sources, OTA Congress of the United States, Washington, DC, 1984; Office of Technology As&sment, Technologies to Maintain Biolooical Diversitv. OTA Con- gress of the united States: ‘Washington, DC, 1967; Gregersen ef al, op tit, Ref 26. Intergovernmental Panel on Climate Change, op tit, Ref 1. 37R.A. Houghton, R.D. Boone, J.M. fvlelillo, C.A. Palm, GM. Woodwell, N. Myers, B. Moore III, and D.K. Skole, ‘Net flux of carbon dioxide from tropical forests in 1980’, Nature, 316, 1985, pp 617-820.


i- *. __ ,.

T&h 1. Average annual deforestation and carbon loss estimate for se&tad forested nations in the lgI3Cls.

Country Deforestation (l@ ha)

Bangladesh 8 Brazil 3897 Cameroon 190 Columbia 890 Costa Rica 40 Ecuador 340 Ghana 281 lndla 1500 Indonesia 790 Ivory coast 500 Laos 200 Libena 88 Madagascar 186 Malaysia 210 Mexico 615 Myanmar 307 Nicaragua 100 Nigeria 400 Papua New Guinea 80 Paraguay 212 Peru 270 Philippines 445 Sudan 504 Thailand 674 Venezuela 245 Zaire 850

Carbon loss (lo6 Mg of carbon)

1 507

19 75

3 29 34

161 84 65 22 12 26 33 52 33

9 52

7 18 23 43 66 72 21

110

tration of carbon in the terrestrial biosphere. The assessment analysed the opportunities to maintain, reduce losses of, and expand forest carbon pools across boreal, temperate, and tropical latitudes.

Maintenance of existing forest carbon pools. The conservation of world forests is estimated to be one of the greatest potential contributions to reducing the accumulation of greenhouse gases in the atmosphere. Slowing deforestation and reducing forest degradation is a goal of the 1989 Noordwijk Declaration. However, since forest-based products play such a key role in the economic viability of certain regions, exploitation and harvesting will continue to be necessary activities, particularly in tropical latitudes. In the boreal and temperate latitudes, forestation practices virtually maintain the existing forest land area.34

Global deforestation and forest degradation are currently estimated to be 15-20 million ha annually. Loss of tropical forests is conservatively estimated at approximately 14 million ha annually according to surveys by UN FAO, the World Resources Institute and Meyers (Table 1). The estimated rates of deforestation vary widely but the greatest losses appear to occur in Brazil, India, Indonesia, and Zaire. The causes of deforestation have been extensively reviewed.35 Broadly, they include shifting agriculture, clearing forests for animal grazing, and tree har- vests. The estimated global loss of carbon from deforestation within the tropical latitudes ranges from 0.5-2.1 Pg annually,36 with emissions from key nations totalling 1.5 Pg (Table 1). These global estimates do not include losses of terrestrial carbon due to forest degradation (ie partial loss of tree productivity in a stand) nor do the calculations completely consider the small portion of carbon which flows into durable wood products.37

By providing food, fibre, and fodder, agroforestry can reduce des- tructive land uses. Integrated sustainable management options such as farm forestry, selected agroforestry systems, and selected forest man-

GLOBAL ENVIRONMENTAL CHANGE June 1993 163


=P.A. Sanchez and JR. Benites, ‘Low- input cropping for acid soils of the humid tropics’, Science, Vol238,1987, pp 1521- 1527. 3sR A Houghton, ‘The future role of troplc- . . al forests in affecting the carbon dioxide concentration of the atmosphere’, Ambio, Vol 19, No 4. 1990, pp 204-209. @‘Gregersen et al, ob.cit, Ref 26. 4’S.A. Boonkird. E.C.M. Femandes, and P.K.R. Nair, ‘Forest villages: An agroforestry approach to rehabilitating forest land degraded by shifting cultivation in Thailand’, in P.K.R. Nair, ed, Agroforestry Systems in the Tropics, Kluwer Academic Publishers, Boston, MA, 1991, pp 21 l- 228: Dixon et al, op cif, Ref 32. 42D.. Howletl and C: Sargent, eds, Techmc- al WorkshoD to Exdore Options for Global Forestry Mknagehenf, lniernational Insti- tute for Environment and Development, London, 1991. 43Brown et a/, op tit, Ref 40; Allan and Lanly, op tit, Ref 34. 44K G MacDicken and N.T. Vergara, Agro- for&y: Classification and Management, Wiley, New York, 1990. 45Gregersen et al, op cil, Ref 27.

agement (eg extractive reserves and biosphere reserves) appear most promising to slow deforestation by resource-poor farmers in tropical latitudes. Sanchez and Benites estimated that 1 ha of agroforestry could offset 5-10 ha of deforestation. 3R Thus, where agroforestry systems can be implemented on a broad scale, the possibility of reducing deforestation appears significant. For example, if each hectare of newly implemented agroforestry did, indeed, offset 5-10 ha of tropical deforestation, then it can be estimated that each hectare of new agroforestry land would save (ie sequester plus conserve) about 12W2300 Mg of atmospheric carbon - [220 MgC/ha x 5 or lOha] + [ 100 MgC/ha). This estimate is based on two assumptions: (1) the median standing stock of carbon in tropical agroforestry systems is 100 Mg of carbon per hectare (MgC/ha); and (2) the mean above-ground biomass of one hectare of tropical forest contains 220 Mg of carbon per hectare (MgC/ha).39

Further, forest management practices, such as forestation and intermediate silvicultural practices, can be implemented on a sustainable basis to provide a continuous flow of goods and services for local populations.a For example, the Forest Village programme introduced by the government of Thailand to slow deforestation has encouraged establishment of agroforestry systems and plantations and provides a sustained flow of goods and services from the forest sector. More than 60 villages have been established, and about 20 000 ha of new forest systems are established on formerly degraded lands each year. The relatively fast-growing multipurpose trees established in Thai forest villages have sequestered 0.01 Pg of above-ground carbon over that past 20 years. Similar programmes have been established across South Asia, Africa and Latin America.41

Expansion of forest carbon pools. A number of promising forest and agroforest management practices that would expand world forests and sequester atmospheric COZ were identified (Figure 1). Based on the median values for carbon sequestration (Mg/ha), the following are the five most promising practice-region combinations, from high to low:

(1)

(2)

(3)

(4)

Natural regeneration in tropical latitudes (Figure lc) . Management of humid tropical forests can result in storage of up to 195 MgC/ha. This reflects the great biomass productivity rates of natural ecosystems in the humid tropics.42 Estimates of forest growth rates and carbon accretion rates in tropical forests vary widely. Afforestation in the temperate latitudes (Figure lb). The relatively high median value (120 MgC/ha) likely reflects the growth rates of plantations established on marginal agricultural lands, which, though medium to poor for agronomic crop productivity, are often quite suitable for forest plantation growth.4” Establishment of agroforestry in tropical latitudes (Figure lc). If tree and agronomic crops are cultivated together, carbon accretion is in the range 60-125 Mg/ha. These practices have been employed by local peoples for centuries. 44 These moderately high carbon sequestration values for agroforestry are encouraging, because this practice is also one that will supply a sustained flow of goods and services to local populations.45 Reforestation in the tropical latitudes (Figure 1~). The high median value of this practice (65 MgC/ha) supports the assertion that reforestation in tropical latitudes has great potential to sequester



Figure 1. Carbon storage for forest management options for (a) boreal; (b) temperate; and (c) tropical biomes.

Median values are indicated by the wide horizontal lines. Boxes represent inter- quartile ranges (middle 66% of observa- tions). Dots are means. Vertical lines indi- cate full range of data.

46Schroeder, op tit, Ref 16.

” Reforestatron Afforestatton Natural Silv0.rlture Short regeneratton rototlon

Management optron

_ Reforestatron Afforestatron Natural Silvtculture Short Agroforestry regeneratron rotatron

Management optlon

700 c

E C 600 _

200 -

100:

-a

71fT

,1 & 17 n ” Reforestation Afforestation Natural Silvlculture Agroforestry

regeneratton

Management option

and store carbon. Mean annual increments (eg up to 60 mj/ha/ year) for eucalyptus and Caribbean pine for rotations of 20 years or less can rapidly store carbon. These plantation crops, however, may not always store the maximum amount of carbon over an extended period because short rotations limit biomass accumulation.46

(5) Reforestation in the temperate latitudes (Figure lb). At a median value of 56 MgUha and an inter-quartile range of 32-96 mgC/ha, this approach is the fifth highest on the list of promising practices for carbon sequestration. Forestation technology and expertise are well developed among nations of temperate latitudes.



47Marland, op tit, Ref 10; K. Andrasko, Climate Change and Global Forests: Cur- rent Know/edge of Potential Effects, Adaptation and MiGgarion Options. FO: MISC/90/7. Forestry Department, UN Food and AgricultureTOrga&ation, Rome, 1990: Sedio and Solomon, OD cit. Ref 10. 48J.N: Swisher and G: ‘Masters, ‘A mechanism to reconcile equity and efficiency in global climate protection: Interna- tional carbon emission offsets’, Ambio, Vol 21, pp 154-159. 4s0.N. Krankina and R.K. Dixon, ‘Forest management in Russia: Challenges and opportunities in the era of perestroika’, Journal of Forestry, Vol90, pp 29-34. =Allan and Lanly, op tit, Ref 34.

The lowest median values among management options evaluated were for silviculture practices in natural forests and plantations (Figure 1). Silvicultural treatments. such as thinning and fertilization, will likely play a role in adapting forests to warmer, drier climates.” These practices can stimulate carbon accretion in the boles of existing trees, and increase the probability that carbon stored could eventually be used as durable wood products.

Across boreal, temperate and tropical biomes, median estimates of the potential to sequester carbon through the establishment and management of agroforest systems are 16 MgC/ha. 38 generally have significantly less carbon per hectare relative to primary forests or plantations. The Wilcoxon non-parametric test indicates that the median values for the temperate and tropical latitudes were significantly greater than for the boreal, and that the temperate and tropical median values are not significantly different (p 5 0.05).

Cost of forest management options at the site level

Initial costs of forest establishment and management are least expensive in boreal regions. As management intensity increases in temperate and tropical regions, initial costs per hectare escalate accordingly (Figure 2). Natural regeneration, silvicultural treatment, agroforestry, and forestation are the least expensive practices within tropical latitudes.48

Boreal latitudes. For boreal forest systems, natural regeneration practices and artificial reforestation could be implemented most effectively at a cost of $9&325/ha (Figure 2). At carbon storage values of approximately 17 Mg/ha and 39 Mg/ha, respectively, the initial cost of carbon sequestration for the two practices is $5 ($4-ll)/Mg and $8 ($3-27)/Mg. Silvicultural treatments are also a cost-effective means to manage boreal forest systems at $74/ha. At a sequestration value of 10.5 MgC/ha for intermediate silvicultural treatments (Figure l), the initial cost of carbon sequestration ranged from $5-76/Mg. Krankina and Dixon49 and Allan and Lanly” also reported that forestation and forest management practices in boreal systems can be sustained and provide a high rate of return on initial investment. The costs of forest establishment in Russia, which contains over 50% of the world’s boreal forests, are a major determinant in calculating global biologic and economic potential to sequester atmospheric carbon.

Temperate latitudes. Within temperate regions, reforestation, afforestation, natural regeneration, and silvicultural practices are the least expensive forest management options for sequestering carbon (Figure 2). Median costs of reforestation ranged up to $350/ha. At a sequestration value of 56 MgC/ha (Figure l), carbon is stored at an initial cost of $6 ($3-29)/Mg depending on site conditions, tree species, and management intensity. Afforestation can store about 120 MgC/ha at a cost of $260/ha or $2 ($0.22-5)/MgC. Natural regeneration can be implemented inexpensively at less than $lO/ha. At 45 Mg/ha, the carbon sequestration cost is less than $l/Mg. Intermediate silvicultural treatments (eg thinning and fertilization) enhance carbon storage in temperate forests at a median cost of about $350/ha (Figure 2b). The initial cost of $13 ($3-158)/MgC. In temperate latitudes, establishment of agroforestry systems costs up to $790/ha, and this practice stores carbon at 34 Mg/ha (Figure lb) for an initial cost of $23 ($14-66)/Mg.



Figure 2. Initial costs for forest management options for (a) boreal; (b) temperate; and (c) tropical biomes.

Median values are indicated by the wide horizontal lines. Boxes represent inter- quartile ranges (middle 50% of observa- tions). Dots are means. Vertical lines indi- cate full range of data.

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Tropical latitudes. The widest range of costs were reported for forest management options within tropical latitudes (Figure 2). Natural regeneration of forests, establishment of short-rotation fuelwood plantations and agroforestry systems can all be established for less than $5OO/ha (50-year basis) - see Figures 1 and 2. Reforestation and agroforestry can sequester carbon at less than $10 ($2-26)/Mg because of high sequestration values - ie about 100 MgC/ha (Figure lc). Intermediate silvicultural treatments (eg thinning and fertilization) stimulate productivity and can sequester carbon at approximately ($lSO-36)/Mg at a sequestration value of 59 Mg/ha (Figure lc). Therefore, in the tropics, natural regeneration, agroforestry, reforestation, and silviculture sequester



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5’National Academy of Science, Policy Im- plications of Greenhouse Warming, US National Academy Press, Washington, DC. 1991.

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carbon at median initial costs of less than $lO/Mg. These initial costs per Mg of carbon sequestered compare favourably to many non-forest options to sequester or conserve carbon that are $30/Mg or more.51

Cost and yield efficiency of national programmes

The previous section revealed that establishment of plantations or agroforestry systems were cost-efficient means of stimulating carbon sequestration compared to other options. A comparison of cost and yield efficiency for nations with significant forest area is presented in Figure 3. Costs of carbon sequestration in forestation programmes were



Flgum 4. Efficiency and yield of reforestation, afforestation, natural regeneration, agroforestry and silvicultural practices in (a) Russia; (b) the USA; and (c) Brazil.

Key to sym&wk Circle = reforestation; triangle = affores~~; square = natursi regeneration; diamond = silviculture; x = agroforestty.

=%id.

highest in Egypt, New Zealand, Zaire, and Venezuela. In contrast, costs were signifi~ntly lower in Australia, Brazil, Chile, China, Congo, Madagascar, Mexico, Thailand, the USA, and Russia. The remaining nations surveyed were intermediate in forestation costs. These calculated values for cost and yield efficiency of national forestation programmes do not consider land rental costs, but are consistent with earlier estimates5’

The efficiency and yield of specific practices (artificial reforestation and afforestation, natural reforestation, intermediate silvicultural practices, and agroforestry) for sequestering carbon are presented in more detail for Russia, the USA, and Brazil (Figure 4). These data represent a range of economic options ($0.5~WMgC) to sequester carbon through forest management in representative boreal, temperate and tropical biomes. Collectively, these nations represent 30% of Earth’s land area, and implementation of these practices on a large scale could

GLOBAL ENVIRONMENTAL CHANGE June 1993

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169

Conservatron and sequestration of carbon

Figure 5. (a) Total initial cost of se- z questering carbon in forest systems i employing forestation and forest man- & agement practices; (b) Marginal initial * costs of sequestering carbon in forest systems employing forestation and forest management practices.

1.8- a 1.6-

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Technxolly suitable land (ho x IO’)

stimulate significant carbon sequestration. Moulton and Richards’” and Swisher54 observed similar carbon sequestration values and cost trends in their assessments of US and Latin American forest management options, respectively.

A global synthesis of carbon sequestration and initial costs at the site level is presented in Figure 5a. Total initial cost rises gradually up to a carbon storage level of approximately 55-70 Pg. Beyond 70 Pg, the total cost begins to escalate at a more rapid rate. The curve reveals that the marginal cost of sequestering 55-70 Pg of carbon in forest systems is approximately $lO/Mg (Figure 5b).

The distribution of technically suitable land among global ecoregions that would be required to achieve different levels of carbon storage is presented in Figure 6a. The analysis was completed by considering the area of land technically suitable for different practices in each ecoregion and the amount of carbon that those practices could store.5” The slope of the lines is relatively gradual up to 55 Pg indicating that relatively large increments of carbon can be stored on relatively small amounts of land. The slope becomes much steeper at 55 Pg as larger increments of land, and therefore higher establishment costs, are needed to store additional increments of carbon. More carbon could be stored, but it becomes less cost-effective. The most productive and least expensive lands would likely be placed under management first.

A global total of approximately 570 million ha of land would be required to store 55 Pg of carbon (Figure 6a). Given current estimates of

53Moulton and Richards, op tit, Fief 12. land availability, the distribution would be 410 million ha in the tropics “Swisher and Masters, op tit, Ref 48. -Grainger, op cif, Ref 28; Houghton et el,

and 160 million ha in the temperate zones. Although over 200 million ha

op c#, Ref 29; Trexler and Haugen, op tit, is available for the establishment of forests in the former USSR, use of

Ref 30. land in the boreal zone would only be considered at higher levels of =Krankina and Dixon, op cit. Ref 49. carbon storage.56 Improved resolution of land avaitability estimates in


Figure 6. (a) Potential distribution of technically suitable land among ecoregions of key forest nations worldwide for different levels of carbon storage; (b) Potential distribution of stored carbon among ecoregions of key forest nations worldwide for different levels of total carbon storage.

57Dixon et al, op tit, Ref 32. =Trexler and Haugen, op tit, Ref 30. 5gHoughton et al, op tit, Fief 29. 60Andrasko et al, op cit. Ref 10; Dixon et al, op cit. Ref 11. 61Dixon et al, op ci& Ref 32. %.M. Peters, A.H. Gentry, and R.O. Men- delsohn, ‘Valuation of an Amazonian rain- forest’, Nature. Vol 339, 1989, pp 855- 858. ‘%loulton and Richards, op tit, Ref 12; Swisher and Masters, op cit. Ref 48. B4Andrasko, op tit, Ref 47; A. Grainger, ‘Strategies to control deforestation and associated carbon emissions in the human tropics’, in Proceedings of the Inter- governmental Panel on Climate Change, Conference on Tropical Forestry Re- sponse Options lo Global Climate Change, US Environmental Protection Agency, Washington, DC, 1990, pp 10+119; National Academy of Science, op tit, Ref

&.A. Mohnen, W. Goldstein, and W. Wang, ‘The conflict over global warming: The application of scientific research to policy choices’, Global Environmental Change, Vol 2, No 2. March 1991, pp 104-123; R. Goodland, E. Asibey, J. Post, and M. Dyson, Tropical Mist Forest Hard- woods: The Urgent Transition to Sustaina- bi/ify, The World Bank, Washington, DC, 1990.




3

future could alter these estimates. Figure 6b illustrates the distribution of carbon storage between coarse ecoregions.57 At the 55 Pg carbon level, 44 Pg would be stored in the tropics and 11 Pg in temperate latitudes over a 50-year period.

Uncertainty is associated with estimates of land areas ecologically suitable and socioeconomically available for forest establishment. For example, Trexler and Haugen5’ estimated that social, demographic, political, and other factors could result in a 70% reduction in the available land estimates for tropical Africa and Asia that were reported by Houghton et ~1.~~ A sensitivity analysis was conducted to determine the possible effects of a 70% reduction in land available for forest establishment and management. A linear reduction in available land area evenly distributed over all nations and ecoregions would result in a reduction of total carbon storage potential to about 16.5 Pg of carbon. Similarly, the technical suitability of ecoregions and site productivity vary widely within a nation or biome.

Discussion and conclusions

Past efforts to develop forest establishment and management cost estimates at the site level for sequestering and conserving carbon in the terrestrial biosphere have been preliminary.60 Site-level,61 regional,62 national,63 and globalbl estimates have been calculated.

Inappropriate land-use practices, such as deforestation and biomass burning, have contributed significantly to the emissions of greenhouse gases in the atmosphere. 65 Several mechanisms to enhance conservation and protection of global forest systems have been proposed, including


Conservarion and sequestration of carbon

=Swisher and Masters, op tit, Ref 40. 67NationaI Academy of Science, op tit, Ref 51. 68HowIett and Sargent, op cit. Ref 42; National Academy of Scierke, op cit. Ref 51: E.S. Rubin. R.N. Coooer. R.A. Frosch. T.kl. Lee, G. Morland, ‘A.& Rosenfeld; D.D. Stine, ‘Realistic mitigation options for global warming’, Science, Vol 257, pp 146-l 49 and 261-265. *‘Swisher and Masters, op tit, Ref 46. “Gregersen et al, op tit, Ref 26. “Allan and Lanly, op tit, Ref 34.

the establishment of agroforestry systems. Many of these forest conservation options are a ‘no-regrets’ (eg multiple benefits) approach to carbon conservation, and provide many ancillary benefits (eg food, fuel and fibre), including the protection of biodiversity.66

The biological and economic opportunity to sequester carbon in forest systems appears significant. This assessment suggests: (1) forest establishment and management practices (eg natural regeneration, reforestation, afforestation, and agroforestry) can stimulate accretion of carbon in forest stands in boreal, temperate and tropical biomes; (2) forest and agroforest establishment and management practices can be used to store carbon temporarily for less than $30/MgC, with median values ranging from $I-S/MgC; (3) technically suitable land can be identified in boreal, temperate and tropical biomes of the world to implement forest management practices; and (4) potential carbon accretion and storage in forest systems may total up to 55 Pg over a 50-year period.

The current assessment of biological and economic information from more than 90 nations worldwide represents the first attempt to develop a bottom-up global analysis of carbon sequestration and conservation potential in forest systems. The forest management practices identified here can be applied to a wide range of ecosystems in boreal, temperate, and tropical biomes. However, before practices can be widely and successfully implemented, consideration must be given to the array of possible economic and sociopolitical constraints.67

From the perspective of forest biomass productivity, afforestation in the temperate latitudes, agroforestry in the tropics, and reforestation in both the temperate and tropical latitudes are among the most promising options. When considering initial costs in dollars per Mg of carbon, attractive options include natural and artificial reforestation in boreal latitudes; natural and artificial reforestation, afforestation and silvicultural practices in the temperate latitudes; and for the tropics, reforestation and agroforestry systems appear the most cost-efficient. The cost estimates are preliminary, and do not reflect benefits associated with goods and services that flow from forest sector. Moreover, rapidly changing labour costs in Russia and other countries will significantly influence economic analyses of forest management options. The ultimate mixture of greenhouse gas reduction options for key nations and the global community (eg forest management, alternative fuels, conservation agriculture) will be driven by the socioeconomic and political factors.@

The costs of carbon sequestration options at the national level have been the focus of several recent research efforts in the USA, Germany, the Netherlands, Brazil, Costa Rica, and other nations. In addition, analyses of the impact of the Tropical Forest Action Plan on carbon sequestration have been completed for some nations and regions.@ The cost estimates of national carbon sequestration efforts in the current study suggest that programmes could be effectively established in many of the major forested nations of the world. The forest-based products which flow from these programmes provide an array of goods and services, as well as offering the opportunity to sequester carbon in durable wood products (eg furniture, housing).‘O

Forest and agroforest management programmes have been implemented in several nations with a range of tree species, site conditions, and financial support. ‘I A number of national forest-based



programmes for the purpose of carbon sequestration and conservation have been announced (eg Australia, USA). The ability of these programmes to stimulate carbon sequestration varies. It must be stressed that a key factor in successful forest management and agroforest programmes is the involvement and support of local populations in the planning and implementation phases.72

Past and current analyses suggest the next step is micro- and macro-modelling of the biological and economic potential of carbon conservation and sequestration efforts. Regional, national and global

72Dixon et al, op cif, Ref 32. carbon budgets (anthropogenic and biogenic pools and flux) can be 731ntergovernmental Panel on Climate Change, op tit, Ref 1.

simulated with various process models.73 The menu of forest establish-

“‘K. Andrasko, in D. Lashof and D. Tirpak, ment and management options developed in this report can be used to eds, Policy Options for Stabilizing Global define appropriate options to reduce accumulation of atmospheric Climate, USEPA 21 P-2003.1, US Environ- mental Protection Agency, Washington,

greenhouse gases on regional, national and global scales. Such an

DC, 1990, pp 175-237; Rubin et a/, op tit, approach has been used for preliminary evaluations of forest-sector Ref 69. policy options at national and global levels.74


conservation and sequestration of carbon

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