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Sustainable Agriculture and the Environment in the HUMID TROPICS APPENDIX Emissions of Greenhouse Gases from Tropical Deforestation and Subsequent Uses of the Land Virginia H. Dale, Richard A. Houghton, Alan Grainger, Ariel E. Lugo, and Sandra Brown Wide-scale land use change is resulting in numerous environmental consequences: degradation of soils, loss of extractive resources, loss of biodiversity, and regional and global climate change, among others. Common land use changes are forest degradation and the conversion of forests to agricultural systems and pastures. Because many agricultural systems in the humid tropics are not sustainably managed, each year large areas of forest are cleared to provide new fertile lands. Sustainable agriculture offers one means of offsetting the global consequences of large-scale land use change. This paper discusses the emissions of greenhouse gases associated with land use change and the potential impact that sustainable agriculture may have on these emissions. Land uses involving intensive deforestation and intensive agricultural practices increase greenhouse gas emissions; in the case of deforestation, by eliminating a Virginia H. Dale is a research scientist in the Environmental Sciences Division at Oak Ridge National Laboratory, Oak Ridge, Tennessee; Richard A. Houghton is a senior scientist at the Woods Hole Research Center, Woods Hole, Massachusetts; Alan Grainger is a bioecographer, resource economist, modeler, and environmental policy analyst and is currently a lecturer in geography at the University of Leeds, Leeds, United Kingdom; Ariel E. Lugo is director and project leader of the Institute of Tropical Forestry, U.S. Forest Service, U.S. Department of Agriculture, Puerto Rico; Sandra Brown is associate professor of Forest Ecology in the Department of Forestry, University of Illinois, Urbana-Champaign, Illinois.
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Sustainable Agriculture and the Environment in the HUMID TROPICS source of oxygen production, carbon dioxide (CO2) conversion, and carbon (CO) sequestration; in the case of agriculture, by increasing sources of methane (CH4) through rice and livestock production. The emphasis here is on CO2, the major contributor to the greenhouse effect, and on tropical deforestation, the major land use change that accounts for the current increase in atmospheric CO2 concentrations. The net flux of carbon from land use changes is calculated by adding the stocks of carbon per unit area for the major land uses of the world to the rates of change in land use. Therefore, this paper reviews estimated carbon content and the rates of change in the carbon content of the major land uses in the humid tropics. That discussion forms a basis for estimating the flux of greenhouse gases from land use changes. Because projections of future impacts are based on particular models, this paper presents and compares the major model structures. Lastly, it discusses how the sustainable uses of land can reduce future emissions of greenhouse gases. The last section also presents a set of priorities for future research. EFFECTS OF LAND USE CHANGE ON GLOBAL CLIMATE Changes in the earth's climate are predicted to cause a 0.3°C warming per decade (range, 0.2° to 0.5°C per decade), which may instigate a 6-cm rise in sea level per decade (range, 3 to 10 cm per decade) in the next century (Houghton et al., 1990). These changes are anticipated as a result of the buildup of radiatively important gases in the atmosphere. Aside from water vapor, the major biogenic gases that contribute to the greenhouse effect (greenhouse gases)—CO2, CH4, nitrous oxide (N2O), chlorofluorocarbons (CFCs), and ozone—result either entirely or in part from human activities. Except for CFCs, these gases are also part of the natural cycles between ocean, land, and atmosphere. The increasing concentrations of these gases in the atmosphere, however, and the enhanced greenhouse effect that may result are due to increased emissions of these gases as a result of human activities, predominantly fossil fuel combustion and the expansion of agricultural lands (for CO2 concentrations, see Figure A-1) (Post et al., 1990). Currently, the burning of fossil fuels is the major contributor, but historically, land use changes have had a larger impact on atmospheric greenhouse gas concentrations (Houghton et al., 1983). Agriculture and the clearance of forests for agricultural use have accounted for about 50 percent of the total emissions of carbon over the past century (Figure A-2). In the past CO2 has accounted for more than half of all gases that contribute to the greenhouse effect and is expected to account for 55 percent over the next century (Houghton et al., 1990).
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Sustainable Agriculture and the Environment in the HUMID TROPICS FIGURE A-1 Carbon dioxide (CO2) released from burning of fossil fuels and expansion of agricultural lands from 1850 to 1980. The lines within the bars indicate standard deviations; Pg, petagram. Source: Dale, V. H., R. A. Houghton, and C. A. S. Hall. 1991. Estimating the effects of land-use change on global atmospheric CO2 concentration. Can. J. Forest Res. 21:87–90. Reprinted with permission. The annual net flux of carbon to the atmosphere from land use change is estimated to have been 0.4 to 2.6 Pg of carbon per year in 1980 (1 Pg = 1015 g) (Detwiler and Hall, 1988a; Houghton et al., 1987). The annual net flux of carbon as a result of fossil fuel emissions was between 5.0 and 5.5 Pg from 1980 to 1988 (Marland and Boden, 1989). Therefore, the recent contribution of CO2 to the atmosphere from land use change in terrestrial ecosystems is between 10 and 50 percent of the flux resulting from fossil fuel emissions. If 10 percent is correct, then land use change is not a major cause of the increases in atmospheric CO2 concentrations. Researchers must accurately identify whether the larger values are correct or whether the rate of land use change is increasing. It is also important to continue research to estimate the carbon flux resulting from the human impact on terrestrial ecosystems. The role of “undisturbed” forests also requires sci-
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Sustainable Agriculture and the Environment in the HUMID TROPICS entific attention because, as these forests regenerate following natural or undetected human disturbances, carbon sequestration could offset some of the emissions resulting from human activities. Note that forests classified as undisturbed have frequently been subject to some human manipulations. This paper discusses the effects of land use change on greenhouse gas emissions and the potential impact that sustainable agriculture may have on the interaction. The emphasis is on CO2, the major contributor to the greenhouse effect (Figure A-3), and on tropical deforestation, the major land use change involved in the current increase in atmospheric CO2 concentrations (Dale et al., 1991). A major finding from this review is that most of the current flux of greenhouse gases to the atmosphere from the tropics is due to the conversion of forests to agricultural uses and that sustainable agricultural practices could be a significant means of controlling the expansion of deforestation. Sustainability —which exists when land can be used for a long period of time without significant declines in the FIGURE A-2 Change in the area of cultivated land and net flux of carbon (Pg, petagram) from terrestrial sources from 1860 to 1980. Source: Houghton, R. A., J. E. Hobbie, J. M. Melillo, B. Moore, B. J. Peterson, G. R. Shaver, and G. M. Woodwell. 1983. Changes in the carbon content of terrestrial biota and soils between 1860 and 1980: A net release of CO2 to the atmosphere. Ecol. Monogr. 53:235–262.
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Sustainable Agriculture and the Environment in the HUMID TROPICS FIGURE A-3 Contributions of different gases to the greenhouse effect calculated for the 1980s. Screened segments indicate the relative contributions of deforestation and land use to the total emissions. White segments represent industrial and natural contributions. For the chlorofluorocarbons (CFCs), all the emissions are industrial. Source: Houghton, J. T., G. J. Jenkins, and J. J. Ephraums, eds. 1990. Climatic Change: The IPCC Scientific Assessment. Cambridge: Cambridge University Press. ecologic attributes of the land—may require inputs of fertilizer, irrigation, use of machines, periods of fallow, adjacent land preserves, or other manipulations. The situation should be seen as sustainable from the viewpoints of both the landholder, who is able to make a living from the land, and the land itself, which maintains soil conditions adequate for growing agricultural or forest crops (Costanza, 1991). Therefore, it is important to evaluate the costs and benefits of particular forms of sustainable agriculture (including greenhouse gas emissions resulting from land use practices). MAJOR LAND USE CHANGES RESPONSIBLE FOR THE FLUX OF GREENHOUSE GASES Forests contain about 90 percent of all the carbon stored in terrestrial vegetation and are being cleared at a very rapid rate. (Table A-1 indicates the variability in estimates of deforestation, and Dale  discusses the methods used to obtain the estimates.) With this clearing, the carbon previously stored in the trees and soils is being re-
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Sustainable Agriculture and the Environment in the HUMID TROPICS TABLE A-1 Rates of Deforestation of Closed Tropical Forests by Source of Information (in Thousands of Hectares per Year) Region Myers, 1980a (1979) FAO and UNEP, 1981b (1976–1980) Grainger, 1984a, c (1976–1980) WRI, 1990b, c (1980s) Myers, 1989a, d (1989) FAO, 1991e (1981–1990) Tropical America 3,710 4,119 3,301 10,859 7,680 7,290 Tropical Africa 1,310 1,319 1,204 1,338 1,580 4,788 Tropical Asia 2,320 1,815 1,608 2,390 4,600 4,707 Total 7,340 7,235 6,113 14,587 13,860 16,785 NOTE: FAO and UNEP, Food and Agriculture Organization of the United Nations and United Nations Environment Program; WRI, World Resources Institute. Numbers in parentheses are years to which deforestation data apply. a Refers only to closed forests in the humid tropics. b Refers to all tropical closed forests. c Uses data from the Food and Agriculture Organization and United Nations Environment Program (1981) only for forests in the humid tropics. d Refers to 34 countries that contain 97 percent of the world's total area of tropical humid forests. e Estimates for 62 of the 76 countries in the tropics; they include almost all of the humid forests along with some dry areas (Food and Agriculture Organization, 1991). The fact that some open forests are included makes a comparison with closed forests somewhat misleading. leased into the atmosphere. The net rate and the completeness of carbon release depend on the fraction of the forest burned, the decomposition rate of downed wood, and the fate of forest products. For example, when wood is burned, it quickly releases carbon into the atmosphere, whereas wood structures retain their carbon for a longer period of time, and some charcoal is essentially a form of permanent carbon storage. Crops and pastures may hold 2 to 5 percent of the carbon in vegetation per unit area, compared with that held by forest vegetation. About half of the mass of vegetation is carbon. Estimates of biomass come from direct measurements (Ajtay et al., 1979; Brown and Lugo, 1982; Olson et al., 1983) or are derived from wood volumes reported in large-scale forest inventories (Brown and Lugo, 1984; Brown et al., 1989) (Table A-2). On average, the soils of the world contain about three times more organic carbon than is contained in vegetation.
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Sustainable Agriculture and the Environment in the HUMID TROPICS The net flux of carbon to the atmosphere from land use changes depends not only on stocks of carbon in forests and rates of land use change but also on uses of agricultural lands (Table A-3). Land use changes can be triggered by natural events (such as fire, hurricanes, or landslides) or by people. Because the land use changes instigated by people have the greatest effect on the net carbon flux, only those changes are discussed here. The land use changes considered below include permanent agriculture and pasture, degradation of croplands and pastures, shifting cultivation, forest plantations and tree crops, logging, and degraded forests (Table A-3). Many surveys have addressed the causes of tropical deforestation. Myers (1980, 1984) emphasized the roles of cattle ranchers, loggers, and farmers. Grainger (1986) distinguished between the land TABLE A-2 Carbon Stocks in Vegetation and Soils of Different Types of Ecosystems Within the Tropics (Megagrams per Hectare) Closed Forestsa Source and Region Forests in Humid Tropics Seasonal Forests Closed Forestsb Open Forests or Woodlandsa Crops Vegetation Tropical America 176, 82 158, 85 89, 73 27, 27 5 Tropical Africa 210, 124 160, 62 136, 111 90, 15 5 Tropical Asia 250, 135 150, 90 112, 60 60, 40 5 Soilsc 100 90 NA 50 NA NOTE: NA, not available. a The first value of each pair of data is based on destructive sampling of biomass (Ajtay et al., 1979; Brown and Lugo, 1982; Olson et al., 1983); the second value is calculated from estimates of wood volumes (Brown and Lugo, 1984; Houghton et al., 1985). It is not evident which estimate is more accurate. b These estimates are also based on wood volumes reported by the Food and Agriculture Organization and United Nations Environment Program (1981) and use the revised conversion factors given by Brown et al. (1989). The first value of each pair of data is for undisturbed forests; the second value is for logged forests. c The values are averaged from estimates by Brown and Lugo (1982), Post et al. (1982), Schlesinger (1984), and Zinke et al. (1986). SOURCES: Houghton, R. A. 1991a. Releases of carbon to the atmospherefrom degradation of forests in tropical Asia. Can. J. Forest Res.21:132–142; Houghton, R. A. 1991b. Tropical deforestation and atmosphericcarbon dioxide. Climatic Change 19:99–118.
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Sustainable Agriculture and the Environment in the HUMID TROPICS TABLE A-3 Initial Carbon Stocks Lost to the Atmosphere When Tropical Forests Are Converted to Different Kinds of Land Use and the Tropical Land Use Areas, 1985 Percentage of Carbon Lost from: Land Use Vegetation Soil Tropical Land Use Area, 1985 (millions of ha) Permanent agriculture 90–100 25 602a Pasture 90–100 12 1,226a , b Degraded croplands and pastures 60–90c 12–25c ? Shifting cultivation 60 10 435d Degraded forests 25–50e ? ? Plantations 30–50f ? 17d Logging 25g (range 10–50) ? 169d Forest reserves 0 0 ? NOTE: For soils, the stocks are to a depth of 1 m. The loss of carbon may occur within 1 year with burning or over 100 years or more with some wood products. The question marks denote unknown information. a Food and Agriculture Organization. 1987. Yearbook of Forest Products. Rome, Italy: Food and Agriculture Organization of the United Nations. b Area includes pastures on natural grasslands as well as those cleared from forest. c Degraded croplands and pastures may accumulate carbon, but their stocks remain lower than the initial forests. d Food and Agriculture Organization and United Nations Environment Program. 1981. Tropical Forest Resources Assessment Project. Rome, Italy: Food and Agriculture Organization of the United Nations. e Houghton, R. A. 1991a. Releases of carbon to the atmosphere from degradation of forests in tropical Asia. Can. J. Forest Res. 21:132 –142. f Plantations may hold as much or more carbon than natural forests, but a managed plantation averages one-third to one-half as much carbon as an undisturbed forest because it is generally regrowing from harvest (Cooper, C. F. 1982. Carbon storage in managed forests. Can. J. Forest Res. 13:155–166). g Based on current estimates of aboveground biomass in undisturbed and logged tropical forests (Brown, S., A. J. R. Gillespie, and A. E. Lugo. 1989. Biomass estimation methods for tropical forests with applications to forest inventory data. Forest Sci. 35:881–902). When logged forests are colonized by settlers, the losses are equivalent to those associated with one of the agricultural uses of the land. SOURCE: Unless indicated otherwise, data are from Houghton, R. A.,R. D. Boone, J. R. Fruci, J. E. Hobbie, J. M. Melillo, C. A. Palm,B. J. Peterson, G. R. Shaver, G. M. Woodwell, B. Moore, D. L. Skole,and N. Myers. 1987. The flux of carbon from terrestrial ecosystemsto the atmosphere in 1980 due to changes in land use: Geographicdistribution of the global flux. Tellus 39B:122–139.
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Sustainable Agriculture and the Environment in the HUMID TROPICS uses that replace forests and the underlying causes of deforestation: socioeconomic factors, environmental factors, and government policy. Fearnside (1987) divided the causes of deforestation in Brazil into proximate and ultimate causes. Repetto (1989) stressed the economic incentives set by government policies. One approach is no more correct than another, although Repetto's approach may be the most useful for determining how to change current incentives. From the perspective of sustainable agriculture, however, there is yet another approach to assigning cause to deforestation—most deforestation in the tropics has been, and still is, due to the development of new agricultural land. The expansion of agricultural land, and thus deforestation, could be reduced by adopting methods of sustainable agriculture. Permanent Agriculture When forests and woodlands are cleared for cultivated land, an average of 90 to 100 percent of the aboveground biomass is burned and immediately released to the atmosphere as CO2. Up to an additional 25 percent of carbon in the 1 m of surface soils is also lost to the atmosphere (Table A-3). Most of the loss occurs rapidly within the first 5 years of clearing; the rest is released over the next 20 years. The wood harvested for products subsequently oxidizes, but it does so much more slowly than does the wood felled for cultivated land. The material remaining above and below the ground decays, as does the organic matter of newly cultivated soil. The rates of decay vary with climate, but in the humid tropics, most material decomposes within 10 years (John, 1973; Lang and Knight, 1979; Swift et al., 1979). However, recent work has indicated that many tropical woods take up to several decades to decompose (S. Brown and A. E. Lugo, personal observations). A small fraction of burned organic matter is converted to charcoal, which resists decay (Comery, 1981; Fearnside, 1986; Seiler and Crutzen, 1980). When croplands are abandoned, the lands may return to forests at rates determined by the intensity of disturbance and climatic factors (Brown and Lugo, 1982, 1990b; Uhl et al., 1988). Cultivation of staple food crops in fields is common in the humid tropics—as it is elsewhere in the world—and is sustainable on good soils. Rice, maize, and cassava are the principal crops. Rice is usually cultivated in flooded fields or paddies, and the productivity and sustainability of wet rice cultivation is enhanced by reducing soil acidity under anaerobic conditions. This improves nutrient availability and the fertilization capabilities of the algae, decayed stubble, and
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Sustainable Agriculture and the Environment in the HUMID TROPICS animal dung that exist in soil. Soil erosion is reduced because the soil surface is covered by water and because of the constraints on soil movement imposed by the mounds of earth surrounding the paddies. Wet rice cultivation is one of the most sustainable land uses in the humid tropics; however, it is not universally applicable. Level, easily flooded sites are required (for example, river floodplains), although slight slopes can be accommodated by terracing. If the water above the sediment is aerobic, it can act as a sink for CH4, another greenhouse gas. However, CH4 is produced in copious amounts under the anaerobic conditions of the flooded fields. So, in addition to the CO2 given off when the forest is cleared initially, there is a continuing emission of CH4. This presents a major and not easily resolvable problem. Although control of deforestation by promoting the spread of wet rice cultivation makes sense because of its high productivity and sustainability, this might be harmful from a climate change perspective. Pastures The changing of forests to pastures results in a 90 to 100 percent loss of carbon from the vegetation, which is similar to that for cultivated lands (Table A-3). Because pastures generally are not cultivated, the loss of carbon from pasture soils is less than the loss from cropland soils (about 12 percent compared with 25 percent). Most studies show a loss of soil carbon (Fearnside, 1980, 1986; Hecht, 1982a), sometimes as much as 40 percent of the carbon originally contained in the forest soil (Falesi, 1976; Hecht, 1982b). However, under some conditions there appears to be no loss of soil carbon (Buschbacher, 1984; Cerri et al., 1988), and there may even be an increase (Brown and Lugo, 1990b; Lugo et al., 1986). Theoretically, cattle ranching on planted pastures is an attractive option because it should maintain a continuous grassy cover on the soil surface and does not involve cultivation, thereby reducing soil degradation. The hydrologic and soil conservation properties of pastures observed on experimental sites are generally favorable. In practice, however, both productivity and sustainability can be low in some tropical areas, causing frequent abandonment of land. The results are a continuing need to clear more forests to provide fresh pastures; overgrazing, which causes widespread, degraded vegetative cover and changes in composition; and soil compaction from constant trampling by animals, which exposes the soil to other forms of degradation. However, in well-managed pasturelands, this pattern of events does
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Sustainable Agriculture and the Environment in the HUMID TROPICS not occur. For example, large areas of productive pasturelands that have been in use for several decades or more exist in Venezuela, Costa Rica, and Puerto Rico. The organic carbon content of the soil of well-managed pasturelands is as high or higher than that of the forests from which the pasturelands were originally derived (S. Brown, personal observation). From a climate change perspective, there are disadvantages to pastures. First, the amount of biomass per unit area is low. Second, frequent burning of pastures to maintain productivity leads to emissions of greenhouse gases in addition to the emissions following the initial clearing. Third, cattle emit CH4 from their guts. In this case, continuing greenhouse gas emissions are not compensated for by high sustainability, as is the case with wet rice cultivation. Degradation of Croplands and Pastures In many areas of the humid tropics, the abandonment of croplands is not followed by forest regeneration. Degraded croplands and pastures may accumulate carbon, but 60 to 90 percent of the carbon in the original forest and 12 to 25 percent of the soil carbon has been lost to the atmosphere (Houghton et al., 1987). Much of the land is abandoned in the first place because it has lost its fertility or has been eroded. These abandoned, degraded lands do not immediately return to forests, yet their degradation requires that new lands be cleared to keep the areas of productive croplands and pastures constant. The new lands are most frequently obtained by clearing forests. Degraded lands are characterized by having been deforested and exposed to factors that reduced the land's productive potential (Lugo, 1988). According to Grainger (1988), the area of degraded lands in the tropics exceeds the area of unspoiled forestlands. The degraded lands have already lost a fraction of the carbon they stored initially and have the potential to serve as carbon sinks, should they be managed properly or rehabilitated by artificial or natural means. Shifting Cultivation The practice of traditional shifting cultivation, in which short periods of cropping alternate with long periods of fallow, during which time forests regrow, is common throughout the tropics. This form of shifting cultivation is sustainable when low population densities exist over large areas and the forests recover during the fallow phase. Shifting cultivation results in about a 60 percent loss of the original
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Sustainable Agriculture and the Environment in the HUMID TROPICS consequence of economic development. Permanent cultivators of cash crops focus on growing the most profitable crops and ensuring that yields are as high as possible. This may not necessarily mean that sustainability is prized, too, however. Cattle ranching in Brazilian Amazonia is a prime example of a case in which the emphasis was on short-term financial gains rather than long-term sustainability. (5) Sustainability can also decline as a result of external influences, as when one land use has an impact on another. For example, expansion of the area of permanent agriculture or logging may jeopardize shifting agriculture by affecting its area and intensity. Such expansion is possible because shifting cultivators often lack legal land rights. The situation is complicated even more by the considerable variation in the degree of sustainability of the resulting agricultural land uses. Overintensive use of land, whether it is due to poor management, inadequate suitability of the land for a given use, inability to invest in required inputs, or socioeconomic and policy factors, can lead to a decline in soil fertility and crop yields and, hence, a decline in the amount of biomass per unit area and the carbon uptake rate. The ultimate result may be a change in land use as land is abandoned. Lack of sustainability also has another effect: more deforestation is required to increase the area of agricultural land so that overall production is maintained, and this leads to further CO2 emissions. A move toward sustainability is therefore a vital consideration if deforestation is to be controlled. It is also an important alternative to consider if greenhouse gas emissions are to be reduced and carbon uptake maximized. Improvements in sustainability go hand in hand with increasing productivity on selected lands. Because low-fertility soils are widespread in the humid tropics, the area of land suited to intensive agriculture under foreseeable socioeconomic conditions is limited. One solution involves increasing the productivity of intensive agriculture on only the best lands so that an increasing share of national food production can be managed. Low-intensity agriculture and forestry could then be allowed to continue elsewhere. Agroforestry One promising way to increase the productivity and sustainability of land use on poorer soils is to use agroforestry systems, a variety of techniques that combine the growing of herbaceous crops and trees and the raising of livestock on the same or adjacent areas of land. This simulates the multilayered structure of natural tropical forests
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Sustainable Agriculture and the Environment in the HUMID TROPICS and maintains the high level of vegetative cover needed to protect the soil from erosion (Grainger, 1980, 1991). Agroforestry also plays a role in efforts to combat the greenhouse effect. So far most attention has been given to large-scale afforestation to increase the rate of carbon uptake by terrestrial biota (Grainger, 1990a,c; Houghton, 1990b; Sedjo and Solomon, 1989). There are practical limits, however, to how fast the establishment rate of timber plantations (for growing industrial wood or fuelwood) can be increased. Foresters identified this problem in the 1970s for reasons unconnected with the greenhouse effect: There were simply insufficient forestry personnel to plant the number of trees needed, and those trees that were planted could be cut down prematurely by local people because young plantations were poorly protected. The best solution was determined to be the establishment of social (or community) forestry programs. Personnel involved with these forestry programs support and assist with the establishment of new tree cover on communal lands or private farmlands, rather than in government forest reserves, with varying degrees of participation by local people. Tree species that satisfy local needs for food, fodder, fuelwood, and other products are chosen. Many social forestry projects involve agroforestry systems of one sort or another. Any new initiative to expand forest cover in the tropics will probably involve a combination of monoculture plantations and agroforestry systems, thereby enhancing the sustainability of land use on the poorer lands on which such activities are likely to be concentrated. Carbon Sinks If sustainable land use practices spread, there is a high potential for carbon sinks to increase in importance in the global carbon budget. For example, carbon accumulation in the biomass and soils of forest fallow could continue for many decades if less of this land had to be recut and burned to meet the demands of food production. At the same time, the need to continue deforesting tropical lands might be reduced, and because many of these forests are still recovering from past disturbances, they could continue to sequester carbon in biomass, necromass, and soil. Therefore, models that project future carbon releases from the tropics must consider the switch to more sustainable uses of the landscape. These uses should encompass an optimum mix of land uses (for example, mature forest, logged forests, secondary forests, and annual and perennial crops). The models should also be able to evaluate the ability of the various land uses to
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Sustainable Agriculture and the Environment in the HUMID TROPICS sequester carbon through biomass accumulation in primary and secondary forests or plantations, woody debris, and soils (Table A-4). Priorities for Future Research To better assess the role of land use changes on climate change, including the impact of sustainability, more research is needed to extend the scope of the data collection, analyses, and modeling approaches outlined here. Some key priorities include the following: There is an urgent need to undertake field studies to gain a better understanding of what constitutes sustainable systems. The critical information needed from field studies is a better understanding of what defines sustainable systems. For example, it would be useful to know the threshold rotation at which shifting cultivation becomes sustainable and what factors influence that threshold. The data should be collected within a well-defined and standardized sampling format so that comparisons can be made between different ecosystems. Further field and remote-sensing research into deforestation and land use change processes is needed so that many of the functional relationships can be quantified at both local and national levels. It is important to develop an index of agricultural sustainability that could be used in land use models and land use planning techniques. The index could be estimated on the basis of such factors as land capability, land use intensity, available investment capital, food yield per hectare, economic rate of return, transportation systems, and cultural factors. More development, testing, and comparisons of the models discussed above are needed. These models can explore the factors that lead to sustainability and project the regional and global repercussions of sustainable agriculture. Because each model emphasizes different aspects of the system and operates at different spatial scales, they are useful for exploring different types of questions. More information is needed on the biomass densities and carbon sequestration rates of alternative tropical land uses and how these vary with productivity and the degree of sustainability. ACKNOWLEDGMENTS W. M. Post, G. Marland, and four anonymous reviewers provided useful comments on the manuscript. J. P. Veillon made available his long-term data for the forests of Venezuela. Research was partially
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Representative terms from entire chapter: