Sustainable Agriculture and the Environment in the Humid Tropics (1993) / Chapter Skim
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Appendix: Emissions of Greenhouse Gases from Tropical Deforestation and Subsequent Uses of the Land
Appendix: Emissions of Greenhouse Gases from Tropical Deforestation and Subsequent Uses of the Land
Pages 215-260
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From page 215... ...
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
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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.
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From page 217... ...
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.
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From page 218... ...
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)
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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)
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From page 220... ...
(1981-1990) Tropical 3,710 4,119 3,301 10,859 7,680 7,290 America Tropical 1,310 1,319 1,204 1,338 1,580 4,788 Africa Tropical 2,320 1,815 1,608 2,390 4,600 4,707 Asia 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.
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From page 221... ...
The land use changes considered below include permanent agriculture and pasture, degradation of croplands and pastures, shifting cultivation, forest plantations and tree crons logging, and degraded forests (Table A-3.
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and pastures Shifting cultivation 60 10 435d Degraded forests 2~50e ?
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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.
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From page 224... ...
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.
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From page 225... ...
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.
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From page 226... ...
Decay rates for the plant material left dead at the time of deforestation and accumulation rates for regrowing vegetation during the fallow periods vary by ecosystem (Brown and Lugo, 1982, 1990a; Saldarriaga et al., 1988; Uhl, 1987; Uhl et al., 1982~. Less soil organic matter is oxidized during the shifting cultivation cycle than during continuous cultivation (Detwiler, 1986; Schlesinger, 1986~.
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In press. Tropical forests as sinks of atmospheric carbon.
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From page 228... ...
However, some agricultural economists disagree and point out that, in practice, increasing intensity often leads to a decline in yields, increased soil degradation, and lower sustainability (Blaikie and Brookfield, 1987~. Third, although the sustainability of shifting cultivation is determined by how well it sustains the yield per hectare over succeeding rotations, it can also be evaluated from a carbon budget perspective with respect to how much carbon is stored, on average, in the fallow vegetation and how much soil carbon is restored after cropping.
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From page 229... ...
The net primary productivity of plantations can be high, with values about 3 and 10 times those of secondary and mature forests, respectively (Brown et al., 1986; Lugo et al., 1988~. Soil organic matter also builds up on tree plantations (Brown and Lugo, 1990a; Cuevas et al., 1991~.
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in tropical forests ranges widely between negative values (when stands are degrading) to more than 15 Mg/ha/year in fast-growing plantations (Lugo et al., 1988~.
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. ESTIMATED FLUX OF GREENHOUSE GASES FROM LAND USE CHANGES The estimated carbon content and rates of change of the major land uses in the tropics reviewed above can be used to estimate the flux of greenhouse gases from those land use changes.
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. The expansion of agricultural lands and pasturelands accounts for most of the carbon loss due to tropical deforestation (Table A-3.
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From page 233... ...
with shifting cultivation and deforestation; however, the production, fate, and half-life of carbon in charcoal are poorly known, so the size of this carbon sink is uncertain. TROPICAL SYSTEMS AS CARBON SINKS The potential for vegetation to be a carbon sink depends on the balance of all natural processes of the carbon cycle and the influence of human and natural disturbances.
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Other Greenhouse Gases Most of the carbon released to the atmosphere from land use changes is released as CO2 (Table A-6 and Figure Am. The emissions of CH4, N2O, and CO to the atmosphere are also of interest because they contribute either directly or indirectly to the heat balance of the earth and have been increasing during recent decades (Figure Am.
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235 ~ — O a, ~ O ~ au ._ U
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Because the average residence time of CH4 in the atmosphere is only about 10 years, whereas that of CO2 is 100 to 250 years, the long-term greenhouse effect due to CO2 is greater than that due to CH4 (Table A-6) (Houghton et al., 1990; Lashof and Ahuja, 1990; Rodhe, 1990~.
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(Luizao et al., 1989~. CARBON MONOXIDE CO is not a greenhouse gas, but it does affect the oxidizing capacity of the atmosphere through interaction with OH and thus indirectly affects the concentrations of other greenhouse gases such as CH4 and N2O.
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From page 238... ...
Total Radiative Effect from All Gases Released as a Result of Tropical Deforestation The global emissions (both total emissions and emissions from tropical deforestation) of the three greenhouse gases previously discussed above (CO2, CH4, and N2O)
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These lags result from the slowness of decay of dead plant material, soils, and wood products or the accumulation of carbon in regrowing forests following shifting cultivation and logging. The annual flux estimation includes timing of the releases and accumulations of carbon stock following land use changes.
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From page 240... ...
The curves marked, Exponential High and Low, are based on two extremes of carbon emissions associated with high and low estimates of biomass in the tropical forest. Abrupt reductions in emissions near 2045, 2060, and 2070 result from an elimination of forests in a major region and, hence, an abrupt reduction in the rate of tropical deforestation.
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National population growth rates are somewhat correlated with deforestation rates in the humid tropics (Allen and Barnes, 1986; Grainger, 1986~. In terms of agriculture, economic development increases per capita food consumption and the need to grow export cash crops.
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Estimates of future impacts of land use changes have been made by expanding the scope of a model previously built to simulate long-term trends in national deforestation rates (Grainger, 1986, 1990b)
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The model simulates a decline in deforestation rates for 43 countries that contain 96 percent of the world's total area of the humid tropical forests and 92 percent of the total deforestation rate in 1980 (Table A-7. Two alternative scenarios the high- and low-deforestation scenarios were simulated with the simpler national deforestation model by using initial population growth rates, which were the same as those for 1970 to 1980, and growth rates in food consumption per capita (a)
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1986. The Future Role of the Tropical Rain Forests in the World Forest Economy.
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The simulations suggest that if governments take steps to ensure that growth In agricultural productivity can outpace the rise in food consumption per capita, like the rate assumed here, then the consequent fall in deforestation rates could lead to a cut In carbon emission rates from the forests in the humid tropics of 40 to 70 percent over the next 30 years. TABLE A-10 Two Scenarios for Future Trends in Carbon Dioxide Emissions from Deforestation in the Tropics (Petagrams of Carbon per Year)
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From page 246... ...
was used to contrast sustainable agricultural practices with the typical scheme of colonists in Rondonia, Brazil, of burning the tropical forest, planting annual and perennial crops and then pastures, and lastly, abandoning their lots. The results from simulations show how these extremes of resource management can affect carbon storage and release in the humid tropics (Southworth et al., 1991~.
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sustainable agriculture scenario. Source: Southworth, F., V
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From page 248... ...
Given the noticeably different results obtained from the two scenarios examined, it is worth asking what needs to be done to implement sustainable shifting cultivation in Rondonia or in other forested regions in the tropics. Activities that reduce the negative exponential decline of carbon in the simulations were represented by planting trees intermixed with annuals.
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From page 249... ...
Thus, a rise in population density may result in more intensive shifting cultivation, and if rotations become too short, it may lead to soil degradation and a decline in yields and sustainability. Although economic development can enable investments that make agriculture more productive and sustainable, the lack of economic development, or poverty, means that many farmers are unable to make such investments.
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From page 250... ...
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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From page 251... ...
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.
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From page 252... ...
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 weR-defined and standardized sampling format so that comparisons can be made between different ecosystems.
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1982. The storage and production of organic matter in tropical forests and their role in the global carbon cycle.
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1991. Biomass of tropical forests of South and Southeast Asia.
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1988a. Tropical forests and the global carbon cycle.
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Food and Agriculture Organization and United Nations Environment Pro~ram. 1981 Tropical Forest Resources Assessment Project.
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1985. Net flux of carbon dioxide from tropical forests in 1980.
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From page 258... ...
In press. Tropical forests as sinks of atmospheric carbon.
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From page 259... ...
1989. Deforestation Rates in Tropical Forests and Their Climate Implications.
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1980. Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning.
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