Sustainable Agriculture and the Environment in the Humid Tropics (1993) / Chapter Skim
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1 Agriculture and the Environment in the Humid Tropics
1 Agriculture and the Environment in the Humid Tropics
Pages 21-65
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From page 21... ...
As scientific understanding of tropical ecosystems has expanded, appreciation of their biological diversity and the vital role they play in the functioning of the earth's biophysical systems has risen. The fate of tropical rain forests, in particular, has come to signify growing scientific and public interest in the impact of human activities on the global environment.
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From page 22... ...
The problems associated with unstable shifting cultivation and tropical monocultures, together with the need to improve productivity on degraded and resource-poor lands, have prompted farmers, researchers, and agricultural development officials to search for more sustainable agricultural and land use systems suitable for the humid tropics. This chapter describes the agricultural resources of the humid tropics, outlines the processes of forest conversion that have affected wide areas, and examines the potential of improved agricultural practices to prevent continued resource degradation.
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From page 23... ...
Estimates of their extent vary. The most current effort to provide reliable and globally consistent information on tropical forest cover, deforestation, and degradation is by the Forest Resources Assessment 1990 Project of the Food and Agriculture Organization (FAO)
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From page 25... ...
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From page 26... ...
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From page 27... ...
1982. Ecological Aspects of Development in the Humid Tropics.
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From page 28... ...
The generally infertile soils are able to support these biologically diverse, high-biomass forests because they have fast rates of nutrient cycling and have reached maturity without frequent disturbances. While the forests of the humid tropics are often referred to generically as tropical rain forests, they in fact include a variety of distinct plant associations.
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From page 29... ...
Although the environmental characteristics and benefits described pertain fundamentally to primary tropical moist forests, they are also provided to varying degrees by secondary forests, regenerating forests, managed forests, forest plantations, and agroforestry systems. These distinctions become important in weighing the impacts of different types of forest conversion and formulating sustainable agricultural systems suited to humid tropic conditions.
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From page 30... ...
Carbon stored within forest biomass and soils is prevented from reaching the atmosphere in the form of carbon dioxide or methane, both of which contribute to global warming. Biological Diversity The unusually high concentration of species in tropical moist forests is widely recognized, and the accelerated loss of that diversityespecially of plant species—has drawn much attention in recent years (Ehrlich and Wilson, 1991; Myers, 1984; Raven, 1988; Wilson and Peter, 1988~.
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From page 31... ...
Products and Commodities The high degree of biological diversity within tropical moist forests is reflected not only in germplasm resources, but also in the array of established and potential products and commodities they contain. Tropical forests are sources not only of widely exploited timber and plantation products, but also of foods (including animal protein)
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From page 32... ...
within sustainable agroecosystems. Nutrient Cycling The vegetation within tropical moist forests thrives by retaining and efficiently recycling scarce but essential nutrients within the ecosystem.
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From page 33... ...
CONVERSION OF HUMID TROPIC FORESTS Forest conversion is the alteration of forest cover and forest conditions through human intervention, ranging from marginal modification to fundamental transformation. At one extreme, forests that have been slightly modified (through, for example, selective extraction, traditional shifting cultivation, or gradual substitution of perennial species)
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From page 35... ...
The current extent of tropical rain forests and tropical moist deciduous forests has been estimated to be 1.5 billion ha, with 1 billion ha considered to be intact or primary forests in which human activity has had little impact (World Bank, 1991~. Apparently Africa has lost the greatest proportion of its original tropical moist forests (about 52 percent)
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From page 36... ...
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From page 37... ...
The final results of the project will be based on uniform remote sensing observations of tropical forests specifically made for the project. Preliminary indications concerning forest degradation indicate that the loss of biomass in the tropical forest is occurring at a significantly higher rate than the loss of area due to deforestation (Forest Resources Assessment 1990 Project, 1991~.
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From page 38... ...
Grainger (1988) estimates that as many as 1 billion ha of degraded land may have accumulated in tropical countries, of which 750 million are suit
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From page 39... ...
In a number of areas, secondary forests may not reach advanced stages of restoration due to the activities of subsistence farmers and the impacts of fires, soil degradation and nutrient depletion, inadequate tree regeneration, and invasion by grasses and shrubs. Causes of Forest Conversion People do not make the enormous investments in capital, time, and energy that forest conversion can entail without valid social, Lumber workers transport dipterocarp logs, which command high prices on the international market, out of the tropical rain forest on the island of Borneo, Indonesia.
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From page 40... ...
Gillis, 1988~. The leading direct causes of forest loss and degradation include large-scale commercial logging and timber extraction, the advancement of agricultural frontiers and subsequent use of land by subsistence farmers, conversion of forests to perennial tree plantations and other cash crops, conversion to commercial livestock production, land speculation, the cutting and gathering of wood Population Issues in the Tropics Population growth is one of many factors contributing to resource degradation in the humid tropics.
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From page 41... ...
For example, shifting cultivation practices In Africa account for 70 percent of the clearing of closed-canopy forests (Brown and Thomas, 1990~. In general, snorting cultivators fall into two broad categories: local or native farmers, who tend to be resource conserving and use susta~nable traditional agricultural practices, and more recent farmers, who have migrated to frontier lands to make a living and tend to be less , , · ~ — In much of Africa and Latin America throughout the 1980s per capita income declined, although it grew in Asia and in industrialized countries (World Health Organization, 1990~.
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From page 42... ...
and Greenberg, 1988; Lanly, 1982; National Academy of Sciences, 1980~. Agricultural expansion, as well as the other immediate causes of forest conversion and degradation, is driven by a network of forces operating at national and international levels.
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From page 43... ...
To meet debt obligations, a number of tropical countries have tried to increase their export earnings through rapid extraction of forest resources and conversion of forestlands. International commodity prices and trade policies have also contributed to forest conversion by failing to reflect social and economic costs and by rewarding land uses that provide higher short-term economic returns.
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From page 44... ...
As a result, their effects on the total area of forest cover and on nutrient cycling, watershed stability, biological diversity, and other ecosystem characteristics were limited. Although forest conversion has expanded steadily over the past five centuries, the three continental expanses of humid tropic forest remained largely intact prior to the late nineteenth century (Tucker, 1990~.
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From page 45... ...
. Forest conversion has followed diverse pathways in the humid tropics, but a general pattern can be discerned.
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From page 46... ...
Because of the nature of land use problems in the humid tropics, many of the negative effects may not be felt until they are irreversible. ENVIRONMENTAL CONSEQUENCES The environmental consequences of forest conversion involve the degree to which ecosystem functions are disrupted, forest biomass and composition altered, and forest cover lost.
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From page 47... ...
Due to the high levels of species diversity, the limited distribution of most of these species, and the specialized relationships and reproductive strategies within tropical forest ecosystems, forest clearing and fragmentation result in high levels of species loss. Because current scientific knowledge can provide only rough estimates of total species diversity within tropical moist forests, the rate at which species are being lost cannot be accurately determined.
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From page 48... ...
The risk of fire rises as forest cover dim~nishes due to hotter and drier microclimatic conditions (Crutzen and Andreae, 1990~. At the global scale, forest conversion affects atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and l Climate Change and Thank Use Emissions of trace gases as a result of human activities could change the atmosphere's radiative properties enough to alter the earth's climate.
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From page 49... ...
. In terms of potential impact on climate change, the most important feature of land use in the humid tropics is the net release of carbon that occurs as a result of forest conversion.
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From page 50... ...
Deforestation activities have also brought new diseases to tribal peoples, especially in areas where previous contacts with outsiders had been infrequent. Forest conversion has consequences for both the forest frontier and the cities in tropical countries.
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From page 51... ...
Resource depletion has often been justified as the only way for nations in the humid tropics, faced with growing populations, large foreign debts, nascent industrial capacity, and an often undereducated rural populace, to develop. Especially in recent decades, a number of tropical countries have depleted forest resources in the effort to solve social, political, and economic problems in their societies, and to reduce large and growing international debt burdens (Ehui, Part Two, this volume; Serrao and Homma, Part Two, this volume; Vincent and Hadi, Part Two, this volume)
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From page 52... ...
Constraints on Agricultural Productivity The development of sustainable production systems suitable for areas with low-quality soil and water resources rests on an appreciation of the constraints on agricultural productivity in the humid tropics (National Research Council, 1982; Savage, 1987~. Agriculture is fundamentally a process of converting solar energy, through photosynthesis, into useful biomass.
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From page 53... ...
Finally, climatic conditions In the humid tropics also result In high postharvest losses to pests and spoilage, and pose special problems for storage, transportation, and processing. SOILS The soils of the humid tropics vary from region to region (Table 1-6)
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From page 54... ...
Misconceptions About Humid Tropic Soils Despite evidence to the contrary, the belief persists that the soils of the humid tropics are incapable of supporting sustainable agriculture and forestry. This belief is based on three main misconceptions about tropical soils: laterite formation, low soil organic matter content, and the role of nutrient recycling in agricultural systems.
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From page 55... ...
SOIL ORGANIC MATTER Organic matter content in soils of the humid tropics compares favorably with soils of temperate forests. Studies indicate that organic carbon and total nitrogen levels in tropical forest soils are somewhat higher than those found in temperate forest soils.
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From page 56... ...
Most tropical moist forests grow on an unpromising soil base, generally Ultisols ent cycling studies that include the entire soil profile indicate a considerable portion of the ecosystem's nitrogen and phosphorus stocks may be located in the soil Jordan, 1985; Sanchez, 1979~. However, additional research is required to determine more accurately the content and availability of these nutrients in the biomass versus in the soils.
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From page 57... ...
. Among the Inceptisols, Aquepts are dominant in humid tropic America and Africa, and Tropepts are dominant in humid tropic Asia.
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From page 58... ...
Many local human populations also depend on nearby biological resources for food, fodder, pharmaceuticals, and other needs. Globally, tropical moist forests are the source of germplasm for many food and industrial crops.
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From page 59... ...
Traditional shifting cultivation systems are being disrupted, modified, and replaced as population pressures rise and as migrants unfamiliar with the humid tropics or indigenous land use practices attempt to farm newly cleared land. Typically, this results in shortened fallow periods, fertility decline, weed infestation, disruption of forest regeneration, and excessive soil erosion.
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From page 60... ...
The fertile alluvial soils of the humid tropics are, in fact, so valuable for raising crops that the distinct and highly diverse lowland forests they once supported have virtually disappeared (Ewel, 1991~. Because monocultures in the tropics concentrate species that under natural conditions were widely dispersed, they are more susceptible to pathogens and other pests than the same species in traditional mixed-crop systems or in natural forests.
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From page 61... ...
Many of the degraded or unproductive pastures or croplands resulting from poor management practices can also be reclaimed. The particular methods that are most appropriate in any given locality will vary both within and among the world's humid tropic regions.
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From page 62... ...
The easing of rigid disciplinary boundaries is of special importance in the humid tropics. During the past century, ecologists and other biologists have endeavored to understand the properties and dynamics of tropical forest ecosystems.
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From page 63... ...
Many traditional resource management techniques and systems, often dismissed as primitive, are highly sophisticated and well suited to the opportunities and limitations facing farmers in the tropics. Traditional land use systems have begun to receive greater attention as the primary goal of agricultural research and development in the humid tropics shifts from maximizing short-term production and economic returns to maintaining the long-term health and productivity of agroecosystems.
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From page 64... ...
Most of the fertile lands in the humid tropics are already being intensively used. Continued conversion of primary forests offers increasingly marginal gains.
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From page 65... ...
Agricultural and nonagricultural land uses can in this way be coordinated to enhance sustainability at the field, landscape, watershed, regional, and even global scales. Operationally, this will entail the adoption of sustainable agricultural technologies on intensively managed lands; the restoration of cleared, degraded, and abandoned lands to biological and economic productivity; improved fallow and secondary forest management; and the protection and careful use of the remaining primary forests.
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