1

Agriculture and the Environment in the Humid Tropics

The wide belt of land and water that lies between the tropics of Cancer and Capricorn is home to half of the world's people and some of its most diverse and productive ecosystems. Citizens and governments within and beyond the tropics are increasingly aware of this region 's unique properties, problems, and potential. 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.

At the same time, the people and nations of the tropics face a difficult future. Most of the world's developing countries are in the tropics, where agriculture is important to rural and national economies. About 60 percent of the people in these countries are rural residents, and a large proportion of these are small-scale farmers and herders with limited incomes (Population Reference Bureau, 1991). The need to stimulate economic growth, reduce poverty, and increase agricultural production to feed a rapidly growing population is placing more pressures on the natural resource base in developing countries (see Part Two, this volume). The deterioration of natural resources, in turn, impedes efforts to improve living conditions. This dilemma, however, has stimulated a growing commitment to sustain-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS 1 Agriculture and the Environment in the Humid Tropics The wide belt of land and water that lies between the tropics of Cancer and Capricorn is home to half of the world's people and some of its most diverse and productive ecosystems. Citizens and governments within and beyond the tropics are increasingly aware of this region 's unique properties, problems, and potential. 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. At the same time, the people and nations of the tropics face a difficult future. Most of the world's developing countries are in the tropics, where agriculture is important to rural and national economies. About 60 percent of the people in these countries are rural residents, and a large proportion of these are small-scale farmers and herders with limited incomes (Population Reference Bureau, 1991). The need to stimulate economic growth, reduce poverty, and increase agricultural production to feed a rapidly growing population is placing more pressures on the natural resource base in developing countries (see Part Two, this volume). The deterioration of natural resources, in turn, impedes efforts to improve living conditions. This dilemma, however, has stimulated a growing commitment to sustain-

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS able development among tropical and nontropical countries alike, with special concern for the world's humid tropics. This report focuses on the humid tropics, a biogeographical area within the tropical zone that contains most of its population and biologically rich natural resources. 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. It stresses the need for a more integrated approach to research, policy, and development activities in managing resources on a more sustainable basis. The definition of agricultural sustainability varies by individual, discipline, profession, and area of concern. Common characteristics include the following: long-term maintenance of natural resources and agricultural productivity; minimal adverse environmental impacts; adequate economic returns to farmers; optimal production with purchased inputs used only to supplement natural processes that are carefully managed; satisfaction of human needs for food, nutrition, and shelter; and provision for the social needs of health, welfare, and social equity of farm families and communities. All definitions embrace environmental, economic, and social goals in their efforts to clarify and interpret the meaning of sustainability. In addition, they suggest that farmers and farm systems must be able to respond effectively to environmental and economic stresses and opportunities. In the humid tropics, priority must be given to soil protection and the efficient recycling of nutrients (including those derived from external sources); to implementation of mixed forest and crop systems; and to secondary forest management that incorporates forest fallow practices (Ewel, 1986; Hart, 1980). THE HUMID TROPICS The humid tropics are defined by bioclimates that are characterized by consistently high temperatures; abundant, at times seasonal, precipitation; and high relative humidity (Lugo and Brown, 1991). Annual precipitation exceeds or equals the potential return of moisture to the atmosphere through evaporation. Total annual rainfall amounts usually range from 1,500 mm to 2,500 mm, but levels of

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS 6,000 mm or more are not uncommon. In general, seasons in the humid tropics are determined by variations in rainfall, not temperature. Most areas experience no more than 4 months with less than 200 mm of precipitation per year. About 60 countries, with a total population of 2 billion, are located partly or entirely within the humid tropics (Table 1-1). About 45 percent of the world's humid tropics are found in the Americas (essentially Latin America), 30 percent in Africa, and 25 percent in Asia. Small portions of the humid tropics can be found in other areas such as Hawaii and portions of the northeastern coast of Australia. The typical vegetation for the humid tropics consists of moist, wet, and rain forests in the lowlands and in the hill and montane uplands. 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) of the United Nations, using remote sensing imagery and national survey data as part of its methodology (Forest Resources Assessment 1990 Project, 1992). It defines forests as ecological systems with a minimum of 10 percent crown cover of trees (minimum height 5 m) and/or bamboos, generally associated with wild flora, fauna, and natural soil conditions, and not subject to agricultural practices. The project estimates that forests cover 1.46 billion ha, or 48 percent of the land area (3.02 billion ha) in the tropical rain forest, moist deciduous forest, and hill and montane forest zones. These forests constitute 30 percent of the land area within the tropical region (4.82 billion ha) and 86 percent of the total tropical forest area (1.7 billion ha). Although they cover only 10 percent of the land area of the world (15 billion ha), they contain one-third of the world's plant matter. Nearly two-thirds of the world's humid forests are found in Latin America, with the remainder split between Africa and Asia. The soils of the humid tropics are highly variable. Table 1-2 shows the geographical distribution of soil orders and major suborders based on the soil classification system developed in the United States. Oxisols and Ultisols are the most abundant soils in the humid tropics, together covering almost two-thirds of the region. Oxisols, found mostly in tropical Africa and South America, are deep, generally well-drained red or yellowish soils, with excellent granular structure and little contrast between horizon layers. As a result of extreme weathering and resultant chemical processes, however, Oxisols are acidic, low in phosphorus, nitrogen, and other nutrients, and limited in their ability to store nutrients, but have relatively high soil organic matter content. Ultisols are the most abundant soils of tropical Asia,

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS TABLE 1-1 Population Data for Selected Countries with Tropical Moist Forests Region or Country Population Estimate, Mid-1991 (millions) Urban Population (%) Rate of Natural Increase (annual %) Number of Years to Double Population Population Projection to 2025 (millions) 1989 Per Capita GNP($) Middle South Asia             Bangladesh 116.6 14 2.4 28 226.4 180 India 859.2 27 2.0 34 1,365.5 350 Sri Lanka 17.4 22 1.5 47 24.0 430 Continental Southeast Asia             Brunei 0.3 59 2.5 27 0.5 14,120 Cambodia 7.1 11 2.2 32 12.9 — Laos 4.1 16 2.2 32 7.4 170 Myanmar 42.1 24 1.9 36 72.2 — Thailand 58.8 18 1.3 53 78.1 1,170 Vietnam 67.6 20 2.3 31 107.8 — Insular Southeast Asia             Indonesia 181.4 31 1.7 41 237.9 490 Malaysia 18.3 35 2.5 28 34.7 2,130 Papua New Guinea 3.9 19 2.3 31 7.6 900 Philippines 62.3 42 2.6 27 100.7 700 Subtotal 1,439.1 26a 2.1a 34a 2,275.7 — Middle America             Belize 0.2 50 3.3 21 0.5 1,600 Costa Rica 3.1 45 2.4 28 5.6 1,790 Dominican Republic 7.3 58 2.3 30 11.4 790 El Salvador 5.4 43 2.8 25 9.4 1,040 Guatemala 9.5 39 3.0 23 21.7 920 Haiti 6.3 28 2.9 24 12.3 400

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Honduras 5.3 43 3.1 23 11.5 900 Mexico 85.7 71 2.3 30 143.3 1,990 Nicaragua 3.9 57 3.4 21 8.2 830 Panama 2.5 52 2.1 34 3.9 1,780 Puerto Rico (U.S.) 3.3 72 1.1 62 4.2 6,010 Trinidad and Tobago 1.3 64 1.6 44 1.8 3,160 Tropical South America             Bolivia 7.5 50 2.6 27 14.3 600 Brazil 153.3 75 1.9 36 245.8 2,550 Colombia 33.6 68 2.0 35 54.2 1,190 Ecuador 10.8 55 2.4 29 19.2 1,040 French Guiana 0.1 81 2.2 31 0.2 — Guyana 0.8 35 1.8 39 1.2 310 Peru 22.0 69 2.3 30 37.4 1,090 Suriname 0.4 48 2.0 35 0.7 3,020 Venezuela 20.1 83 2.3 30 35.4 2,450 Subtotal 382.4 56a 2.4a 31a 642.2 — West Africa             Côte d'Ivoire 12.5 39 3.5 20 39.3 790 Ghana 15.5 32 3.2 22 35.4 380 Guinea 7.5 22 2.6 27 16.0 430 Guinea-Bissau 1.0 27 2.0 35 1.9 180 Liberia 2.7 44 3.2 22 7.4 450 Nigeria 122.5 16 2.8 25 305.4 250 Sierra Leone 4.3 30 2.7 26 10.0 200 Togo 3.8 22 3.7 19 11.3 390 Central Africa             Cameroon 11.4 42 2.6 26 26.1 1,010 Central African Republic 3.0 43 2.6 27 6.6 390 Congo 2.3 41 3.0 23 5.5 930 Equatorial Guinea 0.4 60 2.6 26 0.9 430

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Gabon 1.2 41 2.3 31 2.9 2,770 São Tomé and Principe 0.1 38 2.5 28 0.3 360 Zaire 37.8 40 3.1 22 101.1 260 Eastern Africa             Burundi 5.8 5 3.2 21 15.5 220 Kenya 25.2 22 3.8 18 63.2 380 Madagascar 12.4 23 3.2 22 34.0 230 Mauritius 1.1 41 1.4 51 1.4 1,950 Mozambique 16.1 23 2.7 26 35.4 80 Rwanda 7.5 7 3.4 20 22.9 310 Tanzania 26.9 20 3.7 19 78.9 120 Uganda 18.7 10 3.5 20 55.0 250 Subtotal 339.7 30a 2.9a 25a 876.4 — NOTES: A dash denotes information that was not available; GNP, gross national product. a Average. SOURCE: Population Reference Bureau. 1991. World Population Data Sheet 1991. Washington, D.C.: Population Reference Bureau.

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS TABLE 1-2 Geographical Distribution of Soils of the Humid Tropics (in Millions of Hectares)a Soil Order or Suborder Humid Tropics Total Humid Tropic Americab Humid Tropic Africac Humid Tropic Asiad Oxisols 525 332 179 14 Ultisols 413 213 69 131 Inceptisols         Aquepts 120 42 55 23 Andepts 12 2 1 9 Tropepts 94 17 19 58 Subtotal 226 61 75 90 Entisols         Fluvents 50 6 10 34 Psamments 90 6 67 17 Lithic 72 19 14 39 Subtotal 212 31 91 90 Alfisols 53 18 20 15 Histosols 27 — 4 23 Spodosols 19 10 3 6 Mollisols 7 — — 7 Vertisols 5 1 2 2 Aridisolse 2 — 1 1 Total 1,489 666 444 379 a Based on dominant soil in maps (scale of 1:5 million) of the Food and Agriculture Organization (FAO) of the United Nations. b From Sanchez and Cochrane (1980) plus recent adjustments. c From the FAO (1975) and Dudal (1980). d From the FAO (1977, 1978). Includes 46 million ha of the humid tropics of Australia and Pacific Islands. e Saline soils only (Salorthids). SOURCE: National Research Council. 1982. Ecological Aspects of Developmentin the Humid Tropics. Washington, D.C.: National Academy of Sciences. and are also found in Central America, the Amazon Basin, and humid coastal Brazil. Ultisols are usually deep, well-drained red or yellowish soils, somewhat higher in weatherable minerals than Oxisols but also acidic and low in nutrients. Inceptisols and Entisols account for most of the remaining soils of the humid tropics (about 16 percent and 14 percent, respectively). These are younger soils, more limited in distribution, and range from highly fertile soils of alluvial and volcanic origin to very acidic and nutrient-poor sands.

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS TABLE 1-3 Summary of Forested Bioclimates in the Tropical Zone Bioclimate Mean Annual Biotemperature Mean Annual Precipitation (mm) Other Precipitation Characteristics Lowland >24°C     Moist forest   1,500–4,000 No more than 4 months with <200 mm Wet forest   4,000–8,000 No more than 2 months with <200 mm Rain forest   >8,000 No months with <200 mm Premontane 18°–24°C     Moist forest   1,000–2,000 2–4 months with <100 mm Wet forest   2,000–4,000 No more than 2 months with <100 mm Rain forest   >4,000 No months with <100 mm Lower montane 12°–18°C     Moist forest   1,000–2,000 2–4 months with <100 mm Wet forest   2,000–4,000 No more than 2 months with <100 mm Rain forest   >4,000 No months with <100 mm Montane 6°–12°C     Moist forest   500–1,000 2–4 months with <50 mm Wet forest   1,000–2,000 No more than 2 months with <50 mm Rain forest   >2,000 No months with <50 mm NOTE: At any given latitude, the treeline lies at a mean annual biotemperature of 6°C. Although many humid tropic soils are acidic and low in reserves of essential nutrients, the constant warm temperatures, plentiful rainfall, and even allocation of sunlight throughout the year permit abundant plant growth. Broadleaf evergreen forests are the dominant form of vegetation. 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. Holdridge's (1967) System for the Classification of World Life Zones provides the basis for differentiating forest formations over broad gradients of temperature and rainfall (see Table 1-3). Tropical lowland forests are the most abundant, constituting some 80 percent of humid tropic vegetation. Lowland areas are also significant from the standpoint of human economic activity,

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS environmental impacts, development potential, and scientific interest. Although tropical premontane forest formations comprise only about 10 percent of humid tropic vegetation, they are disproportionately modified by human activity, especially toward the drier end of the gradient, because of their suitability for plantation culture and crop agriculture. The remainder of the humid tropic forests consists of relatively uncommon lower montane and montane formations. Collectively, lowland, premontane, and montane forest formations can be referred to as humid tropic or tropical moist forests. The small nonforest component of humid tropic vegetation includes aquatic and wetland flora and treeless plant communities that exist above timberline on the highest mountaintops. At the latitudinal and climatic limits of the humid tropics, the tropical moist forests grade into more seasonal (monsoonal), semievergreen types and eventually into savannah ecosystems. The term “closed tropical forests” is sometimes used to distinguish the unbroken forests of the humid tropics from drier, more open tropical forest types. FOREST CHARACTERISTICS AND BENEFITS The forests of the humid tropics provide multiple goods, values, and environmental services. At the global scale, tropical moist forests, through photosynthesis, evapotranspiration, decomposition, succession, and other natural processes, play a significant role in the functioning of the atmosphere and biosphere. At local and regional scales, the ecological processes and biological diversity of forests provide the foundations for stable human communities and opportunities for sustainable development. The special characteristics of tropical moist forests, and the direct and indirect benefits they afford, are described in numerous publications (for example, Myers, 1984; National Research Council, 1982; Office of Technology Assessment, 1984; Wilson and Peter, 1988) and summarized below. These characteristics underscore the need to begin with an understanding of ecosystem components and processes in the humid tropics in moving toward more sustainable land uses. 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.

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Local and Global Climatic Interactions Local and global climatic patterns are influenced by the interaction of tropical moist forests and the atmosphere. At the continental scale, forests are thought to influence convection currents, wind and precipitation patterns, and rainfall regimes because of their ability to reflect solar heat back into space and to receive and release large volumes of water (Houghton et al., 1990; Salati and Vose, 1984). It is estimated, for example, that as much as half the atmospheric moisture in the Amazon basin originates in local forests by transpiration (Salati et al., 1983). At the global level, tropical moist forests play an important role in large-scale biogeophysical cycles (especially those of carbon, water, nitrogen, and other elements) that are critical in determining atmospheric conditions. Particularly important is the function of the forests in the carbon cycle. The total biomass accumulations in mature tropical moist forests are the highest in the tropics and among the highest of any terrestrial ecosystem (Brown and Lugo, 1982). In primary forests, carbon exists in essentially a steady state— the amount of carbon accumulated is about equal to the amount released, although there may be a small net accumulation (Lugo and Brown, In press). Secondary and recovering forests act as important carbon sinks (Brown et al., 1992). 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 diversity —especially of plant species—has drawn much attention in recent years (Ehrlich and Wilson, 1991; Myers, 1984; Raven, 1988; Wilson and Peter, 1988). Although tropical moist forests cover about 7 percent of the earth's land surface, they are believed to harbor more than half of the world's plant and animal species. Estimates of the total number of species in tropical moist forests range between 2 million and 20 million (Ehrlich and Wilson, 1991). The majority of these species have yet to be described, much less studied. Basic taxonomic work in tropical moist forests remains a high research priority (National Research Council, 1992). Beyond the high levels of diversity of wild species found in the forests themselves, the humid tropics are also important centers of germplasm diversity for rice, beans, cassava, cocoa, banana, sugarcane, citrus fruits, and other economically important crops. These

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Germplasm collected from the tropics is used in crop improvement research in laboratories around the world. Friable callus of cassava, an important root crop in the tropics, is chopped for suspension in an Austrian laboratory. Credit: Food and Agriculture Organization of the United Nations. germplasm resources include wild relatives of domesticated plants as well as highly localized crop varieties and landraces developed over centuries by farmers. To boost productivity, provide resistance against pests and other environmental stresses, and improve overall quality, plant breeders have already incorporated genetic material from these wild and domesticated strains into breeding lines of rice, cocoa, sugar, and other major crops. 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), spices, medicines, resins, oils, gums, pest control agents, fuels, fibers, and forages for forest dwellers and small-scale farmers. Many of the products used for subsistence purposes at the local level

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Ultisols are found mostly in regions with long growing seasons and ample moisture for good crop production. They are the most abundant soils of humid tropic Asia and are also present in Central America, the Amazon Basin, and humid coastal Brazil. Unlike Oxisols, they exhibit a marked increase of clay content with depth. They also usually contain high levels of aluminum, which is toxic to plants and severely restricts rooting in most crops. However, many Ultisols respond well to fertilizers and good management practices, and are commonly used in both shifting cultivation and intensive cultivation systems. The agricultural production potential of Oxisols and Ultisols is improved if they are properly managed. For example, judicious applications of fertilizer can supplement their limited natural nutrient as a constraint in the humid tropics. Where natural laterite outcrops occur, they are an asset to development. 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. No differences in organic matter content have been found between soils of the tropics and soils of the temperate region in uncultivated, forested ecosystems, or between Oxisols (abundant tropical soils found mostly in Africa and South America) and Mollisols (prairie soils of the U.S. Great Plains). With land clearing and continuous cropping, however, the organic matter content of soils of the humid tropics declines rapidly, because of continuously high temperatures throughout the year (Jenkinson and Ayanaba, 1977). In most forested tropical ecosystems, soil organic matter is concentrated in the topsoil. Even though root growth within tropical forests is concentrated in the topsoil, many roots exploit the usually deep reddish subsoils for water and nutrients. In savannah Oxisols, however, soil organic matter is found in substantial quantities to a depth of 1 m or more. NUTRIENT CYCLING Another commonly held view is that tropical moist forests essentially feed themselves, since their soils are poor in nutrients. Some nutri-

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS stores. In Ultisols, calcium (used to build cell walls) and magnesium (the essential ingredient in chlorophyll) are in short supply and are found primarily in the topsoil, where they have presumably been cycled by vegetation. In some Oxisols, phosphorus, which affects plant growth in many ways, is commonly so low that crops cease growth when they deplete the phosphorus contents of their seeds (Lathwell and Grove, 1986). These soils usually produce crops for only a few years before soil nutrients are exhausted or leached from the soil profile. At this point, farmers must either move to another location, restore nutrients to the soils through rotations or the application of manure or mineral fertilizers, or allow the land to revegetate before replanting. Deforestation often leaves soils in a depleted state. 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. The high efficiency of tropical forest nutrient cycles has long been recognized (Nye and Greenland, 1960; Sanchez, 1976). Agricultural systems generally operate in the same way, with one major exception: biomass is not removed from natural ecosystems, but crop harvests in agroecosystems can remove large quantities of biomass and constitute the main pathway of nutrient loss. In grain crops, about 40 percent of the carbon, 60 percent of the nitrogen, and two-thirds of the phosphorus in crops are removed with the harvest, while most of the potassium, calcium, and magnesium remain in the crop residues (Sanchez et al., 1989). In an agricultural or forestry system, nutrients lost through harvesting must be balanced with nutrient inputs in the form of fertilizers, manures, or biological nitrogen fixation. In agricultural systems dominated by annual crops, the flow of nutrients from soil to crop occurs seasonally and must be extremely rapid if high yields are to be attained. As crop residues are returned to the soil, they are broken down by soil fauna and flora into simple components, which are then available for uptake by the next crop. Losses from the system can occur if crop residues are removed from the field, if soil is lost through erosion, or if soluble nutrients remain in the soil with no crop growth during periods of heavy rain. The use of crop or animal residues as fuel can be a major source of nutrient (and carbon) loss from the system.

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS that are washed by heavy rains. Calcium and potassium are leached from the soil by rain. Iron and aluminum form insoluble compounds with phosphorus and, if present in high concentrations, will decrease the availability of phosphorus to plants. When forests are removed, rapid degradation in soil fertility can occur because of the dependence of these soils on nutrient cycling by deep-rooted plants (Buol et al., 1980). Inceptisols, young soils of sufficient age to have developed distinct horizons, comprise the third most widespread soil type in the humid tropics. Three major kinds occur: Aquepts (poorly drained), Andepts (well drained, of volcanic origin), and Tropepts (well drained, of nonvolcanic origin). Among the Inceptisols, Aquepts are dominant in humid tropic America and Africa, and Tropepts are dominant in humid tropic Asia. Most of the Aquepts, or wet Inceptisols, are of high to moderate fertility and support dense human populations. In tropical America, they occur in the older alluvial plains along the major rivers and inland swamps of the Amazon Basin. About half have high potential for intensive agriculture. In Africa, large areas of wet Inceptisols (known locally as hydromorphic soils) long remained undeveloped because of human health hazards, although many of these hazards have been overcome and settlement has advanced. In Asia, many of the Tropept soils are used for lowland rice production. More than 90 percent of the world's rice is grown and consumed in Asia (where about 55 percent of the earth's people live). Inceptisols of volcanic origin (Andepts) are important in the volcanic regions of Asia, in parts of Central and South America, and in parts of Africa. They are generally fertile and have excellent physical properties. Entisols are soils of recent development that do not show significant horizon layers. Within this soil type, well-drained, young alluvial soils (Fluvents) not subject to periodic flooding are considered among the best soils for agriculture in the world. Fluvents account for only 2.7 percent of the soils of the humid tropics and most are already cultivated; about two-thirds (25 million ha) are found in Asia where they are under intensive lowland rice production. Where forests remain on these soils, their preservation will be difficult due to their high agricultural potential. BIOLOGICAL FACTORS Biological constraints on agriculture in the humid tropics include insect and other pests, pathogens, and weeds; a lack of improved germplasm for the common crops of the region; and the loss of domestic and wild biodiversity. The hot and humid climate provides

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS ideal conditions for pests and diseases. The growing season is essentially continuous and facilitates the development of persistent pests. Losses of crops to pests in the humid tropics are great. Preharvest losses are estimated to be 36 percent of yield, and postharvest losses are estimated to be 14 percent (U.S. Agency for International Development, 1990). The impacts of fungal, viral, and bacterial pathogens in developing countries have been studied less than those of insects, but the most comprehensive studies suggest that losses caused by pathogens are about equal to those caused by insects (Edwards et al., 1990). Weed growth is often so prolific and hard to control that it is thought to be the most important cause of yield depression (MacArthur, 1980; Sanchez and Benites, 1987). Improved varieties of the major food crops grown by the inhabitants of forests in the humid tropics are generally lacking (especially in Africa). Rice, cassava, sweet potatoes, and cocoyams are the principal foods of indigenous populations (Juo, 1989). Root crops, in particular, have received far less attention from plant breeders than have the more conventional cereal crops. At the same time, local varieties and landraces of staple crops, many of which are highly adapted to local climatic and topographic conditions, are disappearing. The loss of germplasm and species diversity is usually regarded as a consequence of development in the forests of the humid tropics. This loss can be seen as a serious constraint on long-term rural and agricultural well-being. The organisms within humid tropic agroecosystems provide vital services as pollinators, plant symbionts, seed dispersers, decomposers, pest predators, and disease control agents. These benefits can be diminished or lost as the diversity within agroecosystems decreases. 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. The local and global potential for using yet untapped plants and animals will remain unknown if their tropical habitats perish (Iltis, 1988). Opportunities for realizing local economic benefits through sustainable uses of biological resources could also be lost. The Path to Sustainable Agriculture Over the centuries, agricultural systems and techniques evolved to meet the special environmental conditions of the humid tropics. These include paddy rice systems; terrace, mound, raised-bed, and drained field systems; and a variety of agroforestry, shifting cultivation, home garden, and modified forest systems. Although these tra-

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS ditional systems are diverse in their particular adaptations, they share many traits: high retention of nutrients; maintenance of vegetative cover; a high level of diversity of crops and crop varieties; complex cropping patterns and time frames; and the integration of domestic and wild animals within the agroecosystem. Shifting cultivation (also known as swidden, slash-and-burn, or slash-and-mulch agriculture) remains in wide use throughout the humid tropics. It is practiced on about 30 percent of the world's arable soils and provides sustenance to more than 300 million people and additional millions of migrants from other regions (Andriesse and Schelhaas, 1987). As traditionally practiced, shifting cultivation protects the resource base through efficient recycling of nutrients, conservation of soil and water, diversification of crops, and the incorporation of long fallow periods in the cultivation cycle. Fallows accumulate nutrients in their biomass and control weeds. 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. Monocultural systems have been successfully introduced over large areas of the humid tropics. Some of the more fertile soils already support monocultural production of coffee, tea, bananas, citrus fruits, palm oil, rubber, sugarcane, and other commodities produced primarily for export. However, the social and economic characteristics of monocultural crop and plantation systems are of concern in many countries where they are important land uses. While they provide productive employment, they often outcompete and, thus, discourage investment in domestic food crop production. At the same time, they occupy most of the high-quality agricultural land, although this is less true in the Asian humid tropics. They often entail concentrated ownership of large areas of land (either in the private sector or by the government), creating social and political instability, especially in densely populated countries. Where these land ownership patterns are pervasive, small-scale farmers who wish to continue farming have no other option but to move toward primary forests and marginal lands (rice farmers are an important exception in that rice production is carried out largely on long-established small farms). Fluctuations in world market prices of the commodities these systems produce, as well as the fertilizers and pesticides on which they depend, make monocultural production more vulnerable to political and macroeconomic trends than small-scale farming. This is evident,

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS for example, in Cuba, where a high proportion of agriculture is devoted to sugar production. The environmental characteristics of monocultural systems also raise important questions about their sustainability. The production and processing methods they employ are significant sources of pollution in many areas (Vincent and Hadi, Part Two, this volume). A high degree of biodiversity loss is incurred in establishing and maintaining monocultures. 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. However, oil palm, rubber, sugarcane, and tea can be stable when grown in monocultures. Despite these problems, monocultural systems are an important part of the mosaic of land uses in the humid tropics. With modifications, including reduced use of pesticides, enhanced recycling of nutrients, and more equitable distribution of productive land, these systems may continue to serve as important sources of food and agricultural production. Some monocultural crops, such as coffee, cacao, and rubber, have been produced in diversified small-landholder systems, making them more desirable both socially and environmentally. In the future, the challenge will be to better manage both the highly productive lands that are already in intensive use and the less productive lands that are used by many small-scale farmers. In advancing toward sustainability, a nation's agricultural system will need to be diverse to take advantage of available markets, to use more effectively its available natural and cultural resources, and to balance social, economic, and environmental needs. The wide array of specific practices associated with sustainable agriculture includes the following: Low-impact land clearing techniques; Mulches, cover crops, and understory crops; Fertilizers and other soil amendments; No- and low-tillage planting techniques; Increased use of legumes as food crops, as cover crops, and in fallows; Improved fallow management techniques; Greater use of specially bred and alternative crops, grasses, shrubs, and trees (especially those tolerant of acidic, salinized, and high-aluminum soil conditions);

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Contour cropping and terracing; Biocontrol and other integrated pest management strategies; A variety of agroforestry systems that mix crops, trees, livestock, and other components; and Intercropping, double cropping, and other mixed cropping methods that allow for more efficient uses of on-farm resources. Sustainable practices to improve productivity and conserve soil, water, and biotic resources can provide farmers with alternatives to continued clearing of forests. Based on recent research in Peru, for example, it is estimated that for every 1 ha of land put into sustainable soil management technologies by farmers, 5 to 10 ha per year of forest could be spared (Sanchez, 1991; Sanchez et al., 1990). The potential of sustainable agricultural practices to reduce deforestation will depend on the location. For example, the sustainable use of secondary forest fallows provides a viable alternative to primary forest clearing. 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. Local needs and opportunities, ecological circumstances, economic opportunities, and social and cultural mores, as well as the status of land and water resources, will determine which methods are most suitable. Sustainable agricultural systems cannot, in this sense, be imported. Although specific technologies can be more freely introduced, they must be adopted to the inherent opportunities and limitations of local agroecosystems. The transition to more sustainable agricultural and land use systems is not without difficulty, particularly in the early stages. In many cases, substantial initial investments of time, labor, and money are required (for example, to construct terraces or to reforest steep slopes). In some cases, the transition requires significant changes in current farming practices and land uses (for example, restrictions on the burning of biomass). Against these short-term effects must be weighed the long-term benefits of these investments and changes. They include the following: Reduced pressure on primary forests and the mitigation of deforestation 's effects; Preservation of species and germplasm diversity within the agroecosystem; Reduction in the amounts of carbon dioxide and other greenhouse gases released into the atmosphere; Conservation of soil, nutrients, and water resources;

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS Increased productivity and a more stable food supply; Greater economic and social stability at local and national levels; Infrastructural developments that benefit small farms and local communities; Greater equity between farmers in resource-rich and resource-poor areas; and Increased training and employment opportunities for small-scale farmers, landless workers, and other people in rural areas. THE NEED FOR AN INTEGRATED APPROACH Improved land use in the humid tropics will require an approach that recognizes the characteristic cultural and biological diversity of these lands, respects their complex ecological processes, involves local people at all stages of the development process, and promotes cooperation among biologists, agricultural scientists, and social scientists. 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. Only recently, however, have they begun to transfer these insights to the study and management of tropical agricultural systems (Altieri, 1987; Gliessman, 1991a). Most public sector agricultural research and development programs in the humid tropics have focused for the past 3 decades on developing and transferring technologies to maximize the production of cereal grains and a limited number of root and pulse crops. These technologies have led to high productivity in areas with good soil and water resources, and they have contributed substantially to national food self-reliance in Asia. Many efforts in Latin America and Africa have been directed toward increasing export earnings. Livestock production technologies have been improved, but not as part of small-scale integrated farming systems. Only recently has the agricultural development community begun to expand its programs to incorporate additional social and environmental considerations, and to devote more attention to the needs of small-scale farmers in resource-poor areas (Consultative Group on International Agricultural Research, 1990; National Research Council, 1991a). Critics of the commodity-oriented approach hold that it has been limited by an inability to embrace all the factors and processes that influence the stability, productivity, and maintenance of tropical agroecosystems. In focusing scientific attention and development programs on particular crops and agroecosystem components, it has tended to neglect the range of physical and biotic interactions that

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS influences crop production, the ecosystem-wide impacts of intensive production practices, the role of the crop in achieving better balanced and more equitable systems of land use, and the long-term social and economic aspects of cropping systems that require purchased inputs (National Research Council, 1991a,b). This commodity-oriented approach has also been criticized for paying too little attention to small farms in resource-poor areas, the diverse crops and animals on which they depend, and the performance of traditional agricultural systems (Dahlberg, 1991). 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. As noted above, their durability, adaptability, diversity, and resilience often provide critical insights into the sustainable management of all tropical agroecosystems. While most of these systems have been greatly modified or abandoned due to economic and demographic pressures, some could, with modification, contribute significantly to the stability and productivity of agriculture in many humid tropic countries. By combining the expanding scientific knowledge of tropical forest ecosystems and the empirical experience of farmers and agricultural scientists, the conceptual foundations of sustainable land use can be strengthened. By applying this knowledge back to the land, many farmers can better provide for their own needs as well as those of society and the ecosystems in which they live (Gliessman, 1990). Agroecology—the application of ecological concepts and principles to the study, design, and management of sustainable agricultural systems—is one possible starting point in developing a more integrated approach. Agroecology tries to understand how physical conditions, soils, water, nutrients, pests, biodiversity, crops, livestock, and people act as interrelated components of agroecosystems, emphasizing the structure and function of the system as a whole. Agriculture is treated not as an independent sector or industry but as a critical element in achieving broader social and economic goals (Gliessman, 1991b). This emphasis allows particular production processes and resource management practices to be understood in their ecological as well as sociocultural contexts. It attempts to enable researchers, resource managers, development officials, and others to understand how multiple ecological, social, economic, and policy factors collectively de-

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS termine the performances of agricultural systems (Conway, 1985; Gliessman, 1985, 1991a; Gliessman and Grantham, 1990). The agroecological approach, if it is to become effective, will require interdisciplinary cooperation not only among tropical ecologists, biologists, foresters, and agricultural scientists but also among anthropologists, economists, political scientists, and other social scientists. Integrated investigations of this type can help ensure that the biophysical and agronomic components of the agroecosystem are to be considered alongside the historical, sociological, economic, political, and other cultural components (Edwards, 1987; Francis, 1986; Grove et al., 1990). However, the institutional structures and scientific environment for accomplishing this goal have yet to evolve. MOVING TOWARD SUSTAINABILITY Many obstacles impede progress toward sustainable land use in the humid tropics. To break the cycle of resource decline, people must be able to meet their needs in ways that are socially, economically, and environmentally viable on a long-term basis. Most of the fertile lands in the humid tropics are already being intensively used. Continued conversion of primary forests offers increasingly marginal gains. The only other alternatives are to enhance, through improved management, the stability and productivity of those lands currently devoted to agriculture, and to rehabilitate previously deforested lands that are now degraded or abandoned. Both strategies are needed. Together with continuing forest protection efforts, they can make land use as a whole more sustainable throughout the humid tropics. There are no easy methods for reversing resource degradation, and no one land use method alone will suffice. Rather, agricultural sustainability will involve a variety of land uses, each of which requires a different strategy and a different degree of management intensity. These diverse efforts, however, rest on several basic realizations: Over the next several decades all land resources in the humid tropics must be more effectively managed to reverse current trends. Success depends not only on making each land use more sustainable but also on coordinating an appropriate mixture of land uses and management strategies for each region. Land use systems must maintain flexibility and allow time for natural processes of ecosystem recovery and change. Building on these premises, a combination of improved land management techniques and innovative policy reforms can contribute to

OCR for page 21
Sustainable Agriculture and the Environment in the HUMID TROPICS a better quality of life for the people of the humid tropics, and to more effective conservation of the natural resources on which they depend. Although there are lands in the humid tropics that are, and will continue to be, devoted exclusively to production agriculture, sustainability necessarily involves a spectrum of land uses, including low-intensity shifting agriculture, mixed cropping and agroforestry systems, perennial tree plantations, and managed pastures and forests, as well as restoration areas, extractive reserves, and strict forest reserves. 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.