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Agriculture Ecosystems & Environment ELSEVIER

Agriculture, Ecosystems and Environment 52 ( 1995 ) 235-249

The ecological sustainability of slash-and-burn agriculture P.J.A. Kleinman a, D. Pimentel b'*, R.B. Bryant c aDepartment of Natural Resources, Cornell University, Ithaca, NY 14853, USA bDepartment of Entomology, Cornell University, Ithaca, NY 14853, USA CDepartment of Soil, Crop and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA Accepted l0 May 1994

Abstract Slash-and-burn agroecosystems are important to rural poor and indigenous peoples in the developing world. Ecologically sound slash-and-burn agriculture is sustainable because it does not depend upon outside inputs based on fossil energy for fertilizers, pesticides and irrigation. One means of demonstrating the soundness of slash-andburn agroecosystems is to prove empirically the ecological compatibility of this system of crop production. This paper examines the ecological sustainability of slash-and-burn agriculture based on the productivity of soil resources. Keywords."Agriculture, slash and burn; Agriculture, sustainable

I. Introduction In 1987, the United Nations' World Commission on Environment and Development (WCED) brought the concept of sustainability to the forefront of global development concerns, reasoning that if development is the goal of all nations, then development must be carried out in a sustainable fashion. However, despite widespread understanding that sustainability must be one of the primary criteria of development, current development policies frequently fail to integrate this factor into the decision-making process (Daly, 1991 ). Nowhere is this failure more apparent than in the inability of domestic and international development agencies to consider slash-and-burn agriculture as a sound food production system. * Corresponding author.

Slash-and-burn agriculture (sometimes referred to as shifting, or swidden agriculture) is sometimes described as non-sustainable (Cramb, 1989; Tobing, 1991 ), and is the primary cause of tropical deforestation (World Resources Institute (WRI), 1990). Certainly, slash-and-burn farming may be associated with poor crop yields and rapid soil degradation (El Moursi, 1984; Christanty, 1986). Not surprisingly, development programs generally promote continuous cropping systems in lieu of slash-and-burn. Furthermore, many developing nations have mounted institutional campaigns condemning all forms of slash-and-burn agriculture (Dove, 1984; Cramb, 1989; Shengji et al., 1990; Myers, 1992 ). In contrast, a growing community of scholars and development experts now argues that some slash-and-burn agroecosystems are sustainable under conditions of low land use pressure, and that there is a role for slash-and-burn agriculture

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in development (Grandstaff, 1980; Dove, 1983; Okigbo, 1984; Pimentel and Heichel, 1991 ). For example, Myers (1992) distinguished between the traditional shifting cultivator and the "shifted cultivator". Shifted cultivators are migrants who have only recently begun practicing slash-and-burn agriculture, and typically practice a form of slashand-burn that does not account for environmental constraints. Because the global population of shifted cultivators has increased dramatically over the last four decades, public perception of slash-and-bum agriculture centers on shifted cultivators rather than on traditional shifting cultivators (Myers, 1992). Traditional shifting cultivators usually practice a more complex form of slash-and-burn agriculture that is adapted to the environment in which it is practiced (Pimentel and Heichel, 199 l; Kidd and Pimentel, 1992). In addition, whereas traditional shifting cultivators often live in areas with low population density where land availability permits low intensity of land use, shifted cultivators typically live in areas of high population density and high land use pressure (Myers, 1980; Okigbo, 1984). In addition to distinguishing between traditional and shifted cultivators, the diversity of slash-and-burn agriculture can be found at many other levels (see Conklin, 1961 ). Slash-and-burn is currently now most prevalent in the tropics and sub-tropics, supporting at least 300 million people (Andriesse and Schelhaas, 1987), but was historically widespread in temperate regions (Okigbo, 1984; Hillel, 1992). Slash-and-burn agriculture is found in a range of ecosystems ranging from forests to grassland and savanna environments (Denevan, 1981). Culturally, slash-and-bum agriculture has been practiced by a diverse range of societies, from isolated tribal communities, such as the Dayaks of Borneo, to large civilizations, such as the early Mayans of Mesoamerica (Dove, 1985; Hillel, 1992). Two arguments justify the imperative to study the sustainability of slash-and-burn agriculture. Firstly, studies of sustainability may serve in the political defense of indigenous swidden societies by clarifying and substantiating the shifted-traditional distinction for governments whose de-

velopment policies neither acknowledge nor support the validity of indigenous slash-and-burn farming. Secondly, evaluating land use sustainability is fundamental to developing and implementing policies that promote sustainability. For example, efforts to develop extractive forest reserves and low-impact, multiple-use buffer zones bordering nature reserves, parks and sensitive ecological areas might include some forms of slash-and-burn farming as appropriate land use options (Kidd and Pimentel, 1992). In all, if any form of slash-and-burn agriculture is to be considered a viable land use option, evidence is required to demonstrate its sustainability. The authors examine the ecological sustainability of slash-and-burn agriculture based on quantitative assessments: ( 1 ) the relationship between slashand-bum and soil productivity; (2) the role of soil parameters in limiting crop productivity. 2. Incorporating sustainability in land evaluation The adoption of sustainability as a primary objective of agriculture and development clearly requires an ability to differentiate between sustainable and non-sustainable human activities. To date, empirical studies of sustainability are conspicuously absent for any land use system. This absence results in part from to an institutional inability to agree upon a functional definition of sustainability. General definitions of sustainability, either implicit or explicit, point to two requirements: ( 1 ) an ability to address near-term human needs; (2) an ability to account for long-term social, economic and ecological limits (Brown et al., 1987; World Commission on Environment and Development (WCED), 1987). This balance of needs with limits lends itself to cost-benefit analysis, and is most illuminating when there is a demonstrable link between both categories. In addition, these definitions emphasize that sustainability is both contextual and dynamic. Factors affecting human needs as well as social, economic and ecological limits vary widely and may change with time (Stewart et al., 1991; Food and Agricultural Organization (FAO), 1992).

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In applying sustainability to agriculture, a specific set of requirements can be defined. For instance, Altieri and Anderson (1986) defined sustainability in agriculture as 'the ability of an agroecosystem to maintain production through time, in face of long-term ecological constraints and socio-economic pressures'. In other words, sustainable agriculture must be economically viable, socially acceptable and ecologically sound (Conway, 1987). Thus, one defines sustainability in agriculture as the ability of an agroecosystem to maintain long-term production stability based on ecological, social and economic soundness. For the purposes of this paper the authors restrict consideration of sustainability to ecological factors only, or ecological sustainability. Undoubtedly, however, the social, economic and ecological dimensions of sustainability are interrelated. One means of evaluating the ecological sustainability of land use is through the use of indicators (Torquebiau, 1992 ). Indicators allow one to reduce the quantity of data required to represent complex relationships (US Environmental Protection Agency (USEPA), 1990). In testing for the ecological sustainability of agriculture, two categories of indicators need to be addressed---indicators of agroecosystem productivity and indicators of ecological limits. It is possible to screen for ecological sustainability by assessing the relationship between these sets of indicators to determine whether agroecosystem productivity can be maintained in the face of ecological impacts. Soil resources are a logical indicator of ecological limits because soil is widely viewed as the greatest limit to long-term productivity in lowinput agroecosystems such as slash-and-burn (FAO, 1984; Wolman, 1985; Pimentel et al., 1987; Kidd and Pimentel, 1992). Crop yield represents an easily quantifiable measurement of agroecological productivity that is central to the needs of subsistance farmers. Selecting soil resources and crop yield as indicators, an ecologically sustainable slash-and-burn agroecosystem would be one in which the edaphic impacts of farming do not significantly reduce long-term crop yields. Consequently, evaluating the eco-

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logical sustainability of slash-and-burn involves ( 1) assessing the relationship between slash-andburn and soil productivity, and (2) assessing the role of soil parameters in limiting crop productivity. 3. Relationship between slash-and-burn agriculture and soil conditions Although there is a relative scarcity in longterm empirical research that charts the relationship between slash-and-burn agriculture and soil parameters across the slash-and-burn cycle (Andriesse and Schelhaas, 1987; FAO, 1988 ), a large number of studies offer basic generalizations regarding this relationship during specific stages of the cycle. Reduced to its basic components, slashand-burn agriculture involves three stages: (1) conversion; (2) cropping; ( 3 ) fallow. In conversion, clearing and burning of native vegetation serve the combined function of removing shading canopies, reducing pest competition and removing other physical impediments to cropping. Clearing and burning also cause some of the nutrients stored in plant biomass to be released, making them available for crop use (Zinke et al., 1978). Cropping practices vary widely, as do their ecological impacts. Traditional agroecosysterns often include soil conservation practices, presumably lending to their sustainability, but they are relatively absent from shifted agroecosystems (Myers, 1992 ). Fallows are employed in response to lowered crop yields and increased pest pressure. In functional terms, fallows serve to halt soil degradation and to restore fertility and other soil conditions that were degraded by conversion and cropping (Kalpagr, 1974; Pimentel and Heichel, 1991 ). 3. I. C o n v e r s i o n

Swidden conversion is initiated with "clearing", a misleading term that implies removal of vegetation although plant debris ("slash") is typically left in situ. A more precise label for this stage is "slashing and felling", because native vegetation is typically slashed or felled if it is too big to be slashed (Padoch, 1985 ). The immediate impacts of slashing and felling initiate soil

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changes that continue through the slash-and-burn cycle. Perhaps the most important and universal impact of slashing and felling in regard to sustainability is the disruption of natural nutrient cycling and the acceleration of nutrient flow out of the agroecosystem. This impact is particularly pronounced in tropical forested systems where natural nutrient cycles are virtually closed and trees are often credited with "mining" insoluble nutrients such as phosphorus from the subsoil (Andriesse and Schelhaas, 1987; Young, 1989). Furthermore, the decay of fine roots immediately following tree death results in the release of associated nutrients which may leach from the forest soil (Fahey et al., 1988). Commonly, the presence of slash serves to protect the soil from erosion; however, bare patches of the soil surface may be directly exposed to incoming precipitation and radiation. The loss of a protective plant canopy may cause a sharp increase in surface soil temperatures (Christanty, 1986; Lal, 1987), thereby stimulating volatilization (i.e. loss of nitrogen to the atmosphere) and increasing rates of organic matter decay (Van Wambeke, 1992 ). Concentrated human activity during slashing and felling, and the physical impact of falling trees, may result in localized increases in bulk density (Lal, 1987 ). Finally, slashing and felling can disrupt populations of soil micro- and macro-fauna and favor the germination of seeds from photophyllic opportunists (Ewel et al., 1981; Uhl, 1987). After clearing, slash is usually allowed to dry before burning. Burning plays an extremely important role in soil fertility. Nutrients released from biomass and even parent material during burning may become available for crop uptake, escape via volatilization, leaching or surface runoff, or remain bound in recalcitrant ash complexes (Kalpagd, 1974; Ramakrishnan, 1992). These outcomes are highly dependent upon the intensity of the burn, which may be the most important factor influencing swidden soil fertility (Andriessc, 1987). Kalpagd (1974), Driessen et al. (1976), Seubert et al. (1977), Andriesse (1980, 1987a), Sanchez et al. (1983), Jordan (1989) and Nakano and Syahbuddin (1989) confirmed the im-

portance of ash in contributing phosphorus, potassium, calcium and magnesium to swidden soils. F5lster (1986) estimated that nutrient contributions from above-ground biomass account for approximately 50-80% of total system fertility in tropical forests, although nitrogen and sulfur are contributed primarily from other sources. This percentage decreases in savanna and grassland agroecosystems (Nye and Greenland, 1960). Burning is also credited with increasing pH and decreasing aluminum saturation, a significant agronomic benefit on acid soils (Ahn, 1974; Ewel et al., 1981; Sanchez et al., 1983; Christanty, 1986). Not all nutrient release is immediate because many nutrients remain bound in ash and soil organic matter and are released after the decomposition of these complexes. In fact, decay of ash and other organic complexes may take years. As a result, available nutrient levels may not peak until 2 or 3 years after conversion (Driessen et al., 1976; Jordan, 1989). Despite these near-term benefits, burning may promote long-term soil degradation. For example, the higher the burning temperatures the more nitrogen is volatilized and lost to the atmosphere (Andriesse, 1987). Sulfur and carbon are also volatilized during burning (Christanty, 1986). Similarly, at higher burning temperatures organic material in the top few centimeters of the soil is also lost (Andriesse and Koopmans, 1984). In soils of inherently poor fertility, the loss of soil organic matter translates to significantly reduced cation exchange capacity (CEC, Driessen et al., 1976). Burning also removes residual soil cover, exposing the soil to rain and wind erosion as well as to direct solar radiation. This exposure may result in surface sealing and crusting, especially in Alfisols and Ultisols, as well as volatilization of nutrients, and ultimately, water and wind erosion (Zinke et al., 1978; Van Wambeke, 1992). Physical changes are also initiated, including the desiccation of the soil via evaporation (Uhl et al., 1981 ), the alteration of soil texture (Ahn, 1974), and the deterioration of soil structure (Christanty, 1986). Finally, burning influences the nature of ecological succession during fallow by damaging or destroying seeds

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and self-propagating root systems and creating fertility conditions favorable to the regeneration of invasive species such as Imperata cylindrica and Chasmopodium caudatum (Ewel et al., 1981; Nyegers, 1989; Seibert, 1990). Several practices offer means of reducing the edaphic impacts of clearing. Although clearing per se does not usually cause severe soil degradation, by avoiding the use of heavy machinery and by selectively cutting trees so that some trees are either coppiced or simply left untouched, impacts on structural characteristics and bulk density are minimized (Lal, 1985; Nyegers, 1989). To minimize the loss of nitrogen and sulfur during burning, temperatures can be reduced. However, lower temperature burns also result in lowered transfer of biomass nutrients to the soil because of limited breakdown of organic matter and ash complexes (Andriesse and Koopmans, 1984).

3.2. Cropping Cropping presents another important set of soil impacts. Cropping practices vary widely between agroecosystems and in relative benefit. The rate of growth and the extent of canopy cover regulate the degree to which the soil surface is exposed to incoming precipitation, wind and sunlight, and hence runoff and erosion (Renard et al., 1991 ). In addition, the combination and diversity of crops grown in a swidden influence soil fertility and susceptibility to erosion. Tillage (e.g. with hoes) may improve near-term agronomic conditions (Lal, 1987), but may also seriously disturb the soil surface, breaking up aggregates and increasing the surface area exposed to rain and wind erosion (Pimentel et al., 1987 ). In fact, given these factors, sediment loss may increase dramatically during the cropping period (Lal, 1987). Soil organic matter generally declines during the cropping period because of increased decay and limited replacement. Driessen et al. (1976) reported a negative correlation between the length of continuous cropping and organic matter content. Such a decline in soil organic matter levels may lead to decreased CEC, increased

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acidity, lowered nutrient levels (especially nitrogen and phosphorus), degradation of soil structure, reduced porosity and aeration, and lowered water infiltration capacity (Ahn, 1974). Combined with the removal of nutrients from the agroecosystem by crop harvest, low-fertility soils, such as those supporting many slash-and-burn agroecosystems, are especially prone to dramatic fertility declines. For example, Uhl and Murphy (1981 ) observed soil nutrient changes in Oxisols and Spodosols under a Venezuelan slash-andburn agroecosystem and found that 3-10% of all nutrients in the agroecosystem were removed by crop harvest. As a result, an important management variable influencing the degree to which soils are degraded by slash-and-burn is the period for which crops are continuously cultivated. A number of soil conservation practices may be incorporated to reduce the degradative effects of cropping and even promote fertility. These include controlling tillage practices, devising protective barriers, mulching, and manipulating cropping characteristics. Particularly on steep slopes, soil tillage can serve to loosen surface soil and disrupt aggregates, making them prone to erosion. However, a lack of tillage may result in increased bulk density (Lal, 1987). Most slashand-burn agroecosystems involve low- or no-till, such that tillage is not a significant factor in swidden soil degradation. In some agroecosysterns tillage in the form of mounding and ridging of soil may be common, especially for the cultivation of root and tuber crops, and may improve crop yields (Marten and Vityakon, 1986). In traditional no-till agroecosystems, the introduction of hoe cultivation has been linked with rapid soil degradation and loss of sustainability (Nyegers, 1989 ). Therefore, the extent and nature of the tillage are factors that need to be addressed on a contextual basis. Protective barriers may also be included in cropping systems with varying degrees of effectiveness and labor requirement. Laying trees along the contour requires limited labor input, but is also less effective than other forms of bartiers (Sabhasri, 1978). Vegetative barriers may be easy to plant, and effective at controlling erosion, but require a level of management input not

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characteristic of many slash-and-burn agroecosystems. In those systems where land use pressure is extremely high, such as in northeastern India, construction of terraces provides an effective, albeit labor intensive, form of soil conservation (Ramakrishnan, 1992). In continuously cropped slash-and-burn agroecosystems, mulching may play an important role in limiting soil degradation. Mulches protect the soil surface from incoming radiation, precipitation and wind, thereby reducing susceptibility to erosion and maintaining relatively constant soil surface temperatures. In addition, mulches may augment soil organic matter levels and increase the infiltration rate (Wilson and Akapa, 1983; Peters and Neuenschwander, 1988). The selection of crops and combinations thereof contributes significantly to soil conservation. Monocultures of field crops may be prone to erosion (Pimentel et al., 1987 ). Conceptually, cropping patterns that mimic ecological complexity are most effective at soil conservation and at promoting sustainability (Altieri and Anderson, 1986). As with mulches, cover crops provide a canopy that protects the soil surface, and, if incorporated into the soil, serve as a green manure (Christanty, 1986). Leguminous green manures are especially effective at replenishing soil nitrogen pools (Gupta and O'Toole, 1986; Nair, 1990). Intercropping tree crops with field crops (agroforestry) also provides soil conservation benefits, and leguminous tree leaves can even be used as green mulches and manures (Kidd and Pimentel, 1992 ). 3.3. Fallow

Although the primary role of the fallow in traditional agroecosystems is increasingly viewed as a weed management strategy, its role in ameliorating soil conditions for future cropping periods by increasing soil organic matter levels is equally important (Van Wambeke, 1992). First and foremost, fallow periods mark the end of the cropping period and the beginning of natural or managed plant succession. The success of fallows in improving soil conditions is modified by the ex-

tent of past soil degradation, characteristics of the successional community, and the length of the fallow. Sabhasri ( 1978 ) concluded that, in the case of forest fallow, trees are "soil builders". Zinke et al. ( 1978 ) reported that 'too short a [ fallow ] period could eventually result in fertility depletion to the point where it would be necessary to abandon land'. They found that although calcium, phosphorus and potassium are returned primarily through residual ash, nitrogen and organic matter are added by forest fallow. Nye and Greenland (1960), Reynders ( 1961 ), Ahn (1974) and Ramakrishnan (1992) also attributed increases in soil fertility to the fallow period. Young (1989) listed additional benefits of bush or forest fallow including reduced wind and water erosion, lowered soil temperatures, closed nutrient cycling, nutrient mining from the subsoil, benefits to soil fauna, reduced acidity, and improved soil structure, texture and moisture characteristics. Managing the fallow is extremely important to maintain agronomically favorable soil conditions in slash-and-bum agroecosystems. This includes controlling the ratio of cropping period to fallow period, and managing the composition of the successional community. In areas of high land use pressure, longer periods of faUow may not be possible, and system sustainability often breaks down. Agroforestry, such as intercropping woody species with field crops or enrichment planting of fallows, is commonly practiced in traditional agroecosystems and contributes to the sustainability of land use intensification by offering an effective means of enhancing soil conditions and reducing the degradative effects of cropping (Christanty, 1986; Kidd and Pimentel, 1992). 3.4. Cyclical changes

Many studies detail trends in soil conditions for varying lengths across the swidden cycle. Although not all studies concur, a number of them point to the ability of an agroecosystem to recover pre-conversion soil conditions, providing insight into what factors are most influential on sustainability. For the sake of generalization,

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edaphic changes are here grouped into two related categories: soil fertility and soil erosion. Figs. 1 and 2 illustrate idealized soil fertility curves for a sustainable slash-and-burn agroecosystem. As noted previously, soil fertility levels generally increase after conversion as a result of burning and may continue to do so for several years. The extent and longevity of the fertility increase is highly variable and is dependent upon many factors. An eventual decline in soil fertility results from continued nutrient removal by crop and fallow species as well as by the transformation of nutrients from plant available to non-extractable forms and the loss of nutrients from the swidden by leaching, runoff and volatilization (Andriesse, 1980; Frlster, 1986 ). Depending on the magnitude of this decline, soil fertility may drop well below pre-conversion levels (Fig. l ) or level off at or above pre-conversion levels (Fig. 2). In the case of Fig. 1, the gradual recovery of soil fertility results from the build-up of soil organic matter during the fallow (Nye and Greenland, 1960 ). Andriesse (1980) observed soil nutrient levels at varying stages across a 20 year slash-and-burn cycle in Sarawak, Malaysia, and compared his findings with those of several other studies carried out in Guatemala (Popenoe, 1960), New Guinea (Reynders, 1961), Thailand (Zinke et al., 1978) and Indonesia (Driessen et al., 1976 ). Although not all of the studies followed soil nutrient levels across the slash-andburn cycle, many of the trends in soil nutrients

mimic those projected in Figs. 1 and 2, demonstrating some consistency in edaphic responses as well the gradual recovery of original soil nutrient levels for some systems. Discrepancies between studies were attributed to variations in sampling design and laboratory analysis of soil samples. Other factors that might contribute to these discrepancies are discussed in the following section of this paper. Jordan (1989) reported on the results of an intensive agroecosystem-level study of slash-andburn in Venezuela in which fluctuations in soil nutrient levels, as well as total agroecosystem nutrient levels, were recorded. With the exception of phosphorus, soil nutrient levels follow the trend presented in Fig. 2. After 2 years of fertility increases, Jordan observed a sharp decline in soil nutrients followed by a general leveling off at levels slightly above those found under forested conditions. Phosphorus levels paralleled those of other nutrients during the first few years after clearing, but eventually fell below pre-conversion levels. Because of the short length of the observation period, no fertility changes were recorded beyond the fourth year of fallow (7 years after clearing). Soil erosion may cause irreversible changes to soil conditions affecting nutrient levels as well as important physical parameters such as texture, structure, bulk density, and rooting depth (Lal, 1974). In many cases, soil erosion is lower under slash-and-burn agroecosystems than under in-

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Fig. !. Projected soilfertilitycurve for a sustainablesystem.

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~__~? ~ N o N . ~

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time(years) Fig. 2. Alternative soil fertility curve for a sustainable system. tensive, permanent cropping systems. Watters ( 1971 ) and Lal ( 1981 ) reported negligible erosion from traditional slash-and-burn agroecosysterns in Venezuela and Nigeria, respectively. However, swidden erosion generally increases as the duration of the cropping period is expanded (Shulda and Agrawal, 1984). In addition, erosion increases greatly with slope gradient and slope length (Renard et al., 1991 ). Notably, because of the size of many slash-and-burn rotation systems, erosion in areas within or outside of the agroecosystem may result in sediment deposition within the agroecosystem. For example, the Dayaks of Borneo sometimes plant swamp rice at the base of steep swiddens (Dove, 1985 ), thereby taking advantage of the deposition of eroded sediments and associated nutrients. Thus, as an agroecosystem, slash-and-burn may be able to withstand greater degrees of localized erosion than can permanent cropping systems, and still remain productive. Studies examining the cumulative losses of sediment from swiddens often point to a marked difference in erosion rates between the fallow and cropping periods, supporting the view that erosion increases with prolonged cropping periods and shortened fallows (Lal, 1987). Ramakrishnan (1992) described a strong positive relationship between the control of swidden erosion and the length of the fallow period in traditional jhum systems of northeastern India. Takashi et al.

( 1983 ) reported an increase in the magnitude of sediment loss during the cropping period from 2.4 to 18.6 times that observed under forested fallow. Significant, but less dramatic increases in erosion from fallow to cropping period were reported by Kellman ( 1969 ) and Jordan (1989). Lal ( 1981 ) calculated erodibility under continuous cropping after forest clearing for a series of experimental plots on an Alfisol in Nigeria. Erodibility increased for the first few years of cropping following swidden conversion. This trend is attributed to the loss of the natural canopy and roots that control rainfall erosion. Subsequently, erodibility decreased as a result of the preferential removal of smaller particle sizes during the first years of erosion, and the predominance of coarser-textured particles thereafter.

3.5. Dependent and independent soil variables The impacts of slash-and-burn farming on soil resources are modified by a number of dependent and independent variables. These include inherent soil characteristics, land use history, topography, and successional community characteristics. These variables may need to be controlled, or at least addressed, when assessing the edaphic impacts of slash-and-burn farming. Inherent soil characteristics define the magnitude and quality of degradation that may occur as well as the potential productivity of low-input

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agroecosystems. Andriesse (1980) claimed 'that the natural variability within one specific soil series may be greater than differences in nutrient level caused by the bush/fallow stage'. This refers to the difficulties in making meaningful comparisons between swiddens on different soils. Intrinsic variability is far greater at the taxonomic level of soil order. For example, Oxisols typically provide good physical characteristics for cropping, but are low in CEC and in saturation of basic cations because of the predominance of one-to-one clays in the fine earth fraction and a long weathering history. Oxisols are therefore most sensitive to chemical degradation. Ultisols, another group of soils commonly found under slash-and-burn, are also poor in natural fertility and are characterized by poor physical conditions including the presence of an argillic horizon that may impede root growth and promote rapid water saturation of the topsoil. For this reason, Ultisols are particularly susceptible to soil erosion and physical degradation as well as to chemical degradation (Van Wambeke, 1992). If studies are to provide insight into agroecosystem level impacts, variability in inherent soil characteristics must be taken into account. Land use history plays a significant role in determining current soil conditions, and thus influences the productive potential and susceptibility to degradation of a soil. Andriesse (1980) attributed discrepancies in calcium, magnesium and potassium levels between swiddens to differences in the number of prior cropping cycles a swidden had undergone. A knowledge of the length of past slash-and-burn cycles and variations in land use practice is especially important to understanding the role of current farming practices in influencing current soil conditions. Because prolonged cropping periods exacerbate soil degradation, and, conversely, longer fallow periods theoretically improve soil conditions relative to shorter fallow periods, the ratio of cropping period to fallow describes the relative ability of an agroecosystem to maintain soil conditions over the long term. Time ratios of cropping period to fallow period have been described by numerous workers for individual slash-and-burn agroecosystems and for individ-

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ual soil types. Zinke et al. ( 1978 ) described a ratio of 1:9 ( 1 year of cropping to 9 years of fallow) as sufficient to maintain soil fertility on moist tropical forest soils in northern Thailand for a traditional slash-and-burn agroecosystem. Sabhasri (1978), also investigating the same agroecosystem in northern Thailand, estimated that a minimal time ratio of 1:6 is required to maintain original soil fertility levels. Greenland and Nye (1959) suggested a ratio of 1:3 to maintain organic matter levels in Alfisols for a traditional slash-and-burn agroecosystem in west Africa. Van Wambeke (1992) listed ratios from 5:12 to 6:15 as most common for long-term organic matter maintenance in Oxisols of high basic cation status. Laudelout (1962) suggested a minimum range of ratios from 3:10 to 4:15 for organic matter maintenance on forested soils of central Africa. Lal (1985) referred to a study conducted by Watters in Venezuela in which a ratio of 3:15 was required to maintain soil fertility. Topographic characteristics of a swidden significantly influence runoff and sediment loss and therefore may confound comparisons of soil degradation between swiddens. Andriesse (1980) stated that the 'slope situation may cause a larger variability than stage in forest regeneration'. Potentially significant factors include slope angle, length and shape of swiddens (Renard et al., 1991 ). Slope shape can be accounted for by partitioning slopes into segments of average slope angle, thus accounting for erosional and depositional surfaces. Catena weathering models that distinguish between erosional and depositional surfaces provide analogous observations, albeit at a grander scale than that of an individual swidden or slash-and-burn agroecosystem (Hall, 1983; Van Wambeke, 1992 ). Given the potential role of fallow vegetation in replenishing swidden nutrients and protecting the soil from surface runoff and erosion, characterization of fallow vegetation can provide insight into long-term changes in soil conditions. Total biomass, rate of biomass production and species composition are all important factors. The first two factors point to the magnitude of above-ground nutrient storage, the relative

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quantity of humus that is generated during fallow, the rate which nutrients accumulate in plant biomass and the rate which a protective canopy will form. The last factor, species composition, reflects the fact that fallow species vary in the quantity and quality of nutrients they accumulate (Uhl, 1987). For example, leguminous species concentrate more nitrogen relative to other species, and deep-rooted trees are especially effective at accumulating phosphorus (Young, 1989). Thus, the successional community influences the effectiveness of the fallow in improving degraded soil conditions for future cropping periods.

4. Edaphic limits to crop yields Impacts on soil resources affect the ecological sustainability of a slash-and-burn agroecosystem as far as they result in reduced crop productivity. Consequently, understanding the relationship between soil conditions and crop yields is critical to evaluating ecological sustainability. For example, Ramakrishnan (1992) described a positive correlation between crop yields and fallow period length because of the role of the fallow in controlling soil erosion. Not only does the relationship between food productivity and soil conditions provide a means of assessing the cost to humans of degrading the soil resource, but it also provides an understanding of the natural production potential of a soil. Knowledge of the natural production potential allows one to determine if adequate long-term yields are simply impossible to achieve under low-input conditions, and if internal characteristics of the agroecosystem may be modified to sustain and/or improve crop yields. Unlike the relationship between slash-and-burn and soil conditions, the relationship between soil conditions and crop yields is better understood and is therefore treated in less detail. To date, most quantitative studies that link productivity with soil conditions have been carried out on experimental plots in temperate regions (Wolman, 1985 ). Parametric models, such as those reviewed by Riquier (1974) and those

developed by the International Benchmark Sites Network for Agrotechnology Transfer (IBSNAT, Caldwell, 1990), can roughly predict crop yields under various cropping conditions using easily acquired soil data. These models, as well as direct experimental and observational studies conducted in the tropics, indicate the role of individual soil parameters in influencing crop yields. Lal (1988) described how ~most parameters that affect crop yields and are affected by soil erosion are interrelated. A change in one variable induces changes in others.' In his review of the association between soil characteristics and crop yields, he included sediment loss, waterholding capacity (which is influenced by soil structure, texture, bulk density, pH and electrical conductivity), soil organic matter and effective rooting depth as factors limiting productivity. A study by Lal ( 1976, 1981 ) on an Alfisol in Nigeria illustrated the negative relationship between yields of several different crops and soil erosion. In addition, Lal described a strong negative relationship between crop yields and loss of soil organic matter, porosity and infiltration capacity. Seubert et al. ( 1977 ) tied reduced yields of various crops to increases in bulk density. Studies in temperate zones have effectively used soil-water balance to predict crop yields under high levels of management (e.g. Timlin, 1986 ). Driessen et al. ( 1976 ) observed swidden yield changes for continuously cropped rice over a 7 year period on an Ultisol in Sarawak, Malaysia. They correlated these changes with changes in soil nutrient levels, especially phosphorus, and soil erosion. Fig. 3 illustrates the observed crop yield changes. The similarity should be noted between observed yields and projected soil fertility changes presented for the first few years after conversion in Figs. 1 and 2. Similar yield curves have been recorded by Allan (1965), Siband (1972), Lal (1974) and Jordan (1989). If any conclusion is to be drawn from the soil productivity studies reviewed here, it should be that a range of soil parameters need to be considered to understand the relationship between crop yields and soil resources, and that the nature of this relationship is highly contextual.

P.ZA. Kleinman et aL / Agriculture, Ecosystems and Environment 52 (1995) 235-249

245

~2

J

2

J

4 6 years of continuous cropping

Fig. 3. Changes in yields of continuously cropped rice.

5. Conclusion

True evaluation of agricultural sustainability must consider market factors, land tenure, appropriate technologies, and other socio-economic variables in addition to ecological concerns. These variables are inextricably linked to the ecological sustainability of an agroecosystem in that they shape farmer behavior and drive decisions. Even so, the ability to evaluate key aspects of sustainability is central to recognizing and promoting sustainable agriculture. Using indicators it is possible to define an ecologically sustainable slash-and-burn agroecosystem as one in which the edaphic impacts of farming do not reduce long-term crop yields. Ecological sustainability can be evaluated by projecting agroecosystem-level impacts on soil conditions and productivity and estimating the effect of these impacts on crop yields. This approach evaluates sustainability under current conditions. At present, several methods exist to assess the ecological sustainability of slash-and-burn agriculture. These methods include on-farm studies, controlled experimental studies, and computer modeling. Because of the tremendous diversity of slash-and-bum agriculture, there is a critical need for empirical on-farm studies, particularly in regard to the edaphic impacts of slash-andburn. New methods for assessing long-term impacts on soil conditions, such as the use of ce-

sium- 137 measurements to estimate soil erosion, show great promise (Quine and Walling, 1991 ). Experimental studies are also valuable to assess the range of variables that influence soil conditions and crop yields. Provided with a better understanding of these processes and availability of data, computer modeling also holds great promise. For example, geographic information systems are now used to map and predict soil erosion (Murty and Venkatachalam, 1992), and computer-driven yield-predicting parametric models are improving dramatically in their ability to assess complex agroecosystems (Caldwell, 1990). The use of computer models may also provide a means of identifying opportunities to promote ecological sustainability through technology transfer. The arguments against the viability of slashand-burn agriculture as an ecologically sustainable form of crop production have pointed to the degradative effects on soil productivity. These arguments, however, do not take into account the great diversity in slash-and-burn systems. With proper management, swidden soil degradation is minimal. Even if there is a small amount of soil erosion, sediment and nutrients will not necessarily be lost from the agroecosystem because swiddens are relatively small compared with the total area in the fallow rotation system. As a result, soil nutrients, water and other factors limiting crop yields may be captured within the managed agroecosystem. Therefore, sound slash-

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and-burn agriculture is one of the few truly ecologically sustainable agroecosystems in the world because crop yields can be maintained without inputs of non-renewable fossil energy resources for fertilizers, pesticides and irrigation (Kidd and Pimentel, 1992 ). Like all agricultural systems, slash-and-burn can be mismanaged and result in serious environmental degradation. However, when properly designed and managed, it serves as a sustainable food production system, and also provides benefits in the form of fuel, building materials, and income. These advantages should receive greater attention. Because slash-and-burn agroecosystems are highly complex, their ecological sustainability depends on several interrelated factors. Numerous direct and indirect soil variables have confounded past studies of slashand-burn, but are now better understood. Addressing these variables, this paper presents a basic strategy for evaluating ecological sustainability through measurements of the impacts of slashand-burn agriculture on soil resources. Although such an approach only accounts for one subset of ecological constraints, it does offer a realistic framework to distinguish between sustainable and non-sustainable slash-and-bum agroecosystems.

Acknowledgments The authors thank S. Bukkens, T.J. Fahey, D.C. Lawrence, M. Leighton, R.J. McNeil, K.M. Perkins, A. Power, T. Scott, H.D. Thurston, N. Uphoff and A. van Wambeke for their critical comments on this manuscript.

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