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Shifting Cultivation Amazon Case Study

"Modernity â€“ the belief that what is newest is best â€“ blinds us from recognizing the ecological knowledge and wisdom that suffuse traditional practices like swidden agriculture which encompass thousands of years of painstakingly accumulated knowledge and insights.  With greater humility, we have much to learn." –David Suzuki. 

"This book represents a multifaceted analysis of the transformation of shifting cultivation. The diverse views of a large team of experts are superbly knit together by the experienced editor to present an authoritative vision of what the future holds for not only shifting cultivation in the Asia-Pacific region, but subsistence farming the world over. The comprehensive book is very timely now when traditional farming systems are drastically impacted by environmental upheavals and economic realities – which could be an opportunity for reinvention rather than a threat of disruption." – P. K. Ramachandran Nair, Distinguished Professor, University of Florida, Gainesville, FL, USA. 

"Those writing about shifting cultivation at the beginning of the modern era must have imagined it as a way of life unlikely to survive a few more decades. What has surprised many is less that it still exists, but that it is so resilient. This attractive and comprehensive book captures the diversity of adaptations, and celebrates the lives of the people involved. Malcolm Cairns is to be congratulated on seeing an awesome publishing project to a magnificent conclusion." – Roy Ellen, Centre for Biocultural Diversity, University of Kent, UK. 

"The appearance of these collectively definitive volumes on swidden cultivation represents an intellectual event of great importance. Finally, a comprehensive account of the form of agriculture most widely practiced in world history; most responsible for changing landscapes, and most grievously misunderstood by high-modernist agriculture. So much to learn here, so much to digest, so much to ponder as we imagine a less catastrophic agricultural future." – James C. Scott, Sterling Professor of Political Science and Anthropology and Co-Director Program in Agrarian Studies, Yale University, USA.

"Once again Cairns has produced a magnus opus at least as valuable as the original: while the immediate impact may be less, the quality is surely even higher. And what makes it all the more remarkable is that the editor – with his constant team - has managed to bring this all together so cohesively from a position of considerable personal indisposition. Read the preface to find out what I mean: a compelling tale of adversity overcome by determination and perseverance." – Dr William Critchley, Pitlochry

"The multiple authors successfully argue in the first two sections that it is the preferred agricultural management system in many environments with respect to resilience to climate change and preservation of biodiversity. Summing Up: Recommended. All readers." – CHOICE, M. S. Coyne, University of Kentucky

"As the contributions to this volume demonstrate, understanding of the environmental and economic benefits of this system in comparison to modern agricultural systems has grown, but there is much still to be demonstrated to improve policy and support the farmer innovations that will enable shifting cultivators to adapt and survive into the future. This volume lays a broad and strong foundation for these efforts." - Danna J. Leaman, Canadian Museum of Nature, Ottawa, Canada

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Tropical Forests

Nutrient losses from undisturbed tropical forests are low (Jordan, 1985). Tropical forests produce a large root biomass that is concentrated near the soil surface. This permits efficient absorption from the soil volume in which the nutrients are concentrated after they are released by decomposing organic matter. It also provides a large surface area on which nutrients can be strongly adsorbed. Intermixing of surface roots with litter and litter-decomposing organisms near the soil surface facilitates nutrient recycling and prevents nutrient losses by leaching from the soil. In addition, mycorrhizal fungi, which are common in tropical forests, attach themselves to decomposing litter and wood, providing a direct pathway for nutrient transport to root systems.

Several other characteristics of tropical forests appear to be involved in nutrient conservation. These include (1) long-lived, tough, and resistant leaves (which prevent breaking of cuticular seals, hence decreasing leaching of minerals), (2) resorption of nutrients from leaves to twigs before the leaves are shed, (3) N fixation and scavenging of nutrients from rainwater by lichens and algae on leaves, (4) production of defensive chemicals against pathogens and herbivores, (5) thick bark that protects trees from invasion by bacteria, fungi, and insects, and (6) storage of a large proportion of the minerals in the biomass, from which they cannot be readily leached, rather than in the soil (Jordan, 1985).

The nutrient balances of tropical forests are much more fragile than those of temperate forests. Nutrient losses from tropical forests range from negligible amounts as a result of minor disturbances such as tree falls to almost total loss of nutrient capital in areas denuded by landslides. Although the effects of disturbances caused by humans are intermediate between these extremes, they often result in serious depletion of mineral nutrients from tropical ecosystems.

Formation of gaps in forests may be expected to increase leaching from the litter layer to the mineral soil because of the increased rates of litter decomposition associated with high temperatures. However, this effect may not be important when the gaps are small, possibly because of nutrient uptake by sprouts, saplings, microbes, and/or new seedlings (Uhl et al., 1988).

On small areas throughout the tropics, forest vegetation is felled, burned, and food crops planted in a system of shifting agriculture (also called “slash and burn” agriculture). The nutrient-rich ash increases the amounts of nutrients available for the first crop but N and S are lost by volatilization. Some of the ash may be blown from the soil surface or leached through the soil by rain. Nevertheless, because availability of soil nutrients is increased by burning, the yield of the first crop usually is high, but declines progressively during subsequent years. The number of crops that can be successfully grown after a tropical forest is cleared will vary with the specific crop, site, and management practices (Kleinman et al., 1995). Eventually fertilizer applications are needed as nutrients in the soil and vegetation become limited. If fertilizers are not added, the unproductive plot usually is abandoned and a new part of the forest is cleared and planted to crops. However, the nutrient capital of a plot can be maintained and soil properties improved by adding fertilizers. Changes in soil chemical properties after eight years of continuous cultivation of crops following clearing of a tropical rain forest were improved by additions of fertilizers and liming (Sanchez et al., 1983).

It is important to separate the effect of shifting cultivation on nutrients in the entire ecosystem from those in the soil compartment. Shifting cultivation results in loss of nutrients from the ecosystem. Nevertheless, nutrients in the soil may show only small changes for some time because nutrients that leach out of the soil are compensated by nutrients that leach into the soil from ash and decomposition of litter. For example, in a slash-and-burn site in an Amazonian rain forest in Venezuela, there was a net loss of nutrients from the ecosystem. Nevertheless, total amounts of nutrients in the soil increased after the burn (Jordan, 1985). Other studies showed that despite progressively decreasing crop yields under shifting cultivation, the nutrient capital in the soil of cultivated fields was as high as or higher than in the soil under undisturbed forest (Nye and Greenland, 1960; Brinkmann and Nascimento, 1973).

The progressive decline in crop yield under slash-and-burn agriculture has been attributed to a decrease in availability of nutrients to plants rather than to low total amounts of soil nutrients. This is emphasized by high productivity of successional species that absorb large amounts of nutrients that are much less available to crop plants. Low availability of N is important in decreasing crop yield on certain sites. Slow mineralization of N may cause N deficiency in plants despite high N levels in the soil.

Nutrients often are depleted early after trees are harvested and at various times thereafter. Ewel et al. (1981) quantified losses of mineral nutrients after cutting and burning of a Costa Rican wet forest. Harvesting of wood removed less than 10% of the total ecosystem nutrients to a soil depth of 3 cm. During drying and mulching (before burning), 33% of the K and 13% of the P were lost. Burning volatilized 31% of the initial amount of C, 22% of the N, and 49% of the S. Only small amounts of C and S were lost after the burn, probably because they had been volatilized during burning. Following the burn and with the onset of rain, losses of nonvolatile elements were high, amounting to 51% of the P, 33% of the K, 45% of the Ca, and 40% of the Mg. Losses of mobile elements including C, N, S, and K in harvested wood and by decomposition of organic matter, burning, and post-burn erosion are shown in Figure 10.8.

Jordan (1985) presented a useful model that summarizes the nutrient dynamics and productivity of tropical ecosystems during disturbances (Fig. 10.9). Mineral nutrients continually enter the ecosystem (a) primarily from precipitation, dry fall, N fixation, and weathering of minerals. Nutrients are concurrently lost (d) by leaching, erosion, and denitrification. In a closed forest (b) gain and loss of nutrients are balanced. Nevertheless, a steady-state is brief because of the impacts of tree fall gaps (c) or more severe disturbances (e.g., wind). Such disturbances release nutrients and make them available to plants although total nutrient stocks change little. When a forest is harvested and slash is burned (e), both N and S are lost by volatilization. Large amounts of macronutrients (Ca, K, Mg) enter the soil. The mineral nutrients in slash and organic matter decompose and are available for absorption by plants. If a cleared forest is used for annual cropping, fruit orchards, pulpwood plantations, or pasture (f, k), initial productivity is high. During cultivation, nutrient stocks are progressively depleted in the harvested crops, as well as by leaching, volatilization, and fixation. After a short period of cropping (g), it may be possible to restore nutrient stocks by fallowing (h). In parts of the Amazon Basin where replacement of nutrients occurs largely by atmospheric deposition, restoration of soil fertility by fallowing may require a long time. In contrast, in areas in which mineral substrates are only slightly weathered, the fallow vegetation restores nutrient stocks much faster. However, when fallow cycles are very short (i, j), replacement of soil nutrients may be inadequate.

With continual disturbances (l) (e.g., annually burned pasture lands), productivity may be expected to decrease over time because burning depletes soil organic matter and N and also converts Ca and K to soluble forms, thereby increasing their losses by leaching (Jordan, 1985).

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