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Agroforestry
Ecological Engineering | Agroforestry
Further Reading Andre´n O, Lindberg T, Paustian K, and Rosswall T (eds.) (1990) Ecological Bulletins 40: Ecology of Arable Land - Organisms, Carbon and Nitrogen Cycling. Copenhagen: Ecological Bulletins. Brussaard L (1994) An appraisal of the Dutch program on soil ecology of arable farming systems (1985–1992). Agriculture, Ecosystems and Environment 51(1–2): 1–6 and following papers. Clements D and Shrestha A (eds.) (2004) New Dimensions in Agroecology, 553p. Binghamton: The Hawort Press, Inc. Eijsackers H and Quispel A (eds.) (1988) Ecological Bulletins 39: Ecological Implications of Contemporary Agriculture. Copenhagen: Ecological Bulletins. Kirchmann H (1994) Biological dynamic farming – an occult form of alternative agriculture? Journal of Agricultural and Environmental Ethics 7: 173–187. Mosier AR, Syers JK, and Freney JR (eds.) (2004) Agriculture and the Nitrogen Cycle, 124p. SCOPE 65 Washington: Island Press. New TR (2005) Invertebrate Conservation and Agricultural Ecosystems, 354p. Cambridge: Cambridge University Press.
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Newman EI (2000) Applied Ecology and Environmental Management, 2nd edn. Blackwell Science. Vanlauwe B, Sanginga N, Giller K, and Merckx R (2004) Management of nitrogen fertilizer in maize-based systems in subhumid areas of subSaharan Africa. In: Mosier AR, Syers JK, and Freney JR (eds.) Agriculture and the Nitrogen Cycle. 124p. SCOPE 65. Washington Island Press. Woomer PL and Swift MJ (1994) The Biological Management of Tropical Soil Fertility. Chichester: Wiley.
Relevant Websites http://www.cgiar.org – Consultancy Group on International Agricultural Research. http://www.fao.org – Food and Agriculture Organization of the United Nations.
Agroforestry P K R Nair, University of Florida, Gainesville, FL, USA A M Gordon, University of Guelph, Guelph, ON, Canada M Rosa Mosquera-Losada, University of Santiago de Compostela, Lugo, Spain ª 2008 Elsevier B.V. All rights reserved.
Introduction Agroforestry in Practice Agroforestry: An Integrated Science and Practice Ecological Foundations of Agroforestry
Ecosystem Services of Agroforestry Ecological Engineering: Agroforestry System Design Future Directions Further Reading
Introduction
Agroforestry is perhaps as old as agriculture itself. The practice has been prevalent for many centuries in different parts of the world, especially under subsistence farming conditions. Homegardening, a major agroforestry practice today and one of the oldest forms of agriculture in Southeast Asia, is reported to have been associated with fishing communities living in the moist tropical region about 13 000–9000 BC. Agroforestry in Europe is reported to have started when domestic animals were introduced in forests for feeding around 4000 BC. The dehesa (animal grazing under trees) system of Spain is reportedly 4500 years old. It has been only during the past three decades, however, that these indigenous forms of growing trees and crops/animals together were brought under the realm of modern, scientific land-use scenarios. The motivations for these initiatives were several. The Green Revolution of the 1970s largely did not reach the poor farmers and those in less-productive agroecological environments. In addition, land-management problems such as tropical deforestation, fuelwood shortage, soil
Agroforestry is the relatively new name for the age-old practice of growing trees and shrubs with crops and/or animals in interacting combinations on the same unit of land. Although defined in various ways, the practice encompasses the concept of on-farm and off-farm tree production in support of sustainable land use and natural resource management. (The World Agroforestry Centre defines agroforestry as ‘‘a dynamic, ecologically based, natural resources management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic and environmental benefits for land users at all levels.’’ The Association for Temperate Agroforestry, AFTA defines it as ‘‘an intensive land management system that optimizes the benefits from the biological interactions created when trees and/or shrubs are deliberately combined with crops and/or livestock.’’)
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degradation, and biodiversity decline were escalating. The search for strategies to address these problems focused the attention on the age-old practices of combined production of trees, crops, and livestock on the same land unit, and an appreciation of their inherent advantages. Agroforestry thus began to be recognized and incorporated into national agricultural and forestry research agendas in many developing countries during the 1980s and 1990s. In the temperate regions, ‘modern’ agroforestry had a slower evolution than in the tropics. It started with the general public’s understanding about the environmental consequences of high-input agriculture and forestry. Their demand for environmental accountability and application of ecologically compatible management practices increased when it became clear that the land-use and land-cover changes associated with the removal and fragmentation of natural vegetation for establishment of agricultural and forestry enterprises and real-estate development were responsible, to a large extent, for the decline in biodiversity, invasion of exotic species, and alterations to nutrient, energy, and water flows that
often result in soil erosion, deterioration of water quality, and environmental pollution. Consequently, the concept of agroforestry gained acceptance as an appropriate approach to addressing some of these problems, and agroforestry applications were developed in North America, China, Australia and New Zealand, and Southern Europe, demonstrating the range of conditions under which agroforestry can be successfully applied.
Agroforestry in Practice A large number of traditional agroforestry systems have been recognized from different parts of the world. Each is a specific local example of the association or combination of the components, characterized by the plant species and their arrangement and management, and environmental and socioeconomic factors; thus, location specificity is an important characteristic of these systems. The major practices that constitute the multitude of location-specific systems in the tropical and temperate regions are listed in Table 1. In a time sequence (temporal) pattern of
Table 1 Major agroforestry practices in the tropics and the temperate regions Agroforestry practice Tropical agroforestry Alley cropping (hedgerow intercropping)
Description
Fast-growing, preferably leguminous, woody species grown in crop fields; the woody species pruned periodically at low height (<1.0 m) to reduce shading of crops; the prunings applied as mulch into the alleys as a source of organic matter and nutrients, or used as animal fodder.
Homegardens
Intimate multistory combinations of a large number of various trees and crops in homesteads; livestock may or may not be present.
Improved fallow
Fast-growing, preferably leguminous, woody species planted and left to grow during fallow phases between cropping years for site improvement; woody species may yield economic products.
Multipurpose trees (MPTs) on farms and rangelands
Fruit trees and other MPTs scattered haphazardly or planted in some systematic arrangements in crop or animal production fields; trees provide fruits, fuelwood, fodder, timber, etc.
Silvopasture: Grazing systems Cut and carry system (Protein banks)
Intergrating trees in animal production systems: Cattle grazing on pasture under widely spaced or scattered trees. Stall-feeding of animals with high-quality fodder from trees grown in blocks on farms.
Shaded perennial-crop systems
Growing shade-tolerant species such as cacao and coffee under or in between overstory shade-, timber-, or other commercial tree crops.
Shelterbelts and windbreaks Taungya
Use of trees to protect fields from wind damage, sea encroachment, floods, etc. Growing agricultural crops during the early stages of establishment of forestry (timber) plantations.
Temperate agroforestry Alley cropping
Trees planted in single or grouped rows in herbaceous (agricultural or horticultural) crops in the wide alleys between the tree rows.
Forest farming
Utilizing forested areas for producing specialty crops that are sold for medicinal, ornamental, or culinary uses.
Riparian buffer strips
Strips of perennial vegetation (tree/shrub/grass) planted between croplands/pastures and streams, lakes, wetlands, ponds, etc.
Silvopasture
Combining trees with forage (pasture or hay) and livestock production.
Windbreaks
Row trees around farms and fields, planted and managed as part of crop or livestock operation to protect crops, animals, and soil from wind hazards.
Ecological Engineering | Agroforestry
component arrangement, these practices may be simultaneous (trees intercropped with crops or livestock), or sequential (trees and crops rotated in sequence). The system complexity varies considerably depending on the number and arrangement of component species ranging from relatively ‘simple’ two-species combinations as in tree þ crop intercropping (Figures 1a and 1b) to multistrata homegardens (Figure 2), and intensive shaded perennial systems (Figure 3) to extensive animal grazing under scattered trees such as the dehesa system of Southern Europe (Figure 4). The nature, complexity, and objectives of agroforestry vary considerably between the tropics and the temperate region. In many parts of the tropics, climatic conditions favor longer production cycles within a year (than in the temperate regions); thus, the tropics have a large diversity of species facilitating the existence of numerous and diverse agroforestry systems. Besides, socioeconomic factors such as human population pressure, more availability of labor, smaller land-holding size, complex land tenure, and less proximity to markets often support more and diverse agroforestry systems in the tropics than in the temperate regions. In general, small family farms, subsistence food crops, and emphasis on the role of trees in improving soil quality of agricultural lands are characteristic of tropical agroforestry systems. On the other hand, environmental protection is the main motivation for agroforestry in the industrialized nations, where monocultural production systems of agriculture and forestry have contributed to reduced biodiversity and loss of forest resources and wildlife habitat, and increased environmental hazards such as erosion, nonpoint source pollution of ground water and rivers, and greenhouse-gas emission. The complexity of agroforestry systems is intense in the lowland humid and subhumid tropics, where the climatic conditions generally favor rapid growth of a large number of plant species. Homegardens, shaded perennial systems (or plantation–crop combinations), and multilayer tree gardens are common in regions with high human population, and less intensive systems such as taungya and shifting cultivation are common in areas with less population density. In the semiarid tropics also, the nature of agroforestry systems is influenced by population pressure: homegardens and multilayer tree gardens are found in the relatively wetter areas, windbreaks, and shelterbelts, and multipurpose trees on croplands are found in the drier regions. In regions of highland tropics that have favorable rainfall regimes, sloping lands and rolling topography make soil erosion an issue of major concern; consequently, soil conservation is one of the main objectives of agroforestry in these regions. Shaded perennial systems, use of woody perennials in soil conservation, improved fallows, and silvopastoral systems are the major forms of agroforestry in such tropical highlands. Several other specific systems also exist in the tropics, for example, apiculture with trees,
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aquaculture involving trees and shrubs, and woodlots of ‘multipurpose trees’ (defined in the next section). Alley cropping, forest farming, riparian buffer strips, silvopasture, and windbreaks are the five major agroforestry practices recognized in North America. Other temperate agroforestry systems include ancient treebased agriculture involving a large number of multipurpose trees such as chestnuts (Castanea spp.), oaks (Quercus spp.), carob (Ceratonia siliqua), olive (Olea europa), and figs (Ficus spp.) in the Mediterranean region. The ‘dehesa’ system (called ‘montado’ in Portugal), which involves grazing under oak trees and has strong linkages to recurrent cereal cropping in rangelands (Figure 4), is also a very old European practice.
Agroforestry: An Integrated Science and Practice Today, agroforestry is recognized as an integrated applied science that has the potential for addressing many of the land management and environmental problems found in both developing and industrialized nations. The essence of agroforestry can be expressed in four key ‘I’ words: intentional, intensive, interactive, and integrative. The term ‘intentional’ implies that systems are intentionally designed and managed as whole unit, and ‘intensive’ means that the systems are intensively managed for productive and protective benefits. The biological and physical interactions among the system’s components (tree, crop, and animal) are implied in the term ‘interactive,’ and ‘integrative’ refers to the structural and functional combinations of the components as an integrated management unit. It is often emphasized that all agroforestry systems are characterized by three basic sets of attributes: productivity (production of preferred commodities as well as productivity of the land’s resources), sustainability (conservation of the production potential of the resource base), and adoptability (acceptance of the practice by the farming community or other targeted clientele). Fundamental to realization of the promise of agroforestry systems is the multitude of lesser-known woody species that have come to be known as ‘multipurpose trees’ or ‘multipurpose trees and shrubs’ (MPTs). The MPTs are the mainstay of most traditional tropical agroforestry systems. The contributions of MPTs and agroforestry systems in general are usually grouped under two broad categories: production of commodities and ecosystem services. The former refers to enhancement of outputs such as food, animal fodder, fuelwood, timber, and nontimber products, whereas the latter refers to treemediated services such as carbon storage, biodiversity conservation, and water-quality enhancement. As a result of major research and development efforts in tropical
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(b)
Figure 1 (a) Integrating nitrogen-fixing, fast-growing trees and shrubs with crops in the same field simultaneously, or in short rotations of 1–2 years of crops followed by 1–2 years of trees, and using the tree biomass as a nutrient source for crops is an agroforestry technology that has gained acceptance among farmers who cannot afford fertilizers. The photo shows G. sepium grown with maize (Zea mays), a practice followed by thousands of farmers in eastern and southern Africa. Photo credit: ICRAF, Nairobi, Kenya. (b) Winter wheat (Triticum aestivum) intercropped with American ash (Fraxinus americana) in a temperate setting, southern Ontario, Canada. American ash is a hardwood timber species that also has esthetic and ornamental values. The growth habits and crown shape of the tree make it amenable to intercropping, causing only minimal shading to the understory species. As explained, environmental protection is the main motivation for agroforestry in the industrialized nations, where tree integration on agricultural landscape can mitigate some of the problems associated with agricultural production systems such as reduced biodiversity, loss of forest resources and wildlife habitat, and increased environmental hazards including soil erosion and nonpoint source pollution of ground water and rivers. Photo: N. Thevathasan.
Ecological Engineering | Agroforestry
Figure 2 Homegardens that consist of intimate, vertically stratified combinations of various food- and fruit-producing trees and crops around homesteads provide food and cash security as well as social and esthetic benefits to millions of farm households around the world, particularly in the tropics. The plants in the photograph, from Jamaica, include: Lower canopy, foreground left: taro (Colocasia esculenta) and greater yam (Dioscorea alata), both edible tuber crops; Lower canopy, foreground right: Guinea grass (Panicum maximum), a fodder grass; Middle canopy, right: banana (Musa spp.) and coffee (Coffea spp.); Upper canopy, consisting of fruit trees: bread fruit (Artocarpus altilis) in background, and mango (Mangifera indica) in front of that. Photo: P. K. R. Nair.
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Figure 4 The dehesa, called ‘montado’ in Portuguese, is a traditional agrosilvopastoral system practiced extensively in the Mediterranean region of southwestern Europe, where animals (cattle or sheep) graze under scattered trees. The most common tree species are oaks: Quercus ilex (‘encina’ in Spanish) and Q. suber (‘alcornoque’ in Spanish). Their acorns are relished by pigs; Q. suber is excellent for cork production as well. Q. ilex dehesas used to be valued more in the past (40–50 years ago) because of the higher animal-feed value of its acorns compared to those of Q. suber; but, today Q. suber dehesas fetch higher value because of the high value of cork. Portugal and Spain are the two major cork-producing countries of the world, and most of this cork is produced in dehesa agroforestry system. Photo credit: David Howlett.
agroforestry during the past three decades, a large number of indigenous MPTs have been identified and their multiple role in providing food and nutritional security, medicines, cash income, and a whole host of other products and benefits have been recognized. (Several comprehensive databases of MPTs are available, including the Agroforestree and other databases by ICRAF, various tree data bases by FAO, tropical fruits database, and Forestry Compendium and Forest Products Abstracts database by CABI.) A vast majority of MPTs, however, have not been domesticated let alone exploited commercially. Undoubtedly, a major opportunity as well as challenge in agroforestry lies in domesticating, improving, and exploiting the multitude of these indigenous MPTs.
Ecological Foundations of Agroforestry
Figure 3 Shaded perennial-crop systems involve growing shade-tolerant commercial species such as coffee (Coffea spp.) and cacao (Theobroma cacao) under overstory shade trees. The photo shows young cacao plants in between rows of peach palm (Bactris gasipaes) and black pepper (Piper nigrum) in Bahia, Brazil. Photo: P. K. R. Nair.
The concept of agroforestry is based on the premise that land-use systems that are structurally and functionally more complex than either crop or tree monocultures result in greater efficiency of resource (nutrients, light, and water) capture and utilization, and greater structural diversity that entails tighter nutrient cycles. While the above- and belowground diversity provides more system stability and resilience at the site level, the systems provide connectivity with forests and other landscape features at the landscape and watershed levels. A common thread found in the many historical definitions of agroforestry is the reference to the systems nature of this multifaceted land-use system. However, the
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multitude of ways in which trees may be incorporated into agricultural production systems – intercropping as opposed to silvopastoralism, for example – is problematic in terms of conceptualizing all agroforestry systems in one standardized system model. Nonetheless, there is great merit in doing so because it allows comparison with natural forested or agroecosystems as to the relative extent that ecological properties are maintained or relinquished by agroforestry systems. For example, compared with the net primary productivity (NPP) of 2–6 Mg dry matter (biomass) ha1 yr1 (depending upon species) for temperate coniferous forest plantations, certain agroforestry systems in the tropics such as the multistrata (vertically stratified) homegardens (Figure 2) and shaded perennial systems (Figure 3) can exhibit in excess of 15 Mg ha1 yr1. Indeed, the ecological indexes for species similarity, diversity, and richness (Sorenson’s, Shannon-Wiener, and Margalef, respectively) of multispecies homegardens are similar to those of nearby primary forests. These similarities with natural ecosystems are strong indicators of ecological sustainability of agroforestry systems – assuming, of course, that natural ecosystems are ecologically sustainable. The forced integration of trees into agricultural production systems adds considerable interspecific competition to whatever existing intraspecific competition is present for water, nutrients light, and CO2, thereby creating a more complex agroecosystem. Both positive (e.g., enhanced productivity, cycling of nutrients, soil fertility, and microclimate) and negative (e.g., allelopathic, pest, and disease vectors) may be created and ironically, single management activities undertaken in agroforestry systems may change both negative and positive interactions simultaneously. For example, the pruning of tree branches to add value to the tree component of the system, especially in temperate situations, will decrease shading of the crop component (a negative interaction) but will also decrease litterfall inputs (a positive interaction) to crop alleys, an important pathway for increasing soil carbon. The knowledge of interspecific interactions in agroforestry systems is considered currently inadequate in the temperate zone as compared to the tropics where considerably more is known. Competition for light in both regions by understory crops is a productivity-limiting factor, even in silvopastoral systems where forage quality and quantity can suffer under extreme shade. In the tropics, nutrient and water availability is often as important, and competition for these resources has actually severely limited broader adoption of improved agroforestry technologies in these regions. Four major ecological properties recently identified as critical to the understanding of agroforestry system design, development, management, and evaluation are discussed as follows.
1. Spatial and temporal heterogeneity. Considerable variation exists in agroforestry systems where system components can vary considerably in size, lifespan, and phenology. 2. Disturbance. In many agroforestry systems, some components occupying specific spatial territory are continually being returned to earlier stages of ecosystem succession (crops) while others are developing along much longer successional trajectories (trees). 3. Perennialism. The perennial nature of certain system components contributes to feedback systems which minimize nutrient losses and provide stability to soil chemical, physical, and biological properties. 4. Structural and functional diversity. Agroforestry systems increase vertical heterogeneity, providing niches for a multitude of organisms not normally associated with monocropped agricultural systems. This will generally make energy flow more complex within agroforestry systems, although the overall understanding of this process remains low in both temperate and tropical systems. An understanding of biogeochemical cycles in many tropical and temperate agroforestry systems is emerging globally; this, in concert with an understanding of NPP, carbon sequestration, and aspects of biodiversity, including genetic diversity, is key to embracing the aforementioned ecological properties, thus allowing for the evaluation of the numerous environmental benefits and services that will result from efficiently designed agroforestry systems.
Ecosystem Services of Agroforestry Arising from the above ecological foundations, agroforestry systems have shown to provide several ecosystem services and benefits. While the effects of many of these services and benefits cannot realistically be measured in quantitative terms in relatively short time periods that are common for agricultural production systems, available methods that have been developed for single-commodity-oriented production systems are not sensitive enough to measure the interactive benefits of mixed species, multiple-benefit-oriented, integrated agroforestry systems. Soil Productivity and Protection One of the tree-mediated benefits of considerable advantage in the tropics is that trees and other vegetation improve the productivity of the soil beneath them. Research results during the past two decades show that three main tree-mediated processes increased nitrogen (N) input by N2-fixing trees (NFTs), enhanced availability of nutrients resulting from production and decomposition of tree biomass, and greater uptake and
Ecological Engineering | Agroforestry
utilization of nutrients from deeper layers of soils by deep-rooted trees determine the extent and rate of soil improvement in agroforestry systems. Furthermore, the presence of deep-rooted trees in the system can contribute to improved soil physical conditions and higher soil microbiological activities under agroforestry. NFTs that are common in the tropics are a particularly valuable resource for soil improvement. Farmlands in many parts of the developing world generally suffer from the continuous depletion of nutrients as farmers harvest without fertilizing adequately or fallowing the land. One promising way for overcoming the acute problem of low nutrient status of African soils is to enable smallholders to use fertilizer tree systems that increase on-farm food production. After years of experimentation with a wide range of soil-fertility replenishment practices, three major types of simple, practical, fertilizer-tree systems have been developed: 1. improved fallows using trees and shrubs such as sesbania (Sesbania sesban) or tephrosia (Tephrosia vogelii); 2. mixed intercropping with species such as gliricidia (Gliricida sepium); and 3. biomass transfer with species such as wild sunflower (Tithonia diversifolia) or gliricidia. These practices can provide 50–200 kg N ha1 yr1 to the associated cereal crops. The other major avenue of soil improvement with agroforestry is through soil conservation. About 1.9 billion hectares of land, a third of total farmland, in developing nations are estimated to be degraded through erosion, salinity, and fertility depletion. The potential of agroforestry to reduce the hazards of erosion and desertification as well as to rehabilitate such degraded land and to conserve soil and water has been well recognized. Although this approach is more recent in the temperate zone, research has demonstrated the ameliorative potential of agroforestry. For example, research results show that the agroforestry designs of grass–shrub–tree buffers (riparian buffer) are superior to grass buffers in reducing sediment losses, and trees used as windbreaks around agricultural fields reduce soil erosion caused by wind and water. Agroforestry practices have also been recognized for their role in reducing the hazard of forest fires. Biological Diversity in Working Landscapes The well-being of the land is directly tied to the well-being of its inhabitants. Providing rural people and poor farmers with the opportunity to earn sustainable, stable livelihoods will help conserve the planet’s biodiversity. As much as 90% of the biodiversity resources in the tropics are located in human-dominated or working landscapes. When landscapes are increasingly being fragmented and remaining patches of natural vegetation are reduced to isolated
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habitat islands consequent to population pressure and human activities, mixed species agroforestry systems could play a significant role in helping to maintain a higher level of biodiversity and provide greater landscape connectivity. Agroforestry impinges on biodiversity in working landscapes in at least three ways. First, the intensification of agroforestry systems can reduce exploitation of nearby or even distant protected areas. Second, the expansion of agroforestry systems into traditional farmlands can increase biodiversity in working landscapes. Third, agroforestry development may increase the species and withinspecies diversity of trees in farming systems. Another promising aspect of agroforestry in the context of biodiversity conservation is in growing commercial crops such as coffee (Coffea spp.) and cacao (Theobroma cacao), known as shaded-perennial systems in the agroforestry literature. This is projected to start a trend of combining environmental research with consumer products, which could then have a large impact on global conservation. In the temperate regions too, agroforestry can augment the supply of forest habitat and provide greater landscape connectivity. Where croplands occupy most of the landscape, riparian forest buffers and field shelterbelts can be essential for maintaining plant and animal biodiversity, especially under a changing climate scenario. A comprehensive assessment of shelterbelt agroforestry systems in the northern Great Plains of the USA has clearly demonstrated their importance on breeding bird species richness and community composition at both the farm- and landscape levels.
Carbon Storage Agroforestry systems have the potential to sequester substantial quantities of carbon (C). In addition to sequestering C in biomass and soil, these systems can contribute to both carbon conservation (conservation of carbon stocks in forests by alleviating the pressure) and carbon substitution (reducing fossil fuel burning by producing fuel wood). In temperate regions, agroforestry practices have been estimated to have the potential to store C in the range of 15–198 Mg C ha1 (mode: 34 Mg C ha1). The C sequestration potential of agriculture in the USA through the application of agroforestry practices by 2025 is estimated as 90.3 Tg C yr1 (Tg = 1012 g = million tons). In the tropics, agroforestry systems are estimated to have helped to regain 35% of the original C stock of the cleared forest, compared to only 12% by croplands and pastures. It has been estimated that an increase of 1 ton (Mg) of soil carbon pool of degraded cropland soils may increase crop yields by 2040 kg ha1. A projection of C stocks for smallholder agroforestry systems indicates C sequestration rates ranging from 1.5 to 3.5 Mg C ha1 yr1 and a tripling of C stocks in a 20-year period, to 70 Mg C ha1. Global deforestation, which is estimated to occur at the rate of 17 million ha yr1,
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is expected to cause the emission of 1.6 Pg C ha1 yr1 (Pg = 1015 g = billion tons). Global warming is now accepted as a real issue the world over. Ways of reducing CO2 in the atmosphere will undoubtedly be receiving increasing attention in the future. More than twice as much carbon is held in soils as in vegetation or the atmosphere; changes in soil carbon can have a large effect on the global carbon budget. Assuming that 1 ha of agroforestry could save 5 ha from deforestation, carbon emission caused by deforestation could be reduced substantially by establishing agroforestry systems. Along with these efforts, carbon markets will become stronger and more active. The opportunity offered by agroforestry systems to sequester carbon and market it for real money is real, even in poor countries. Water Quality and Environmental Amelioration Agricultural nonpoint source pollution is a significant cause of stream and lake contamination in many regions of industrialized world. A major causative source of this pollution is nutrients such as phosphorus (P) and nitrogen (N) that are lost from soils of fertilized agricultural and forestry operations, particularly in coarse-textured, poorly drained soils where drainage water ultimately mixes with surface water. It has been estimated that beef cattle operations on improved pastures (184 000 ha), unimproved pastures (33 500 ha), and rangeland (47 000 ha) on ranches are a large contributor of P loads to Lake Okeechobee, Florida, one of the largest freshwater lakes in the USA. Recent studies have shown that agroforestry practices such as silvopasture and riparian buffer could be a means of addressing the problem of environmental impact of nonpoint source pollution. The deeper and more extensive tree roots will invariably be able to take up more nutrients from the soil compared to crops with shallower root systems – the so-called ‘safety-net’ effect that has been affirmed in various agroforestry situations. Consequently, nutrient-leaching rates from soils under agroforestry systems where trees are a major component can be lower than those from treeless systems. Other studies documenting the effects of planted riparian forests on stream quality in temperate agricultural settings have also been reported, for example, in northwestern USA. The water-quality enhancement resulting from the reduction of nutrient loading could be a substantial environmental benefit of agroforestry in heavily fertilized agricultural landscapes. With increasing realization of the adverse impacts of chemical agriculture and climate change on availability and quality of water in many parts of the world, water is now a critical issue in natural resource management. Time-tested integrated land-use practices such as agroforestry could be appropriate approaches to addressing the problem. However, the science of this is nonexistent and needs to be explored and established.
The connectivity and buffering effect offered by the presence of agroforestry systems on the landscape is another important environmental benefit of agroforestry that is little appreciated. It is common knowledge that the increasing presence of trees of similar or different species in an agricultural setting will have many positive biophysical influences (see the sections, titled ‘Soil productivity and protection’ and ‘Carbon storage’). As the trees age, these influences increase in magnitude; therefore, it is to be expected that when trees are introduced into degraded agricultural landscapes (intercropping) and stream banks (riparian buffer), there will be gradual changes at the system (landscape) level. Long-term studies of this nature are rare, but some convincing examples are available (see the section titled ‘Ecological engineering: agroforestry system design’). The fire-protection benefit offered by extensive tree-based systems such as the dehesa in the Mediterranean region (Figure 4) is another benefit that has traditionally been enjoyed but never valued in economic terms.
Food and Nutritional Security The MPTs play a major role in food production in two ways: directly by providing edible products such as fruits, and indirectly by supporting food production through enhancing the soil’s ability to support agriculture. A large number of fruit-producing trees are integral parts of traditional homestead and other agroforestry farming systems with their characteristic multistrata canopies in many developing countries. Although several of these fruit trees have not been studied scientifically and are thus underexploited and little known outside their habitat, they make significant contributions to food and nutritional security. Another group of underexploited species of immense cultural and economic value are the natural medicinal plants (‘medicinals’). In Africa, more than 80% of the population depend on medicinal plants to meet their medical needs, and about two-thirds of the species from which such medicines are derived are trees. While the majority of these tree products are obtained by extraction from natural forests, some ‘well-known’ agroforestry tree species grown on farms for other uses (such as fodder, food, or fuelwood) are also used for their medicinal values. Examples include Acacia nilotica used in India and Africa; Azadirachta indica, the neem tree, used throughout Asia and Africa; Parkia biglobosa (nere´ or the locust bean tree) used in Africa; and Tamarindus indica, the tamarind tree, used in India and Africa. There is also increasing interest in natural medicines in the developed world, creating new or expanded markets for these products. This puts further extraction pressure on the natural forests. Many of the medicinal tree species are already overexploited. Some species are so depleted that their gene pools are greatly eroded (e.g., Prunus africana), and some are in danger of extinction.
Ecological Engineering | Agroforestry
Ecological Engineering: Agroforestry System Design Many agroforestry systems lend themselves to the concept of ecological engineering, whereby the ecological processes are used to solve engineering problems in such a way that ecosystems are designed, constructed, and managed for both environmental and societal benefits. The premise that structurally and functionally more complex agroforestry systems have greater efficiency of resource capture and utilization than either crop or tree monocultures provides a strong foundation for such efforts. An important consideration in design of agroforestry systems, however, is the interplay and importance of ecological, economic, and social attributes and objectives of agroforestry systems. As illustrated schematically in Figure 5, enhanced and sustained production of many typical ‘services’ normally associated with food or timber production systems is possible largely as a result of forced integration of trees, crops, and/or animals in such a way that interactions are created both above- and belowground. The resultant ‘production system’ feeds back at a number of scales to extra- and intrasystem components to provide a number of ecosystem services typical of those found in natural, undisturbed systems. For example,
recent studies that tree incorporation in pastures leads to retention of more phosphorus in the system and thus prevention or reduction of the chances of its transport from the coarse textured soils of Florida to adjacent water bodies corroborate the premise of silvopasture-system design. Another example is the intercropping of trees with agricultural crops in the temperate zone (Figure 1b), where trees are planted in widely spaced single or double rows in agricultural fields. Although the trees use up about 10% of land, this ‘loss’ will be more than offset economically and ecologically by production and service value coming from the tree component. Ecological benefits of intercropping that have been monitored in such a system in eastern Canada during a 20-year period include enhanced biodiversity (of birds, earthworms, and beneficial insects), soil organic matter buildup, synchronous development of tight nutrient cycles that prevent nutrients leaching into adjacent waterways, and higher sequestration of carbon both above- and belowground. Most economic models demonstrate equality between agroforestry and monocropping systems, based on the sometimes extended length of period associated with obtaining a long-term product from the tree component. When short-term, mostly nontimber products from the tree components are considered in conjunction with
Sustainable agroforestry systems
Production Enhancing production of food, fodder, fuelwood, timber, and non-timber products Interactions
Ecosystem services
Soil conservation Carbon storage
Aboveground
Radiation interception
Belowground N2 fixation nutrient cycling & deep capture
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Water quality enhancement Biodiversity conservation
Soil productivity improvement
Figure 5 A schematic presentation of the major mechanisms and processes involved in production and service attributes of sustainable agroforestry systems.
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large-scale societal benefits (e.g., improved water quality and reduced pesticide use), the economic models tip very much in favor of agroforestry systems.
Future Directions Agriculture and forestry are too often treated separately, yet these two sectors are often interwoven on the landscape and share many common goals and ecological foundations. The multitude of agroforestry systems, be they practiced in the tropics or temperate regions, are firmly grounded on strong ecological principles, and through provision of many basic needs and ecosystem services, they contribute to attainment of many regional developmental goals. These underexploited systems have the potential to develop into a set of major land-use options in the twenty-first century. The gains and developments of nearly three decades of agroforestry research and development are certainly impressive. Undoubtedly, agroforestry is now on a firm scientific footing. Today agroforestry is no longer a mysterious enigma that defies science and scientific principles, as it was perceived three decades ago. Agroforestry is well on its way to becoming a specialized science at a level comparable to those of crop science and forestry science. These hard-earned scientific gains need to be put to practice for solving the problems we had set out to address when we started research. The adoption rates of these systems have, however, been rather low so far, perhaps because of the emphasis on sustainability, rather than as an opportunity for immediate monetary gains. See also: Classical and Augmentative Biological Control; Ecological Engineering: Overview.
Further Reading Bene JG, Beall HW, and Coˆte A (1977) Trees, Food and People. Ottawa, Canada: International Development Research Centre. Brown LR (2004) Outgrowing the Earth: The Food Security Challenge in an Age of Falling Water Tables and Rising Temperatures. New York: W.W. Norton.
Buresh RJ and Tian G (1998) Soil improvement by trees in sub-Saharan Africa. Agroforestry Systems 38: 51–76. Evenson RE and Gollin D (2003) Assessing the impact of the Green Revolution, 1960 to 2000. Science 300: 758–762. Garrett HE, Rietveld WJ, and Fisher RF (eds.) (2000) North American Agroforestry: An Integrated Science and Practice. Madison, WI: American Society of Agronomy. Gordon AM and Newman SM (eds.) (1997) Temperate Agroforestry Systems. Wallingford: CAB International. Jose A and Gordon AM (eds.) (2007) Advances in Agroforestry 4: Towards Agroforesty Design: An Ecological Approach. Dordrecht: Springer Science. Kumar BM and Nair PKR (eds.) (2006) Advances in Agroforestry 3: Tropical Homegardens: A Time-Tested Example of Sustainable Agroforestry. Dordrecht, The Netherlands: Springer Science. Lal R (2004) Soil carbon sequestration impacts on global climate change and food security. Science (Washington, DC) 304: 1623–1627. Mosquera-Losada MR, McAdam J, and Rigueiro-Rodriguez A (eds.) (2005) Silvopastoralism and Sustainable Land Management. Wallingford, UK: CAB International. Nair PKR, Rao MR, and Buck LE (eds.) (2004) Advances in Agroforestry 1: New Vistas in Agroforestry: A Compendium for the 1st World Congress of Agroforestry 2004. Dordrecht, The Netherlands: Kluwer. Ong CK and Huxley P (eds.) (1996) Tree-Crop Interactions: A Physiological Approach. Wallingford, UK: CAB International. Palm CA, Tomich T, Van Noordwijk M, et al. (2004) Mitigating GHG emissions in the humid tropics: Case studies from the alternatives to slash-and-burn program (ASB). Environment, Development and Sustainability 6: 145–162. Rao MR, Nair PKR, and Ong CK (1998) Biophysical interactions in tropical agroforestry systems. Agroforestry Systems 38: 3–50. Rigueiro-Rodriguez A, McAdam J, Mosquera-Losada MR (eds.) (in press) Advances in Agroforestry 5: Agroforestry in Europe. Dordrecht: Springer. Schroth G, da Fonseca AB, Harvey CA, Gascon C, Vasconcelos HL, and Izac N (eds.) (2004) Agroforestry and Biodiversity Conservation in Tropical Landscapes. Washington, DC: Island Press. Steppler HA and Nair PKR (eds.) (1987) Agroforestry: A Decade of Development. Nairobi, Kenya: ICRAF (World Agroforestry Centre).
Relevant Websites http://www.cabi-publishing.org/AbstractDatabases – Abstracts Databases, CABI Publishing. http://www.aftaweb.org – Association for Temperate Agroforestry. http://www.fao.org – Food and Agriculture Organization of the United Nations. http://www.tradewindsfruit.com – Tropical Fruit Database, Trade Winds Fruit. http://www.icraf.cgiar.org – World Agroforestry Centre.
Further Reading Andre´n O, Lindberg T, Paustian K, and Rosswall T (eds.) (1990) Ecological Bulletins 40: Ecology of Arable Land - Organisms, Carbon and Nitrogen Cycling. Copenhagen: Ecological Bulletins. Brussaard L (1994) An appraisal of the Dutch program on soil ecology of arable farming systems (1985–1992). Agriculture, Ecosystems and Environment 51(1–2): 1–6 and following papers. Clements D and Shrestha A (eds.) (2004) New Dimensions in Agroecology, 553p. Binghamton: The Hawort Press, Inc. Eijsackers H and Quispel A (eds.) (1988) Ecological Bulletins 39: Ecological Implications of Contemporary Agriculture. Copenhagen: Ecological Bulletins. Kirchmann H (1994) Biological dynamic farming – an occult form of alternative agriculture? Journal of Agricultural and Environmental Ethics 7: 173–187. Mosier AR, Syers JK, and Freney JR (eds.) (2004) Agriculture and the Nitrogen Cycle, 124p. SCOPE 65 Washington: Island Press. New TR (2005) Invertebrate Conservation and Agricultural Ecosystems, 354p. Cambridge: Cambridge University Press.
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Newman EI (2000) Applied Ecology and Environmental Management, 2nd edn. Blackwell Science. Vanlauwe B, Sanginga N, Giller K, and Merckx R (2004) Management of nitrogen fertilizer in maize-based systems in subhumid areas of subSaharan Africa. In: Mosier AR, Syers JK, and Freney JR (eds.) Agriculture and the Nitrogen Cycle. 124p. SCOPE 65. Washington Island Press. Woomer PL and Swift MJ (1994) The Biological Management of Tropical Soil Fertility. Chichester: Wiley.
Relevant Websites http://www.cgiar.org – Consultancy Group on International Agricultural Research. http://www.fao.org – Food and Agriculture Organization of the United Nations.
Agroforestry P K R Nair, University of Florida, Gainesville, FL, USA A M Gordon, University of Guelph, Guelph, ON, Canada M Rosa Mosquera-Losada, University of Santiago de Compostela, Lugo, Spain ª 2008 Elsevier B.V. All rights reserved.
Introduction Agroforestry in Practice Agroforestry: An Integrated Science and Practice Ecological Foundations of Agroforestry
Ecosystem Services of Agroforestry Ecological Engineering: Agroforestry System Design Future Directions Further Reading
Introduction
Agroforestry is perhaps as old as agriculture itself. The practice has been prevalent for many centuries in different parts of the world, especially under subsistence farming conditions. Homegardening, a major agroforestry practice today and one of the oldest forms of agriculture in Southeast Asia, is reported to have been associated with fishing communities living in the moist tropical region about 13 000–9000 BC. Agroforestry in Europe is reported to have started when domestic animals were introduced in forests for feeding around 4000 BC. The dehesa (animal grazing under trees) system of Spain is reportedly 4500 years old. It has been only during the past three decades, however, that these indigenous forms of growing trees and crops/animals together were brought under the realm of modern, scientific land-use scenarios. The motivations for these initiatives were several. The Green Revolution of the 1970s largely did not reach the poor farmers and those in less-productive agroecological environments. In addition, land-management problems such as tropical deforestation, fuelwood shortage, soil
Agroforestry is the relatively new name for the age-old practice of growing trees and shrubs with crops and/or animals in interacting combinations on the same unit of land. Although defined in various ways, the practice encompasses the concept of on-farm and off-farm tree production in support of sustainable land use and natural resource management. (The World Agroforestry Centre defines agroforestry as ‘‘a dynamic, ecologically based, natural resources management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic and environmental benefits for land users at all levels.’’ The Association for Temperate Agroforestry, AFTA defines it as ‘‘an intensive land management system that optimizes the benefits from the biological interactions created when trees and/or shrubs are deliberately combined with crops and/or livestock.’’)
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degradation, and biodiversity decline were escalating. The search for strategies to address these problems focused the attention on the age-old practices of combined production of trees, crops, and livestock on the same land unit, and an appreciation of their inherent advantages. Agroforestry thus began to be recognized and incorporated into national agricultural and forestry research agendas in many developing countries during the 1980s and 1990s. In the temperate regions, ‘modern’ agroforestry had a slower evolution than in the tropics. It started with the general public’s understanding about the environmental consequences of high-input agriculture and forestry. Their demand for environmental accountability and application of ecologically compatible management practices increased when it became clear that the land-use and land-cover changes associated with the removal and fragmentation of natural vegetation for establishment of agricultural and forestry enterprises and real-estate development were responsible, to a large extent, for the decline in biodiversity, invasion of exotic species, and alterations to nutrient, energy, and water flows that
often result in soil erosion, deterioration of water quality, and environmental pollution. Consequently, the concept of agroforestry gained acceptance as an appropriate approach to addressing some of these problems, and agroforestry applications were developed in North America, China, Australia and New Zealand, and Southern Europe, demonstrating the range of conditions under which agroforestry can be successfully applied.
Agroforestry in Practice A large number of traditional agroforestry systems have been recognized from different parts of the world. Each is a specific local example of the association or combination of the components, characterized by the plant species and their arrangement and management, and environmental and socioeconomic factors; thus, location specificity is an important characteristic of these systems. The major practices that constitute the multitude of location-specific systems in the tropical and temperate regions are listed in Table 1. In a time sequence (temporal) pattern of
Table 1 Major agroforestry practices in the tropics and the temperate regions Agroforestry practice Tropical agroforestry Alley cropping (hedgerow intercropping)
Description
Fast-growing, preferably leguminous, woody species grown in crop fields; the woody species pruned periodically at low height (<1.0 m) to reduce shading of crops; the prunings applied as mulch into the alleys as a source of organic matter and nutrients, or used as animal fodder.
Homegardens
Intimate multistory combinations of a large number of various trees and crops in homesteads; livestock may or may not be present.
Improved fallow
Fast-growing, preferably leguminous, woody species planted and left to grow during fallow phases between cropping years for site improvement; woody species may yield economic products.
Multipurpose trees (MPTs) on farms and rangelands
Fruit trees and other MPTs scattered haphazardly or planted in some systematic arrangements in crop or animal production fields; trees provide fruits, fuelwood, fodder, timber, etc.
Silvopasture: Grazing systems Cut and carry system (Protein banks)
Intergrating trees in animal production systems: Cattle grazing on pasture under widely spaced or scattered trees. Stall-feeding of animals with high-quality fodder from trees grown in blocks on farms.
Shaded perennial-crop systems
Growing shade-tolerant species such as cacao and coffee under or in between overstory shade-, timber-, or other commercial tree crops.
Shelterbelts and windbreaks Taungya
Use of trees to protect fields from wind damage, sea encroachment, floods, etc. Growing agricultural crops during the early stages of establishment of forestry (timber) plantations.
Temperate agroforestry Alley cropping
Trees planted in single or grouped rows in herbaceous (agricultural or horticultural) crops in the wide alleys between the tree rows.
Forest farming
Utilizing forested areas for producing specialty crops that are sold for medicinal, ornamental, or culinary uses.
Riparian buffer strips
Strips of perennial vegetation (tree/shrub/grass) planted between croplands/pastures and streams, lakes, wetlands, ponds, etc.
Silvopasture
Combining trees with forage (pasture or hay) and livestock production.
Windbreaks
Row trees around farms and fields, planted and managed as part of crop or livestock operation to protect crops, animals, and soil from wind hazards.
Ecological Engineering | Agroforestry
component arrangement, these practices may be simultaneous (trees intercropped with crops or livestock), or sequential (trees and crops rotated in sequence). The system complexity varies considerably depending on the number and arrangement of component species ranging from relatively ‘simple’ two-species combinations as in tree þ crop intercropping (Figures 1a and 1b) to multistrata homegardens (Figure 2), and intensive shaded perennial systems (Figure 3) to extensive animal grazing under scattered trees such as the dehesa system of Southern Europe (Figure 4). The nature, complexity, and objectives of agroforestry vary considerably between the tropics and the temperate region. In many parts of the tropics, climatic conditions favor longer production cycles within a year (than in the temperate regions); thus, the tropics have a large diversity of species facilitating the existence of numerous and diverse agroforestry systems. Besides, socioeconomic factors such as human population pressure, more availability of labor, smaller land-holding size, complex land tenure, and less proximity to markets often support more and diverse agroforestry systems in the tropics than in the temperate regions. In general, small family farms, subsistence food crops, and emphasis on the role of trees in improving soil quality of agricultural lands are characteristic of tropical agroforestry systems. On the other hand, environmental protection is the main motivation for agroforestry in the industrialized nations, where monocultural production systems of agriculture and forestry have contributed to reduced biodiversity and loss of forest resources and wildlife habitat, and increased environmental hazards such as erosion, nonpoint source pollution of ground water and rivers, and greenhouse-gas emission. The complexity of agroforestry systems is intense in the lowland humid and subhumid tropics, where the climatic conditions generally favor rapid growth of a large number of plant species. Homegardens, shaded perennial systems (or plantation–crop combinations), and multilayer tree gardens are common in regions with high human population, and less intensive systems such as taungya and shifting cultivation are common in areas with less population density. In the semiarid tropics also, the nature of agroforestry systems is influenced by population pressure: homegardens and multilayer tree gardens are found in the relatively wetter areas, windbreaks, and shelterbelts, and multipurpose trees on croplands are found in the drier regions. In regions of highland tropics that have favorable rainfall regimes, sloping lands and rolling topography make soil erosion an issue of major concern; consequently, soil conservation is one of the main objectives of agroforestry in these regions. Shaded perennial systems, use of woody perennials in soil conservation, improved fallows, and silvopastoral systems are the major forms of agroforestry in such tropical highlands. Several other specific systems also exist in the tropics, for example, apiculture with trees,
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aquaculture involving trees and shrubs, and woodlots of ‘multipurpose trees’ (defined in the next section). Alley cropping, forest farming, riparian buffer strips, silvopasture, and windbreaks are the five major agroforestry practices recognized in North America. Other temperate agroforestry systems include ancient treebased agriculture involving a large number of multipurpose trees such as chestnuts (Castanea spp.), oaks (Quercus spp.), carob (Ceratonia siliqua), olive (Olea europa), and figs (Ficus spp.) in the Mediterranean region. The ‘dehesa’ system (called ‘montado’ in Portugal), which involves grazing under oak trees and has strong linkages to recurrent cereal cropping in rangelands (Figure 4), is also a very old European practice.
Agroforestry: An Integrated Science and Practice Today, agroforestry is recognized as an integrated applied science that has the potential for addressing many of the land management and environmental problems found in both developing and industrialized nations. The essence of agroforestry can be expressed in four key ‘I’ words: intentional, intensive, interactive, and integrative. The term ‘intentional’ implies that systems are intentionally designed and managed as whole unit, and ‘intensive’ means that the systems are intensively managed for productive and protective benefits. The biological and physical interactions among the system’s components (tree, crop, and animal) are implied in the term ‘interactive,’ and ‘integrative’ refers to the structural and functional combinations of the components as an integrated management unit. It is often emphasized that all agroforestry systems are characterized by three basic sets of attributes: productivity (production of preferred commodities as well as productivity of the land’s resources), sustainability (conservation of the production potential of the resource base), and adoptability (acceptance of the practice by the farming community or other targeted clientele). Fundamental to realization of the promise of agroforestry systems is the multitude of lesser-known woody species that have come to be known as ‘multipurpose trees’ or ‘multipurpose trees and shrubs’ (MPTs). The MPTs are the mainstay of most traditional tropical agroforestry systems. The contributions of MPTs and agroforestry systems in general are usually grouped under two broad categories: production of commodities and ecosystem services. The former refers to enhancement of outputs such as food, animal fodder, fuelwood, timber, and nontimber products, whereas the latter refers to treemediated services such as carbon storage, biodiversity conservation, and water-quality enhancement. As a result of major research and development efforts in tropical
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(b)
Figure 1 (a) Integrating nitrogen-fixing, fast-growing trees and shrubs with crops in the same field simultaneously, or in short rotations of 1–2 years of crops followed by 1–2 years of trees, and using the tree biomass as a nutrient source for crops is an agroforestry technology that has gained acceptance among farmers who cannot afford fertilizers. The photo shows G. sepium grown with maize (Zea mays), a practice followed by thousands of farmers in eastern and southern Africa. Photo credit: ICRAF, Nairobi, Kenya. (b) Winter wheat (Triticum aestivum) intercropped with American ash (Fraxinus americana) in a temperate setting, southern Ontario, Canada. American ash is a hardwood timber species that also has esthetic and ornamental values. The growth habits and crown shape of the tree make it amenable to intercropping, causing only minimal shading to the understory species. As explained, environmental protection is the main motivation for agroforestry in the industrialized nations, where tree integration on agricultural landscape can mitigate some of the problems associated with agricultural production systems such as reduced biodiversity, loss of forest resources and wildlife habitat, and increased environmental hazards including soil erosion and nonpoint source pollution of ground water and rivers. Photo: N. Thevathasan.
Ecological Engineering | Agroforestry
Figure 2 Homegardens that consist of intimate, vertically stratified combinations of various food- and fruit-producing trees and crops around homesteads provide food and cash security as well as social and esthetic benefits to millions of farm households around the world, particularly in the tropics. The plants in the photograph, from Jamaica, include: Lower canopy, foreground left: taro (Colocasia esculenta) and greater yam (Dioscorea alata), both edible tuber crops; Lower canopy, foreground right: Guinea grass (Panicum maximum), a fodder grass; Middle canopy, right: banana (Musa spp.) and coffee (Coffea spp.); Upper canopy, consisting of fruit trees: bread fruit (Artocarpus altilis) in background, and mango (Mangifera indica) in front of that. Photo: P. K. R. Nair.
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Figure 4 The dehesa, called ‘montado’ in Portuguese, is a traditional agrosilvopastoral system practiced extensively in the Mediterranean region of southwestern Europe, where animals (cattle or sheep) graze under scattered trees. The most common tree species are oaks: Quercus ilex (‘encina’ in Spanish) and Q. suber (‘alcornoque’ in Spanish). Their acorns are relished by pigs; Q. suber is excellent for cork production as well. Q. ilex dehesas used to be valued more in the past (40–50 years ago) because of the higher animal-feed value of its acorns compared to those of Q. suber; but, today Q. suber dehesas fetch higher value because of the high value of cork. Portugal and Spain are the two major cork-producing countries of the world, and most of this cork is produced in dehesa agroforestry system. Photo credit: David Howlett.
agroforestry during the past three decades, a large number of indigenous MPTs have been identified and their multiple role in providing food and nutritional security, medicines, cash income, and a whole host of other products and benefits have been recognized. (Several comprehensive databases of MPTs are available, including the Agroforestree and other databases by ICRAF, various tree data bases by FAO, tropical fruits database, and Forestry Compendium and Forest Products Abstracts database by CABI.) A vast majority of MPTs, however, have not been domesticated let alone exploited commercially. Undoubtedly, a major opportunity as well as challenge in agroforestry lies in domesticating, improving, and exploiting the multitude of these indigenous MPTs.
Ecological Foundations of Agroforestry
Figure 3 Shaded perennial-crop systems involve growing shade-tolerant commercial species such as coffee (Coffea spp.) and cacao (Theobroma cacao) under overstory shade trees. The photo shows young cacao plants in between rows of peach palm (Bactris gasipaes) and black pepper (Piper nigrum) in Bahia, Brazil. Photo: P. K. R. Nair.
The concept of agroforestry is based on the premise that land-use systems that are structurally and functionally more complex than either crop or tree monocultures result in greater efficiency of resource (nutrients, light, and water) capture and utilization, and greater structural diversity that entails tighter nutrient cycles. While the above- and belowground diversity provides more system stability and resilience at the site level, the systems provide connectivity with forests and other landscape features at the landscape and watershed levels. A common thread found in the many historical definitions of agroforestry is the reference to the systems nature of this multifaceted land-use system. However, the
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multitude of ways in which trees may be incorporated into agricultural production systems – intercropping as opposed to silvopastoralism, for example – is problematic in terms of conceptualizing all agroforestry systems in one standardized system model. Nonetheless, there is great merit in doing so because it allows comparison with natural forested or agroecosystems as to the relative extent that ecological properties are maintained or relinquished by agroforestry systems. For example, compared with the net primary productivity (NPP) of 2–6 Mg dry matter (biomass) ha1 yr1 (depending upon species) for temperate coniferous forest plantations, certain agroforestry systems in the tropics such as the multistrata (vertically stratified) homegardens (Figure 2) and shaded perennial systems (Figure 3) can exhibit in excess of 15 Mg ha1 yr1. Indeed, the ecological indexes for species similarity, diversity, and richness (Sorenson’s, Shannon-Wiener, and Margalef, respectively) of multispecies homegardens are similar to those of nearby primary forests. These similarities with natural ecosystems are strong indicators of ecological sustainability of agroforestry systems – assuming, of course, that natural ecosystems are ecologically sustainable. The forced integration of trees into agricultural production systems adds considerable interspecific competition to whatever existing intraspecific competition is present for water, nutrients light, and CO2, thereby creating a more complex agroecosystem. Both positive (e.g., enhanced productivity, cycling of nutrients, soil fertility, and microclimate) and negative (e.g., allelopathic, pest, and disease vectors) may be created and ironically, single management activities undertaken in agroforestry systems may change both negative and positive interactions simultaneously. For example, the pruning of tree branches to add value to the tree component of the system, especially in temperate situations, will decrease shading of the crop component (a negative interaction) but will also decrease litterfall inputs (a positive interaction) to crop alleys, an important pathway for increasing soil carbon. The knowledge of interspecific interactions in agroforestry systems is considered currently inadequate in the temperate zone as compared to the tropics where considerably more is known. Competition for light in both regions by understory crops is a productivity-limiting factor, even in silvopastoral systems where forage quality and quantity can suffer under extreme shade. In the tropics, nutrient and water availability is often as important, and competition for these resources has actually severely limited broader adoption of improved agroforestry technologies in these regions. Four major ecological properties recently identified as critical to the understanding of agroforestry system design, development, management, and evaluation are discussed as follows.
1. Spatial and temporal heterogeneity. Considerable variation exists in agroforestry systems where system components can vary considerably in size, lifespan, and phenology. 2. Disturbance. In many agroforestry systems, some components occupying specific spatial territory are continually being returned to earlier stages of ecosystem succession (crops) while others are developing along much longer successional trajectories (trees). 3. Perennialism. The perennial nature of certain system components contributes to feedback systems which minimize nutrient losses and provide stability to soil chemical, physical, and biological properties. 4. Structural and functional diversity. Agroforestry systems increase vertical heterogeneity, providing niches for a multitude of organisms not normally associated with monocropped agricultural systems. This will generally make energy flow more complex within agroforestry systems, although the overall understanding of this process remains low in both temperate and tropical systems. An understanding of biogeochemical cycles in many tropical and temperate agroforestry systems is emerging globally; this, in concert with an understanding of NPP, carbon sequestration, and aspects of biodiversity, including genetic diversity, is key to embracing the aforementioned ecological properties, thus allowing for the evaluation of the numerous environmental benefits and services that will result from efficiently designed agroforestry systems.
Ecosystem Services of Agroforestry Arising from the above ecological foundations, agroforestry systems have shown to provide several ecosystem services and benefits. While the effects of many of these services and benefits cannot realistically be measured in quantitative terms in relatively short time periods that are common for agricultural production systems, available methods that have been developed for single-commodity-oriented production systems are not sensitive enough to measure the interactive benefits of mixed species, multiple-benefit-oriented, integrated agroforestry systems. Soil Productivity and Protection One of the tree-mediated benefits of considerable advantage in the tropics is that trees and other vegetation improve the productivity of the soil beneath them. Research results during the past two decades show that three main tree-mediated processes increased nitrogen (N) input by N2-fixing trees (NFTs), enhanced availability of nutrients resulting from production and decomposition of tree biomass, and greater uptake and
Ecological Engineering | Agroforestry
utilization of nutrients from deeper layers of soils by deep-rooted trees determine the extent and rate of soil improvement in agroforestry systems. Furthermore, the presence of deep-rooted trees in the system can contribute to improved soil physical conditions and higher soil microbiological activities under agroforestry. NFTs that are common in the tropics are a particularly valuable resource for soil improvement. Farmlands in many parts of the developing world generally suffer from the continuous depletion of nutrients as farmers harvest without fertilizing adequately or fallowing the land. One promising way for overcoming the acute problem of low nutrient status of African soils is to enable smallholders to use fertilizer tree systems that increase on-farm food production. After years of experimentation with a wide range of soil-fertility replenishment practices, three major types of simple, practical, fertilizer-tree systems have been developed: 1. improved fallows using trees and shrubs such as sesbania (Sesbania sesban) or tephrosia (Tephrosia vogelii); 2. mixed intercropping with species such as gliricidia (Gliricida sepium); and 3. biomass transfer with species such as wild sunflower (Tithonia diversifolia) or gliricidia. These practices can provide 50–200 kg N ha1 yr1 to the associated cereal crops. The other major avenue of soil improvement with agroforestry is through soil conservation. About 1.9 billion hectares of land, a third of total farmland, in developing nations are estimated to be degraded through erosion, salinity, and fertility depletion. The potential of agroforestry to reduce the hazards of erosion and desertification as well as to rehabilitate such degraded land and to conserve soil and water has been well recognized. Although this approach is more recent in the temperate zone, research has demonstrated the ameliorative potential of agroforestry. For example, research results show that the agroforestry designs of grass–shrub–tree buffers (riparian buffer) are superior to grass buffers in reducing sediment losses, and trees used as windbreaks around agricultural fields reduce soil erosion caused by wind and water. Agroforestry practices have also been recognized for their role in reducing the hazard of forest fires. Biological Diversity in Working Landscapes The well-being of the land is directly tied to the well-being of its inhabitants. Providing rural people and poor farmers with the opportunity to earn sustainable, stable livelihoods will help conserve the planet’s biodiversity. As much as 90% of the biodiversity resources in the tropics are located in human-dominated or working landscapes. When landscapes are increasingly being fragmented and remaining patches of natural vegetation are reduced to isolated
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habitat islands consequent to population pressure and human activities, mixed species agroforestry systems could play a significant role in helping to maintain a higher level of biodiversity and provide greater landscape connectivity. Agroforestry impinges on biodiversity in working landscapes in at least three ways. First, the intensification of agroforestry systems can reduce exploitation of nearby or even distant protected areas. Second, the expansion of agroforestry systems into traditional farmlands can increase biodiversity in working landscapes. Third, agroforestry development may increase the species and withinspecies diversity of trees in farming systems. Another promising aspect of agroforestry in the context of biodiversity conservation is in growing commercial crops such as coffee (Coffea spp.) and cacao (Theobroma cacao), known as shaded-perennial systems in the agroforestry literature. This is projected to start a trend of combining environmental research with consumer products, which could then have a large impact on global conservation. In the temperate regions too, agroforestry can augment the supply of forest habitat and provide greater landscape connectivity. Where croplands occupy most of the landscape, riparian forest buffers and field shelterbelts can be essential for maintaining plant and animal biodiversity, especially under a changing climate scenario. A comprehensive assessment of shelterbelt agroforestry systems in the northern Great Plains of the USA has clearly demonstrated their importance on breeding bird species richness and community composition at both the farm- and landscape levels.
Carbon Storage Agroforestry systems have the potential to sequester substantial quantities of carbon (C). In addition to sequestering C in biomass and soil, these systems can contribute to both carbon conservation (conservation of carbon stocks in forests by alleviating the pressure) and carbon substitution (reducing fossil fuel burning by producing fuel wood). In temperate regions, agroforestry practices have been estimated to have the potential to store C in the range of 15–198 Mg C ha1 (mode: 34 Mg C ha1). The C sequestration potential of agriculture in the USA through the application of agroforestry practices by 2025 is estimated as 90.3 Tg C yr1 (Tg = 1012 g = million tons). In the tropics, agroforestry systems are estimated to have helped to regain 35% of the original C stock of the cleared forest, compared to only 12% by croplands and pastures. It has been estimated that an increase of 1 ton (Mg) of soil carbon pool of degraded cropland soils may increase crop yields by 2040 kg ha1. A projection of C stocks for smallholder agroforestry systems indicates C sequestration rates ranging from 1.5 to 3.5 Mg C ha1 yr1 and a tripling of C stocks in a 20-year period, to 70 Mg C ha1. Global deforestation, which is estimated to occur at the rate of 17 million ha yr1,
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is expected to cause the emission of 1.6 Pg C ha1 yr1 (Pg = 1015 g = billion tons). Global warming is now accepted as a real issue the world over. Ways of reducing CO2 in the atmosphere will undoubtedly be receiving increasing attention in the future. More than twice as much carbon is held in soils as in vegetation or the atmosphere; changes in soil carbon can have a large effect on the global carbon budget. Assuming that 1 ha of agroforestry could save 5 ha from deforestation, carbon emission caused by deforestation could be reduced substantially by establishing agroforestry systems. Along with these efforts, carbon markets will become stronger and more active. The opportunity offered by agroforestry systems to sequester carbon and market it for real money is real, even in poor countries. Water Quality and Environmental Amelioration Agricultural nonpoint source pollution is a significant cause of stream and lake contamination in many regions of industrialized world. A major causative source of this pollution is nutrients such as phosphorus (P) and nitrogen (N) that are lost from soils of fertilized agricultural and forestry operations, particularly in coarse-textured, poorly drained soils where drainage water ultimately mixes with surface water. It has been estimated that beef cattle operations on improved pastures (184 000 ha), unimproved pastures (33 500 ha), and rangeland (47 000 ha) on ranches are a large contributor of P loads to Lake Okeechobee, Florida, one of the largest freshwater lakes in the USA. Recent studies have shown that agroforestry practices such as silvopasture and riparian buffer could be a means of addressing the problem of environmental impact of nonpoint source pollution. The deeper and more extensive tree roots will invariably be able to take up more nutrients from the soil compared to crops with shallower root systems – the so-called ‘safety-net’ effect that has been affirmed in various agroforestry situations. Consequently, nutrient-leaching rates from soils under agroforestry systems where trees are a major component can be lower than those from treeless systems. Other studies documenting the effects of planted riparian forests on stream quality in temperate agricultural settings have also been reported, for example, in northwestern USA. The water-quality enhancement resulting from the reduction of nutrient loading could be a substantial environmental benefit of agroforestry in heavily fertilized agricultural landscapes. With increasing realization of the adverse impacts of chemical agriculture and climate change on availability and quality of water in many parts of the world, water is now a critical issue in natural resource management. Time-tested integrated land-use practices such as agroforestry could be appropriate approaches to addressing the problem. However, the science of this is nonexistent and needs to be explored and established.
The connectivity and buffering effect offered by the presence of agroforestry systems on the landscape is another important environmental benefit of agroforestry that is little appreciated. It is common knowledge that the increasing presence of trees of similar or different species in an agricultural setting will have many positive biophysical influences (see the sections, titled ‘Soil productivity and protection’ and ‘Carbon storage’). As the trees age, these influences increase in magnitude; therefore, it is to be expected that when trees are introduced into degraded agricultural landscapes (intercropping) and stream banks (riparian buffer), there will be gradual changes at the system (landscape) level. Long-term studies of this nature are rare, but some convincing examples are available (see the section titled ‘Ecological engineering: agroforestry system design’). The fire-protection benefit offered by extensive tree-based systems such as the dehesa in the Mediterranean region (Figure 4) is another benefit that has traditionally been enjoyed but never valued in economic terms.
Food and Nutritional Security The MPTs play a major role in food production in two ways: directly by providing edible products such as fruits, and indirectly by supporting food production through enhancing the soil’s ability to support agriculture. A large number of fruit-producing trees are integral parts of traditional homestead and other agroforestry farming systems with their characteristic multistrata canopies in many developing countries. Although several of these fruit trees have not been studied scientifically and are thus underexploited and little known outside their habitat, they make significant contributions to food and nutritional security. Another group of underexploited species of immense cultural and economic value are the natural medicinal plants (‘medicinals’). In Africa, more than 80% of the population depend on medicinal plants to meet their medical needs, and about two-thirds of the species from which such medicines are derived are trees. While the majority of these tree products are obtained by extraction from natural forests, some ‘well-known’ agroforestry tree species grown on farms for other uses (such as fodder, food, or fuelwood) are also used for their medicinal values. Examples include Acacia nilotica used in India and Africa; Azadirachta indica, the neem tree, used throughout Asia and Africa; Parkia biglobosa (nere´ or the locust bean tree) used in Africa; and Tamarindus indica, the tamarind tree, used in India and Africa. There is also increasing interest in natural medicines in the developed world, creating new or expanded markets for these products. This puts further extraction pressure on the natural forests. Many of the medicinal tree species are already overexploited. Some species are so depleted that their gene pools are greatly eroded (e.g., Prunus africana), and some are in danger of extinction.
Ecological Engineering | Agroforestry
Ecological Engineering: Agroforestry System Design Many agroforestry systems lend themselves to the concept of ecological engineering, whereby the ecological processes are used to solve engineering problems in such a way that ecosystems are designed, constructed, and managed for both environmental and societal benefits. The premise that structurally and functionally more complex agroforestry systems have greater efficiency of resource capture and utilization than either crop or tree monocultures provides a strong foundation for such efforts. An important consideration in design of agroforestry systems, however, is the interplay and importance of ecological, economic, and social attributes and objectives of agroforestry systems. As illustrated schematically in Figure 5, enhanced and sustained production of many typical ‘services’ normally associated with food or timber production systems is possible largely as a result of forced integration of trees, crops, and/or animals in such a way that interactions are created both above- and belowground. The resultant ‘production system’ feeds back at a number of scales to extra- and intrasystem components to provide a number of ecosystem services typical of those found in natural, undisturbed systems. For example,
recent studies that tree incorporation in pastures leads to retention of more phosphorus in the system and thus prevention or reduction of the chances of its transport from the coarse textured soils of Florida to adjacent water bodies corroborate the premise of silvopasture-system design. Another example is the intercropping of trees with agricultural crops in the temperate zone (Figure 1b), where trees are planted in widely spaced single or double rows in agricultural fields. Although the trees use up about 10% of land, this ‘loss’ will be more than offset economically and ecologically by production and service value coming from the tree component. Ecological benefits of intercropping that have been monitored in such a system in eastern Canada during a 20-year period include enhanced biodiversity (of birds, earthworms, and beneficial insects), soil organic matter buildup, synchronous development of tight nutrient cycles that prevent nutrients leaching into adjacent waterways, and higher sequestration of carbon both above- and belowground. Most economic models demonstrate equality between agroforestry and monocropping systems, based on the sometimes extended length of period associated with obtaining a long-term product from the tree component. When short-term, mostly nontimber products from the tree components are considered in conjunction with
Sustainable agroforestry systems
Production Enhancing production of food, fodder, fuelwood, timber, and non-timber products Interactions
Ecosystem services
Soil conservation Carbon storage
Aboveground
Radiation interception
Belowground N2 fixation nutrient cycling & deep capture
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Water quality enhancement Biodiversity conservation
Soil productivity improvement
Figure 5 A schematic presentation of the major mechanisms and processes involved in production and service attributes of sustainable agroforestry systems.
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large-scale societal benefits (e.g., improved water quality and reduced pesticide use), the economic models tip very much in favor of agroforestry systems.
Future Directions Agriculture and forestry are too often treated separately, yet these two sectors are often interwoven on the landscape and share many common goals and ecological foundations. The multitude of agroforestry systems, be they practiced in the tropics or temperate regions, are firmly grounded on strong ecological principles, and through provision of many basic needs and ecosystem services, they contribute to attainment of many regional developmental goals. These underexploited systems have the potential to develop into a set of major land-use options in the twenty-first century. The gains and developments of nearly three decades of agroforestry research and development are certainly impressive. Undoubtedly, agroforestry is now on a firm scientific footing. Today agroforestry is no longer a mysterious enigma that defies science and scientific principles, as it was perceived three decades ago. Agroforestry is well on its way to becoming a specialized science at a level comparable to those of crop science and forestry science. These hard-earned scientific gains need to be put to practice for solving the problems we had set out to address when we started research. The adoption rates of these systems have, however, been rather low so far, perhaps because of the emphasis on sustainability, rather than as an opportunity for immediate monetary gains. See also: Classical and Augmentative Biological Control; Ecological Engineering: Overview.
Further Reading Bene JG, Beall HW, and Coˆte A (1977) Trees, Food and People. Ottawa, Canada: International Development Research Centre. Brown LR (2004) Outgrowing the Earth: The Food Security Challenge in an Age of Falling Water Tables and Rising Temperatures. New York: W.W. Norton.
Buresh RJ and Tian G (1998) Soil improvement by trees in sub-Saharan Africa. Agroforestry Systems 38: 51–76. Evenson RE and Gollin D (2003) Assessing the impact of the Green Revolution, 1960 to 2000. Science 300: 758–762. Garrett HE, Rietveld WJ, and Fisher RF (eds.) (2000) North American Agroforestry: An Integrated Science and Practice. Madison, WI: American Society of Agronomy. Gordon AM and Newman SM (eds.) (1997) Temperate Agroforestry Systems. Wallingford: CAB International. Jose A and Gordon AM (eds.) (2007) Advances in Agroforestry 4: Towards Agroforesty Design: An Ecological Approach. Dordrecht: Springer Science. Kumar BM and Nair PKR (eds.) (2006) Advances in Agroforestry 3: Tropical Homegardens: A Time-Tested Example of Sustainable Agroforestry. Dordrecht, The Netherlands: Springer Science. Lal R (2004) Soil carbon sequestration impacts on global climate change and food security. Science (Washington, DC) 304: 1623–1627. Mosquera-Losada MR, McAdam J, and Rigueiro-Rodriguez A (eds.) (2005) Silvopastoralism and Sustainable Land Management. Wallingford, UK: CAB International. Nair PKR, Rao MR, and Buck LE (eds.) (2004) Advances in Agroforestry 1: New Vistas in Agroforestry: A Compendium for the 1st World Congress of Agroforestry 2004. Dordrecht, The Netherlands: Kluwer. Ong CK and Huxley P (eds.) (1996) Tree-Crop Interactions: A Physiological Approach. Wallingford, UK: CAB International. Palm CA, Tomich T, Van Noordwijk M, et al. (2004) Mitigating GHG emissions in the humid tropics: Case studies from the alternatives to slash-and-burn program (ASB). Environment, Development and Sustainability 6: 145–162. Rao MR, Nair PKR, and Ong CK (1998) Biophysical interactions in tropical agroforestry systems. Agroforestry Systems 38: 3–50. Rigueiro-Rodriguez A, McAdam J, Mosquera-Losada MR (eds.) (in press) Advances in Agroforestry 5: Agroforestry in Europe. Dordrecht: Springer. Schroth G, da Fonseca AB, Harvey CA, Gascon C, Vasconcelos HL, and Izac N (eds.) (2004) Agroforestry and Biodiversity Conservation in Tropical Landscapes. Washington, DC: Island Press. Steppler HA and Nair PKR (eds.) (1987) Agroforestry: A Decade of Development. Nairobi, Kenya: ICRAF (World Agroforestry Centre).
Relevant Websites http://www.cabi-publishing.org/AbstractDatabases – Abstracts Databases, CABI Publishing. http://www.aftaweb.org – Association for Temperate Agroforestry. http://www.fao.org – Food and Agriculture Organization of the United Nations. http://www.tradewindsfruit.com – Tropical Fruit Database, Trade Winds Fruit. http://www.icraf.cgiar.org – World Agroforestry Centre.