| P. K. Ramachandran Nair, University of Florida
| Andrew M. Gordon, University of Guelph
Agroforestry has been defined in various ways. The World Agroforestry Centre (http://www.icraf.cgiar.org) defines it 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 (http://www.aftaweb.org) 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.” Several other definitions are also available. In essence, they all refer to the practice of the purposeful growing of trees and crops, and/or animals, in interacting combinations, for a variety of benefits and services.
The practice of growing trees and crops together 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 to 9 000 B.C.
Agroforestry in Europe is reported to have started when domestic animals were introduced in forests for feeding around 4000 B.C. 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 have been brought under the realm of modern, scientific land-use management scenarios.
The motivations for these initiatives were several. In the tropics, the Green Revolution of the 1970s largely did not reach the poor farmers and those in less-productive agroecological environments. In addition, defective land-management practices resulted in increased tropical deforestation, fuelwood shortage, soil degradation and biodiversity decline. The search for strategies to address these problems focused the attention on the age-old practice of combining 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 an increased perception on the part of the general public about the environmental consequences of high-input agriculture and forestry. For example, soil erosion by water and wind is estimated to affect 102 million hectares (252 million acres) or approximately 30% of crop lands in the United States alone causing a loss of 1.93 Pg (billion metric tons) of soil annually. The energy dependency of US agriculture is also well documented: when the entire American food system, from field to table, is considered, 42 kJ of energy input is estimated to be required for each kJ consumed.
The single-species emphasis of food and wood production in commercial systems has caused considerable decline of biological (including genetic) diversity: compare the diversity of corn (Zea mays), soybean (Glycine max) or pine (Pinus spp.) production systems with the approximate 100 species found in an oak (Quercus spp.) – hickory (Carya spp.) forest.
The list of drawbacks to modern agriculture practices can be long. It is 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 are 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. Public demand for environmental accountability and application of ecologically compatible management practices increased when these problems associated with row-crop agriculture and forestry became clearer. Consequently, the concept of agroforestry gained acceptance as an approach to addressing some of these problems. That led to the development of agroforestry applications in North America and other temperate zones such as Australia and New Zealand, Europe, and China, demonstrating the range of conditions under which agroforestry can be successfully applied.
Agroforestry: An Integrated Science and Practice
The concept of agroforestry stems from the expected role of on-farm and off-farm tree production in supporting sustainable land-use and natural resource management. This concept 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 (nutrient, light, water) capture and utilization, and greater structural diversity that entails tighter nutrient cycles. While the above- and below-ground 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.
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 is commonly expressed in four key “I” words: intentional (intentionally designed system), intensive (managed intensively for productive and protective benefits), interactive (biological and physical interactions among the system’s components – tree, crop, and animal), and integrated (the structural and functional combinations of the components as an integrated unit). 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) are the cornerstones of all agroforestry systems.
A large number of traditional as well as improved agroforestry systems have been recognized from different parts of the world (Table 1). Numerous and diverse agroforestry systems can be found in the tropics, partly because of their favorable climatic conditions, and partly because of the socioeconomic factors such as human population pressure, more availability of labor, smaller land-holding size, complex land tenure, and less proximity to markets. Fundamental to the realization of the promise of tropical 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). 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.
| Unlike the production emphasis in the tropics, environmental protection and, to some extent monetary return, are the main motivating factors for agroforestry in the industrialized nations. Alley cropping, forest farming, riparian buffer strips, silvopasture, and windbreaks are the five major agroforestry practices recognized in North America. Other temperate-zone agroforestry systems include the ancient tree-based 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. As noted, the dehesa system, grazing under oak trees with strong linkages to recurrent cereal cropping in rangelands, is also a very old European practice.
Ecosystem Service of Agroforestry
A common thread found in the many historical definitions of agroforestry is the reference to the systems nature of this multi-faceted land-use system. However, the 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 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 to 6 Mg dry matter (biomass) ha-1 yr-1 (depending upon species) for temperate coniferous forest plantations, certain agroforestry systems in the tropics such as the multistrata (vertically stratified) homegardens and shaded perennial systems can exhibit NPP in excess of 15 Mg ha-1 yr-1.
Agroforestry systems, regardless of type, are capable of providing numerous ecological goods and services, of a range of complexities, over long periods of time. Indeed, agroforestry systems can be designed and engineered to provide specific quantities of particular goods and services. Nonetheless, the universal application of ecological principles to agroforestry system design and management is nearly impossible as a result of the many varied types of systems in existence – from riparian management systems that link terrestrial and aquatic systems to more traditional systems that integrate perennial plants with annual crops, with or without animals. The broad geographical range over which agroforestry systems may be successfully implemented and the scale at which interactions occur – from landscape to individual plant – also complicates the development of a universal understanding of nutrient and energy flows and the relationship of these to system productivity.
Yet another difficulty in dealing with agroforestry systems is that the effects of many of the ecosystem services and productivity benefits cannot realistically be measured in quantitative terms in the 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.
Although the systems differ in the nature and types of environmental services provided, some generalizations can be stated. Most agroforestry systems will tend to improve soils, including productivity, largely through the incorporation of organic matter and carbon into upper soil profiles, resulting from the production of annual litterfall from the tree component. As a result of the presence of perennial root systems, soil erosion relative to monocropped agroecosystems is often minimized. With respect to the maintenance and proliferation of biodiversity, agroforestry systems often enhance components of biodiversity, at scales ranging from the stand (farm-level) to regional landscapes. This is especially true in the fragmented agricultural landscapes of North America.
In intercropping systems, microclimate modification is common, and although the competition for water, light and nutrient resources between the tree and crop component is complicated, improved and sustained crop yields have been noted. In all systems, the storage of carbon is enhanced, not only in the perennial nature of the trees, but as also reflected in increased soil carbon. Improving the quality of surface water that is adversely impacted by runoff from heavily fertilized pasture systems and improved carbon sequestration are additional environmental benefits of silvopastoral agroforestry systems that are just beginning to be realized in quantitative terms.
Another important benefit of adopting these types of systems may actually be found in the benefits provided in terms of animal welfare – sheltering crop animals from extremes in weather. Integrated riparian management systems address the interaction of terrestrial and aquatic environments in farming landscapes and can make major contributions to water quality at local scales and provide connectivity in agricultural landscapes at much larger scales.
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 is the interplay of ecological, economic, and social attributes and objectives.
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 intra-system components to provide a number of ecosystem services typical of those found in natural, undisturbed systems.
For example, recent studies in Florida have shown 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 to adjacent water bodies. Another example is the intercropping of trees with agricultural crops 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 10-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 below ground.
Many 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 non-timber products from the tree components are considered in conjunction with large-scale societal benefits (e.g. improved water quality, reduced pesticide use, etc.), the economic models tip very much in favor of agroforestry systems. However, the simplicity of single species for large scale operations and perhaps the excessive focus of much research and extension on monocultures have favored their adoption despite many demonstrable agroforestry advantages.
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 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 under-exploited 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. Agroforestry is now on a firm scientific footing, and is well on its way to becoming a specialized science at a level comparable to those of crop science and forestry science. 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. A deserving appreciation of the ecosystem and environmental services provided through agroforestry could lead to development of rigorous environmental economic assessment and eventually to modification of community tax structures and environmental legislation such that environmental benefits become bankable and environmental integrity becomes a societal norm.
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P. K. Ramachandran Nair is Distinguished Professor, School of Forest Resources and Conservation, IFAS, University of Florida, Gainesville, Florida 32611, USA; email@example.com
Andrew M. Gordon is Professor, Department of Environmental Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada; firstname.lastname@example.org
Posted 27 February 2008