Agroforestry is the deliberate incorporation of trees and other woody species of plants into other types of agricultural activities. By definition the use of woody species must result in the enhancement of either the biological productivity or the economic return of the system, or both. There are many types of agroforestry, which are usually defined by what type of agricultural activity is involved, but this can be a very broad definition and includes what we normally think of as agriculture (agroforestry), but also other combinations such as livestock production (sylvo-pastoral agroforestry) and even aquaculture (sylvo-aqua agroforestry). Even more complicated versions are possible such as agricultural systems that incorporate livestock, trees and aquaculture (sylvo-pastoral-aqua agroforestry).
Classification of Agroforestry Systems
In addition, agroforestry systems may be classified based on four interrelated criteria .
This criterion refers to the composition and the spatial and temporal arrangement of the system. Adding woody species can greatly alter the horizontal and vertical arrangement of plants in a system, often increases the diversity of the system, and typically increases the length of time that the system is in use.
This refers to the function of the woody component, (such as timber, fruit, fodder, etc). Typically, the inclusion of woody species increases the number of products generated by the system, but also includes functions such as the use of trees as windbreaks or to control soil erosion.
This refers to the purpose of the system, usually broken down into subsistence, commercial, or intermediate. In addition, agroforestry may be promoted to meet specific social goals such as poverty alleviation (Social forestry) or to improve community access to resources on communal lands (Community forestry).
This refers to the suitability of the agroforestry system for a given environment. Thus there are different types of agroforestry for tropical, temperate and arid environments which take into account the environmental, ecological, and biological conditions of each area.
The Biological Basis of Agroforestry
There are many ways in which agroforestry practices may enhance biological productivity. Soil fertility may be enhanced by planting nitrogen-fixing woody species between rows of crop plants. The foliage of many woody species may also be harvested to provide "green manure" for crop plants which provides nutrients, but also helps prevent soil erosion and water loss, and may help deter pests. Woody species also may be planted to provide shade for crop plants, to prevent soil erosion, or to act as windbreaks. All of these practices however may be summarized by saying that the addition of woody species to an agroecosystem has the potential to change both the physical structure of the ecosystem as well as the flow and retention of nutrients in the ecosystem.
The Economic Basis of Agroforestry
In addition to enhancing the biological productivity of agroecosystems, the addition of woody species often results in enhanced economic returns. The use of woody foliage as fodder may reduce the cost of feeding livestock, or may prevent the economic damage done by the loss of livestock during famine or drought. Usually, the woody species used in agroforestry themselves are valuable. Woody species may provide fruit, fiber, nuts, building and craft materials, medicines, timber, charcoal, and a host of other products which can be used on-farm or sold. Longer-growing tree species used in agroforestry are often used by farmers as a "bank account" which can provide cash income during years with bad harvests. As such, agroforestry practices not only diversify the economic products available to farmers, but may serve to increase economic security as well. Because agroforestry practices improve economic returns, agroforestry has long been seen as an ecologically and socially "friendly" means of sustainable development. In 1975, thre International Development Research Centre (IDRC) of Canada commissioned a study (Bene et al., 1977) to identify forestry methods to improve land use in low-income tropical countries (Nair, 1993). One result of this study was the establishment in 1977 of what would become the International Centre for Research in Agroforestry (ICRAF). In addition to conducting research and pioneering new methods, since 1982, ICRAF has helped disseminate information through its journal Agroforestry Systems published by Springer Press.
The Ecological Importance of Agroforestry
Farmers typically adopt agroforestry practices because those practices increase productivity and provide economic benefits. But, agroforestry practices can also have significant ecological effects which many consider as, or more, important than the potential agricultural and economic benefits. The basis of these ecological benefits lies in the fact that including trees and other woody species in agroecosystems changes the structure, diversity, and functioning of these systems, making them more like natural ecosystems than traditional types of agriculture which do not include trees and other woody species. One of the best examples of how agroforestry changes the structure and function of agroecosystems may be seen by comparing two type of coffee cultivation commonly referred to as "shade coffee" and "sun coffee".
Shade Coffee vs Sun Coffee
For the last three hundred years, most coffee traded on the international market has been produced under the shade of trees, but starting in about the 1970's new types of coffee that could tolerate higher exposure to the sun were promoted throughout the tropics. This so-called "sun" coffee could be planted in higher densities because shade trees which took up space were no longer needed. Sun coffee was promoted in tropical countries as a means of economic development. But, to take advantage of the increased sunlight and higher planting densities, additional inputs of fertilizer were often needed to achieve sustained increases in productivity per hectare. So too, with the elimination of shade trees, there came a need for increased applications of pesticides to control insect pests that previously had been kept in check by birds and other predators that could be found in shade coffee plantations, but were not present at the same densities in sun coffee plantations. As long as farmers were willing and able to pay for fertilizer and pesticides, sun coffee was more productive than shade coffee. But, with the increased production of sun coffee the international coffee market was eventually flooded with coffee beans, effectively lowering the price paid to producers and making it harder for farmers to pay for external inputs. Most importantly, even though this sounds like an economic story, the real story is ecological. The inclusion of trees in shade coffee plantations helped maintain productivity without costly inputs by making the structure and function of the shade coffee plantations more like the surrounding natural ecosystems. Although productivity per hectare is lower, fewer external inputs are needed because nutrients are capable of cycling through the system at about the required rate for shade coffee to grow well. Also, shade coffee plantations have been shown to harbor more bird and arthropod species than sun coffee plantations. In turn, those bird and arthropod species help control pest species. Furthermore, the realization that shade coffee provides habitat for migratory birds has led to shade coffee being marketed in North America and Europe as "songbird friendly" coffee, for which consumers pay a premium price. In this way, "song-bird" coffee demonstrates the full range of biological, ecological, and economic benefits possible in agroforestry
Agroforestry and Biodiversity Conservation
Shade coffee is just one example of how agroforestry can enhance biodiversity. Studies of other agroforestry systems have shown similar results leading to the realization that agroforestry practices have a tremendously important role to play in the current battle to conserve global biodiversity. Worldwide, the greatest threat to biodiversity is the conversion of more or less undisturbed habitat into agroecosystems for human use. Much of the remaining undisturbed habitat exists in the tropical forests of South America and Southeast Asia which also contain the greater portion of global biodiversity. The promotion of agroforestry practices in South America and Southeast Asia may help to conserve that biodiversity in at least four ways. First, in many places in the tropics, agroforestry is more productive, affordable, and sustainable than other agricultural practices. Because it is more productive, both biologically and economically, farmers need less land to survive. Secondly, because agroforestry systems more closely mimic natural ecosystems, they typically harbor higher invertebrate and vertebrate biodiversity. Third, agroforestry practices have been shown to enhance habitat for bird and arthropod species that act as pollinators and as the natural enemies of pest species protecting these two very important ecosystem services. And finally, agroforestry practices may protect and enhance other ecosystem services such as erosion prevention and groundwater recharge, which in turn helps prevent the degradation and loss of surrounding habitat.
Cacao Agroforestry and the Atlantic Forests of Brazil
Perhaps the greatest success story so far concerning the role of agroforestry in conserving biodiversity is in the lowland tropical forests on the Atlantic coast of Brazil. These forests are both highly diverse and widely used for agriculture. As of 2006, about ???% of these forests have been converted into agriculture of one type or another. The region however also contains about ??? hectares of shade-grown cacao plantations; which function much like shade tree coffee plantations elsewhere in the tropics. Farmers in the region have successfully resisted converting their plantations to what could be called "sun-cacao" for many of the same reasons described above concerning the differences between sun- and shade-coffee. If it were not for these extensive shade-grown cacao plantations, the Atlantic forest ecosystems and the remaining rich biodiversity they harbor, would be almost completely lost.
- ^Nair, 1993.