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  Introduction to Agroforestry

Agroforestry is the tradition, science, and art of growing trees and shrubs in combination with herbaceous crops and livestock. Agroforestry combines the sciences of agriculture and forestry, and requires attention to many economic factors and biological interactions. The primary motivation of agroforestry is reducing poverty in tropical and subtropical regions.

Techniques of agroforestry build on the considerable expertise of indigenous peoples developed over many centuries throughout the world. Especially in tropical and subtropical regions, native peoples have discovered sophisticated and locally effective techniques for growing trees and crops together. In the 1960s, after the green revolution, agroforestry came to the attention of scientists who were seeking agricultural innovations to benefit the poor. The scientific farming techniques of the green revolution had been very successful and yet produced little or no benefit for the poorest farmers. These people lacked the capital to buy seed, fertilizer, pesticides, and irrigation. Attempting to meet the needs of these people, agroforestry researchers look for low-input and sustainable farming methods. Proposed methods must also be economically and culturally acceptable for the particular society. Besides improving the standard of living, it is hoped that agroforestry and other sustainable farming methods will conserve soil, improve environmental quality, and prevent further destruction of tropical rainforests.


Classifying agroforestry systems

There are three main structural types of agroforestry:

  • Agrisilvicultural -- systems that include agricultural crops and trees

  • Silvopastoral -- systems that include trees and livestock

  • Agrisilvopastoral -- systems that include agricultural crops, trees, and livestock

Other criteria for classifying agroforestry systems include natural vegetation and climate. Vegetation in tropical regions consists of savannas and other grasslands (43%), broadleaf evergreen rainforest (30%), drier semi-deciduous and deciduous forest (15%), desert shrubs and scattered grasses (7%), and barrens (5%). Because temperatures are never cold in the tropics, the timing and success of crops depends mostly on seasonal rainfall patterns. One quarter of the tropical regions have high rainfall throughout the year. Fifty percent have seasonal rainy and dry seasons. Dry climates cover 16% and the remaining 9% are deserts.

Agroforestry systems are also differentiated based on the desired function, especially the function of the woody component. The main functions are economic production and environmental protection.

The goal of the system may be primarily commercial production, primarily subsistence, or a combination. Based on structure, climate, and function, an agroforestry project might be classified as, for example, a silvopastoral system in the sub-humid tropical savanna for cattle production for subsistence.


Shifting cultivation and improved fallows

Shifting cultivation, also known as "slash and burn agriculture", is an ancient farming system in which natural vegetation is cut down and burned. This releases a pulse of nutrients and reduces acidity of the soil. During this period, good crops can be produced. The period of cultivation is usually two or three years. During cultivation, fertility declines as nutrients are leached and organic matter decomposes. At the same time, it is progressively harder to prevent regrowth of wild vegetation. At some point, the farmers decide their efforts are better spent on clearing and burning a fresh area.

After cultivation, the area is allowed to regenerate to natural vegetation. The regeneration phase, or fallow period, usually lasts ten to twenty years, much longer than the cultivation period. While the land is fallow, the action of woody vegetation rebuilds soil fertility. Fallow periods in shifting cultivation should be long enough to allow full replenishment of nutrients so that productivity will be sustained over the long term. With increases in human population, fallow periods tend to be shortened in an effort to produce more food. If the fallow period is too short, the fertility of the soil will not be fully restored, and productivity will gradually decline.

A variation of shifting cultivation involves managing the fallow period. In this improvement, species that are known to contribute to soil regeneration are retained during the farming period. A further improvement is to plant preferred tree species so that they dominate the area during the fallow period. An ideal fallow species grows rapidly, efficiently recycles available nutrients, fixes nitrogen, and provides other benefits such as food. Species with these characteristics are useful in many agroforestry situations. Sesbania fixes nitrogen and most of the nitrogen goes to the foliage. Leucaenna diversiflora and L. leucophala provide many benefits including nitrogen fixation, erosion control, livestock feed, clean-burning firewood, poles, and seed pods for human consumption. Other important trees for nutrient enrichment are Acacia, Casuarina, and Gliricidia.


Alley cropping

Alley cropping is a method of growing food crops between hedgerows of shrubs and trees, preferably nitrogen-fixing species. The hedges are pruned periodically to prevent excessive shading of the growing crops and to provide biomass for various purposes. Alley cropping may be used with terraces or contour ditches. Ditches capture rain water and increase water infiltration into the soil, thereby making more water available to plants. On slopes, dense hedgerows are helpful for erosion control. When trees in the hedgerow need to be spaced farther apart, such as for production of timber and poles, grass species are often planted in between the trees to control erosion. Fruit trees are often planted in the hedgerows and sometimes in the ditch.

The most common mistake in alley cropping is failure to prune hedgerows. Pruning is necessary to allow sunlight to reach the other crops. Hedgerows may include trees species that are “coppiced.” Coppicing means regularly pruning to a height of around three feet. Another problem in alley cropping, especially in drier areas, is root competition between hedgerows and other crops. Also the ditches may wash out, creating gullies.


Taungya

Taungya ("tawn-ya") is a colonial form of agroforestry that is effective in replanting forest plantations. Farmers are permitted to grow crops following timber harvest during the reforestation period. The farmers help the young trees grow. The emphasis is on producing timber which is eventually sold by government or industry. The farmers do not own the land, and this system is considered exploitive by some.


Plantation crop combinations

Agroforestry multi-crop systems are rarely used in large scale commercial plantations. However, smaller plantations are less able to afford expensive technology and methods, so they turn to agroforestry. Furthermore, on smaller plantations, intensive management of multiple crops is more practical. Some small plantations focus primarily on the cash crop with incidental other crops. Other farms rely heavily on subsistence crops and livestock while also producing a perennial cash crop.

A wide range of species are used in plantations. Tree crops of cacao are grown in combination with other crops such as corn, cassava, banana, cucumber, or sweet potato. Rubber trees are grown with soy beans, corn, banana, peanuts, or poultry. Coffee is commonly combined with banana, corn, plantain, or dairy cattle. Coconut is one of the most versatile of agroforestry tree species. More than 32 annual and perennial crops can be interplanted with coconuts. Cashew is another popular tree because it tolerates a wide range of conditions and grows in combination with many crops. Coconut or cashew production can also be combined with grazing.


Multi-story home gardens and treegardens

Most impressive are the multi-story home gardens that produce a virtual supermarket of food and other household necessities. Common in rural areas of the tropics and subtropics, home gardens consist of herbaceous plants, trees, shrubs, vines, and livestock, all managed by family labor. A typical home garden produces food, medicine, fodder, firewood, and timber for family use. Home gardens also generate income through sale of surplus products. Of course, they also provide welcome shade and ornamental values.

Multi-story tree gardens are similar to multi-story home gardens but they are managed less intensively. They are usually away from houses and may be located on communally owned land. These forests contain a wide variety of multipurpose trees and other plants. Tree gardens often provide more nutritious foods to supplement a diet based mainly on field crops. These forests also provide medicines, firewood, timber for construction, and forage crops.


Tropical soils and agroforestry

Soils of the tropics can be challenging. These soils may have problems such as acidity, aluminum toxicity, high phosphorus fixation by iron oxides, low cation exchange capacity, and salinity. Soil nutrient deficiencies are common and have important effects such as influencing nitrogen fixation by soil microbes. Many of the problems are due to high rainfall which has leached and weathered the soil for thousands of years.

By maintaining trees on the land, agroforestry helps maintain ecosystem functions that reduce the effects of assorted soil problems. For example, tree roots help overcome nutrient deficiencies by bringing nutrients from deeper soil layers. Tree litter fall makes these nutrients available at the soil surface. Tree crops also help maintain soil organic matter, which is very important especially where cation exchange capacity is low. When planning an agroforestry project, tree species should be selected in consideration of the nutrient content of the soil. Also pruning of the trees should be timed to release the nutrients just when the herbaceous crop requires them. For example, when nitrogen-fixing trees are severely cut back, nitrogen is released from the root nodules and becomes available for the other crops.


Agroforestry in temperate regions

In temperate regions, agroforestry is not widely practiced. Farmers in these regions are highly successful using mechanized agriculture and monocultural (single crop) methods. They find that agroforestry methods are less profitable. For example, in mechanized farms, tree crops create obstacles for farm machinery and decrease efficiency.

Although mechanized monocultural agriculture dominates the temperate regions, agroforestry is useful for reducing erosion, improving fish and wildlife habitat, improving quality of life, and supplementing income. In temperate conifer forests, grazing can be combined with timber production. For instance, in ponderosa pine plantations of Oregon and northern California, grazing by cattle is timed to coincide with the palatability of the grasses that compete with tree seedlings. By bringing in livestock to control the grasses, tree seedlings have less competition and grow faster, reducing or eliminating the need for herbicides. Similarly, in southern New Zealand, timber production and sheep grazing are combined.

Other agroforestry techniques are used in the temperate regions, including windbreaks, shelter belts, riparian plantings, and special forest products. Windbreaks and shelter belts are important in arid regions such as the Great Plains. Another agroforestry success story in North America is production of annual crops or livestock in combination with nut trees including black walnut and pecan.


Diverse influences on agroforestry

  • Scientific Research

An important area for scientific research is development of shade tolerant crops. For example, in alley cropping or when producing timber products incombination with other crops, there is ample space for growing shade tolerant crops. Some crops grown for storage roots or leaves maintain a high level of productivity in the shade. Although research has made great strides in growing crops in full sunlight, a future challenge is development of shade tolerant crops that provide income and/or meet subsistence needs.

  • Economic viability

As we have seen, agroforestry attempts to reduce poverty in developing countries by providing alternatives to mechanized and chemically dependent agriculture. At the same time, agroforestry needs to be practical and economically viable. In the some cases, purist ideas can obstruct economic progress and may need to be abandoned. For example, nitrogen fixation by trees is often not competitive with simply purchasing nitrogen fertilizer. Synthetic fertilizers, applied wisely, have a role to play in sustainable agriculture. This is especially important in areas where land is scarce and high food production is imperative.

Using conventional economic analysis, the trees in the agroforestry system may appear to be less profitable than the other crops. However, conventional analysis doesn't tell the whole story. Another consideration is that a tree crop provides subsistence farmers with a kind of bank account that can be liquidated in emergencies or as a retirement fund. For example, when disease destroyed the cacao plantations near the Atlantic coast of Costa Rica and Panama, farmers who had interplanted with timber trees had a source of funds to finance their recovery. Although the "bank account" of trees provides an economic cushion, other crops must provide enough food and income to allow the farmer and family to survive until the timber harvest.

  • Overcoming cultural barriers

Agroforestry also faces cultural constraints. For example, a Peace Corps volunteer in Cameroon, Africa, was beginning a tree-planting project. In this area, the women were not allowed to own land. Planting trees was understood as establishing ownership of the land. Therefore women were not allowed to plant trees, despite the fact that everyone would benefit if women planted trees. Solution: Do not call the trees "trees", instead call them "flowers". Women are allowed to plant flowers. Problem solved.


References

P. K. Nair. 1993. An introduction to agroforestry. Norwell, Massachusetts: Kluwer Academic Publishers. 499 p.

Fletcher, Richard et al. Agroforestry in western Oregon. Corvallis, Oregon: Oregon State University Extension Service. 16 p.

Young, Anthony. 1989. Agroforestry for soil conservation. No. 4 in International Center for Research in Agroforestry (ICRAF) agroforestry series. United Kingdom: CAB International. 276 p.

Sanchez, P. A. 1976. Properties and management of soils in the tropics. New York: Wiley. 618 p.

Sanchez, P. A. 1987. Soil productivity and sustainability in agroforestry systems. In H. A. Steppler and P. K. Nair, eds. Agroforestry: a decade of development. Nairobi: ICRAF p 205-223.

Von Platen, H. H. 1990? Economic evaluation of agroforestry systems of cacao (Theobroma cacao) with laurel (Cordia alliodora) and poro’ (Erythrina poeppigiana) in Costa Rica. Agroforestry Project, Tropical Agricultural Research and Training Center, Turrialba, Costa Rica.

Whitmore, T. C. 1990. An introduction to tropical rain forests. Oxford: Clarendon Press. 226 p.


Periodicals related to agroforestry

  • Agroforestry Today

  • Agroforestry Systems

  • The Journal of Agroforestry

  • International Tree Crops Journal

  • Journal of Sustainable Forestry

Other sources of information

  • New Forests Fund - A project of the International Center for Development Policy

731 Eighth Street, S.E. Washington, D.C. 20003

Engaged in reforestation in 43 countries in Africa, Latin America, and Asia. Their training centers and nurseries spread knowledge and help rural people to escape from poverty and environmental deterioration. They help farmers adopt new species such as leucaena. Their strategy for reforestation and fighting poverty includes market plans for cashews, firewood, and other products.


  • Institute for Sustainable Forestry

P.O. Box 1580 Redway California 95560 707-923-4719 Fax 707-923-4257

Engaged in the study, practice, and promotion of sustainable tree growing and harvesting in the Northern California and the Pacific Northwest. They develop standards for environmentally sound wood products while also reducing forest fire hazard. Their approach is voluntary, community-oriented, and creates jobs.


  • Tree People

12601 Mulholland Drive, Beverly Hills California 90210

Engaged in tree planting in the cities and mountains of Southern California.


  • California Oak Foundation

1212 Broadway, Suite 810 Oakland California 94612 510-763-0282

A non-profit organization protecting and perpetuating native oak woodlands in California. They help local, regional, and state agencies develop ordinances and policies to protect oak woodlands. For example, they educate landowners about how estate taxes threaten oak woodlands and other open space. They also engage in education in schools and technical conferences.


  • US Forest Service, Rocky Mtn. Forest and Range Experiment Station

National Agroforestry Center, University of Nebraska - East Campus Lincoln, NE 68583-0822 (402) 437-5178


  • On-line photos of agroforestry applications

http://www.cgiar.org/icraf/res_dev/prog_5/tr_mat/slides/ag_tech/1_1.htm


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