| Reforestation in the Pacific Islands |
|7. Agroforestry strategies for the community forester|
Agroforestry is a sustainable land management system that integrates the production of crops and forest plants and/or animals simultaneously or sequentially, on the same land. Agroforestry applies management practices that are compatible with the cultural practices of the local population, with the intent of increasing the overall yield of land through production of a variety of products, improving the local standard of living, and improving or protecting the quality of the environment.
Agroforestry techniques provide for the use of trees in traditional ways such as timber, fuelwood, and fruit farming. They also encourage new uses: intercropping nitrogen-fixing trees (NFTs) with agricultural crops, creating sustainable, year-round food production, and controlling soil erosion. Useful introductory references include Combe, 1982; Fillion and Weeks, 1984; King, 1980; Mongi, 1975; and Spurgeon, 1979.
Some agroforestry systems have been in place for thousands of years. Most, however, have relied on shifting cultivation (swidden agriculture) techniques and are therefore not sustainable over long periods of time in the same location. Traditional systems have also rarely incorporated fast growing NFT species, which offer many advantages over other types of trees. (See ICE Manual FC046; Haines and DeBell, 1979; and NAS, 1977, for details on NFTs.)
Most NFTs used are members of the legume family, often of the genus Mimoseae. Results from controlled experiments and field tests with NFTs in mixed agricultural systems have demonstrated that the overall productivity of the mixed system can be increased over that of traditional cropping or forestry systems alone. Of course, the agroforestry species mix must be carefully selected, proper care taken, and maintenance procedures followed.
The ability of certain trees to fix nitrogen is key to the success of many agroforestry projects. Nitrogen is abundant in the atmosphere, but not in a form readily usable by plants. NFTs fix nitrogen through the action of Rhizobia bacteria, which infect root hairs and remain in root nodules (see Ill us. 7-1). The Rhizobia are capable of fixing atmospheric nitrogen and then releasing it to the plant in a usable form. The plant absorbs the nitrogen, and some is incorporated into the leaves. Fallen leaves can then be collected and used as mulch to provide nitrogen to crops or as a protein rich fodder for livestock. Of the nitrogen in the leaves, 60% is generally lost to the air (though renitrification) or washed into streams. The other 40% is available as green manure to crops. This quantity is adequate to provide all of the nitrogen needs of most crops and is sufficient to increase yields by 89-100% (in controlled tests).
For NFTs to be effective, the correct Rhizobia strain must be available, healthy, and vigorous. The presence of healthy Rhizobia is indicated by bright red nodules. White nodules indicate an absence or an inadequate number of Rhizobia colonies.
Inoculants can be obtained from extension agents or the forestry service. If necessary, they can be grown in a laboratory using only basic equipment and simple culturing techniques. If lab culture is not possible, soil can be taken from the area around healthy stands of the same tree species that have active nodules. This soil can be used to treat seeds and other soil to initiate new colonies. Rhizobia do not grow well under acidic conditions. If the soil has a low pH, add lime.
Phosphate may also be a limiting factor for NFTs. Superphosphate or rock phosphate may be applied if signs of phosphate deficiency appear.