| CERES No. 99 - May-June 1984 |
Deforestation causes a serious production problem for yam growers in developing countries. Poles, which are essential to support the sunlight demanding yam plant and promote high yields, are becoming scarce and expensive. In some areas, poles are ought in from remote forested areas and purchased by yam farmers at an most prohibitive price. Similar problems are faced by growers of :her climbing food crops (such as beans) that require poles for support.
A scarcity of yam poles - and poles for other climbing food crops - not a problem for some farmers in e Philippines and Indonesia, who use living yam poles. On Mt. Makiling near the University of the Philippines Los Banos this unique agroforestry stem has been traced back to 1906, when farmers planted a common type of Leucaena leucocephala (called ipil-ipil in the Philippines) to shade out a noxious grass Imperata cylindrica (cogan grass) that had colonized e mountainsides. Once the mountain sides were reclaimed, yams were planted near the base of the Leucaena.
Yams in their initial stage of growth better under partial shade and the shade of the Leucaena promotes the early growth of the yams, explains Michael D. Benge, a USAID agroforestry officer who has studied the stem. When the yam reaches the stage of growth where energy is being converted to tuber growth, the Leucaena tree is girdled at about 40 cm from the ground to kill the tree canopy. Once girdled, Leucaena quickly drops its leaves, allowing full sunlight on the yam vines. Only the top portion of the Leucaena is killed, and the dead stem provides a study pole to support the yam. The Leucaena beneath the girdle coppices, and the new sprouts, are cut and applied as a mulch or carried home to feed livestock. When the yam is ready for harvest, the dead pole is cut into firewood lengths and used for household consumption or sold at the roadside. One strong coppice is allowed to grow to form a new pole.
Other crops, such as onions, garlic, and squash, are intercropped among the Leucaena and yams, which in creases the farm's productivity. Diversification minimizes insect and disease problems, and the fertile soil - enhanced by the nitrogen-fixing ability, the deep tap root system (which draws nutrients up from the subsoil), and the nutrient rich leaves of the Leucaena which increase the soil organic matter) - make these among the most productive hillside farms in the Philippines, according to a study prepared by Benge for USAID's Bureau of Science and Technology.
A similar type of cropping system is evolving in the Tana Ai area in the eastern part of Sikka District, Nusa Tenggara Timur Province, southeastern Indonesia. There, slash-and-burn cultivators are establishing more sedentary fields by growing contour hedgerows combined with a fallow cover crop of the giant type of Leucaena.
The contour hedgerows (referred to as wash tops) are composed of the common type of Leucaena. A natural terrace forms behind the contour hedgerows, which control erosion and reduce runoff into the terrace below. The mass of lateral roots, as well as the deep tap roots of the hedgerow-wash-tops, form a network that binds the soil, and the stems provide a barrier on which biomass and soil collect. This results in a natural back slope forming on the terrace.
Somewhat similar to the Philippine system, the trunks of the Giant Leucaena are girdled by cutting a ring of bark from the tree 20 cm wide and 1.5 cm above the ground before the planting of upland rice, maize, and other food crops. Because the leaves wither and fall in a short time, competition of the trees with the food crops for light is not a problem. New re-growth (coppice) emerges below the girdle, a single strong sprout being allowed to form a new tree after the killed stem has been harvested.
The Giant Leucaena provides an ideal fallow cover crop' which readily fertilizes the field while protecting soil from erosion and increasing water percolation into the soil. The Leucaena canopy also shades out most of the undesirable weeds, grasses, and under story. Girdling the trees requires much less labour than felling trees and bunching and burning the slash. Burning is not necessary - once the trees are girdled, the top portions die and the leaves soon drop to the ground and decay rapidly; thus no trunks and branches remain on the field. The killed and girdled trunks serve as poles for climbing plants, such as yams and beans, and after the yams are harvested, the trunks are cut for firewood.
The International Institute of Tropical Agriculture (IITA) in Nigeria has also experimented with living yam poles, but has concluded from its research that yam yields were reduced. However, in that system, Benge points out, instead of girdling the stem near the ground, they lopped the top of the Leucaena at a height of over 2 m. Thus, the coppice re-growth created a shade canopy. which reduced yam yields. When girdled closer to the ground, coppice does not create a high shade canopy.
From their research, IITA concluded in its report, Research Highlights for 1979, "The major disadvantage observed in the improved system is the excessive growth retardation caused by weeds on the fallow plant's growth during the early stage of development. Weed control thus becomes essential if the trees are to reach usable sizes in the shortest time possible." Benge says that this refers to the initial establishment and growth of tree seedlings, since in Nigeria weed growth is profuse in both the wet and dry seasons. The people of Tana Ai in Indonesia took care of this "disadvantage" by interplanting the Leucaena among their food crops, which they weeded anyway. The coppice on girdled trees has a height advantage over the weeds and coppice grows more rapidly (because of the already established extensive root system) than newly established seedlings. If the trees are not spaced too far apart, the canopy closes during the food crop fallow, shading out the weeds.