|Agro-forestry in the African Humid Tropics (UNU, 1982, 162 pages)|
|Current agro-forestry activities|
Since the papers presented covered a wide variety of ideas and techniques, the discussion also touched on a number of topics, many of which had been discussed previously but in a different context. In particular, a number of comments on plant-plant interactions were made, and these again served to emphasize the need for further research and observations. Great care must be taken when interpreting results, and an example was cited where the tending had had a greater effect on the maize yield than the presence or absence of a tree crop. In another example the growth of Gmelina arborea was found to vary according to the annual crop, but this could have been due to the different weeding practices for maize and cassava rather than the type of crop. Cassava, in particular, was noted to be weed-tolerant and to have a negative effect on tree crops when planted too close. In short, management provides yet another set of variables that must be examined when evaluating agro-forestry practices.
Experience with intercropping Terminalia superba with cocoa, coffee, and banana was found to be relatively similar in Gabon, Congo, and Sierra Leone. Cocoa performance under T. superba in the Congo was particularly poor after 20 years because of excessive shading. As in Sierra Leone, both the overstorey timber crop and the understorey tree crop will be clear-felled after 40 years.
The better performance of food crops under former teak (Tectona grandis) plantations as compared to Cassia siamea stimulated much discussion. A number of factors were mentioned which could have caused this result, and these included the nitrogen-fixing ability of Cassia, the faster breakdown of Cassia leaves as compared to teak leaves, the relative abundance of ground fires in teak plantations, and the fact that the Cassia had been coppiced several times, a management operation which leads to more herbaceous growth, higher levels of soil organic matter, and greater soil aeration.
The successful introduction of improved varieties in southern Nigeria was further expounded upon. The basic technique used was to graft adult buds of fruiting trees on to young stock, and this led to early fruiting at reduced heights. At present the work is concentrating on native species, and the demand from the local people for improved material exceeds supply. Since the new varieties are intended for use in combination with other food crops, there is no substitution of fruit products for essential food crops. It was noted that relatively few crops have been successfully introduced, but since agro-forestry aims at sustainability rather than high, short-term yields, it may prove to be a more successful vehicle for introducing new crops or varieties. In the case of Rwanda, the dissemination of agro-forestry techniques to the local farmers was simplified by the fact that they were required to work on demonstration plots in each community. In this way they were exposed to new techniques and new crops, and over time many of the farmers adopted these.
In response to questions on the role of animals and forage species in agro-forestry systems in the humid tropics, one speaker noted that pasture and browse species could play an important role in reducing erosion and fires, but this depended on proper management and species selection. In the case of erosion one must maintain a sufficiently dense groundcover under the forest canopy and control grazing accordingly. The possibility of competition between browse and forage species was raised, but at least in the case of Gliricidia septum this was not felt to be a problem, probably because of its relatively deep rooting habits. The use of leguminous species for browse was also discussed, with particular attention to the question of toxicity. In general it was felt that the problem could be avoided by including the protein-rich legume with a variety of other feeds; specific tolerance levels had to be set for each animal with regard to each leguminous species. Toxicity can also be reduced by processing and plant breeding, and location is still another important variable.
In noting the high loss through volatilization of certain nutrients, particularly nitrogen when Leucaena cuttings are used as mulch, it was suggested that crop residues and other plant material might best be fed to ruminants. This would allow more efficient use of nitrogen and perhaps the organic matter as well.