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Effect of food crops on tree growth in Tanzania
J.F. Redhead and J.A. Maghembe
Division of Forestry, University of Dar es Salaam, Morogoro, Tanzania
Village afforestation must meet the needs of farmers for quick tangible returns. The growth of food crops among trees appears to be a promising incentive. Research to ascertain appropriate species and mixtures, and appropriate espacements and rotations to maximize production, are described.
Good food crop yields are obtained in the first year of intercropping but fall off in the second and subsequent years with trees at close espacement. Wider tree espacement is likely to prolong useful food cropping. Trees intercropped with food crops show growth as good, or nearly as good, as trees grown under clean-weeded conditions and much superior growth to trees raised under spot-weeded or unweeded conditions
It is recommended that similar trials be established for various tree and food crops over a range of ecological sites. These will serve also as valuable demonstration plots.
King and Chandler (1978) emphasize that the term agro-forestry may be applied to a wide spectrum of land-use combinations. These range from taungya (or agrisilviculture) to the selective use of trees as shelterbelts in a predominantly agricultural system.
Many traditional farming systems already include the permanent cultivation of a balanced mixture of tree species and agricultural crops. The trees often provide an immediate economic return in terms of direct products in addition to the indirect benefits of improving the ecological conditions for the food crops and animals.
Examples in high rainfall areas are the integrated farming systems developed in Latin America, where Cordia alliodora and Cedrela odorata are grown over lemon, bananas, and plantains, which are in turn underplanted with coffee, or the Kandy spice gardens of Sri Lanka, where tree crops such as nutmeg, cloves, and jackfruit are underplanted with coffee, bananas, pepper, and vegetables (cf. the paper by Watson in this volume, pp. 6-12). In Africa a good example is the land use on the slopes of Mount Kilimanjaro, where coffee, bananas, and annual food crops are intermixed with timber trees such as Grevillea robusta.
Trees have also been combined with a more limited agricultural cropping programme. For example, the taungya system was designed primarily for the establishment of permanent timber crops. It was most successful in areas where there was a shortage of good agricultural land. King (1968) expresses concern that the peasant labour involved is exploited because the forest services benefit from their efforts and pay no remuneration. However, it is noteworthy that despite dilution with the tree crop, per hectare yields to the farmers are higher than on land available to them outside the reserves. In fact, Hofstad (1978) has calculated that in north-eastern Tanzania the value of the food crops grown under the taungya system may far outweigh the discounted value of the subsequent tree crops.
Peasant farmers in all countries are naturally conservative: change usually involves risk, and they are unwilling to undertake change without convincing evidence of guaranteed personal financial or material advantages. They have very limited capital reserves to sustain them; thus, change should provide immediate, or short-term, returns. Furthermore, peasant farmers belong to closely integrated communities that are economically interdependent. Suggested changes should benefit the whole community and cause minimal disruption to existing social structures and customary practices.
The "felt needs" of a rural community are for a sustainable system of land management that will produce acceptable food for people and animals, maintain fertility and water balance, and provide a renewable source of firewood and building poles within walking distance of the village. A successful agro-forestry system for a farming community must meet specific needs in these areas, must provide obvious short-term personal advantages for the farmer, and, as far as possible, should be consistent with local traditional working habits and patterns of family labour,
Intercropping of selected tree species and agricultural crops may be competitive or collaborative. Traditional systems have developed over time to select effective mixtures; development of new systems will require research into many factors, including appropriate species and mixtures, and appropriate spacings and rotations to maximize production.
The Division of Forestry at Morogoro, Tanzania, is currently initiating research to assess the effects and productivity of intercropping selected tree species with staple food crops over an extended period.
Agro-forestry Research at Morogoro, Tanzania
The rainfall at Morogoro is about 800 mm a year, falling mainly between March and May, but sometimes with a substantial amount in November. Unfortunately for the farmer it is very unpredictable in its regularity. Although Morogoro does not lie within the humid tropics, the research being carried out is of wider application. The investigations are simple in design, the field work is easy to carry out and applicable over a wide range of ecological conditions. Very important also, this type of trial gives quick interim results that can be easily seen and understood.
Trials of Eucalyptus melliodora with Different Crops
The layout of the trial and early results have been previously described by Maghembe and Redhead (1981). Containergrown Eucalyptus melliodora were planted in February 1978 in farmland that had been ploughed and harrowed. The tree seedlings were planted at a spacing of 2.5 x 2.5 m and intercropped with maize, sorghum, and beans, which were planted at 90 x 30 cm, 60 x 15 cm, and 40 x 20 cm, respectively. The same three crops were planted again in 1979 and 1980. Yields of maize and beans were recorded; sorghum was not harvested as the crop was eaten by birds. Both weed-free and unweeded plots of E melliodora were also grown as controls. No fertilizers were used, and the experiment was arranged as a latin square. Each plot contained 5 x 5 trees, and the central core of 3 x 3 trees was periodically measured. The area was clear-felled in March 1981, and the leaf litter and standing biomass assessed. The same food crops were sown again so that yields along with coppice could be ascertained.
The yield of maize was 1,280 kg/ha in the first year and 100 kg/ha in the second year. In the third year it did not flower. Sorghum followed a similar pattern, although yields were not recorded. At 2.5 x 2.5 m spacings, the trees were too close to permit sufficient light for maize and sorghum to grow after the first year. The yields of beans for the three years were poor; in the first year, when good yields were expected, the crop was badly attacked by an unidentified fungus. In the second year the beans appeared healthy but the yield was only 150 kg/ha. In the third year the beans were etiolated and the yield was insignificant. At the time of the first harvest, the mean height of the Eucalyptus in unweeded plots was significantly lower than Eucalyptus in intercropped or weed-free treatments, in which heights did not differ significantly. The trees in the beans and clean-weeded plots were noticeably more robust and more heavily branched than the trees among maize and sorghum, which were spindly due to competition for light (figs. 1-2). Survival of the trees has been approximately 90 per cent, except in the unweeded plots, where half died during the first two years.
FIG. 1. Mean Height of Eucalyptus melliodora Inter" cropped with Maize, Sorghum, and Beans, Compared with Clean-Weeded and Unweeded Trees, Morogoro, Tanzania
FIG. 2. Volume of Eucalyptus melliodora Intercropped with Maize, Sorghum, and Beans, Compared with Clean-Weeded and Unweeded Trees, Morogoro, Tanzania
At three years of age, the stand has yielded more than 1,000 poles/ha suitable for house building and more than 6 m³ /ha of fuelwood. The mean size of trees grown among beans is not significantly different from that of those grown in weed-free plots, and the trees intercropped with maize and sorghum are two-thirds the size of clean-weeded trees. The maize and sorghum yield would clearly make up in value for the reduced volume of the tree crop.
It was apparent from this trial that normal yields of agricultural crops could be expected in the first year with little effect on the tree crop in the case of Eucalyptus melliodora, but for food production in subsequent years wider spacing of the trees is necessary.
Trials of Trees at Different Spacings with Crops
It was primarily to determine how far apart the trees could be and still produce an acceptable volume of firewood or appropriately shaped poles that a series of more elaborate long-term investigations was laid down in 1980. Another objective was to test tree species that would provide both fuel and fodder. Four trials were established: (1) Eucalyptus camaldulensis (planted for fuel and pole production) with maize and beans; (2) Acacia albida (planted for fodder and fuel production) with maize and beans; (3) Leucaena leucocephala (planted for fuel production) with maize and beans; (4) Leucaena leucocephala (planted for fodder production) with maize and beans.
The layout of these investigations has been described by Maghembe and Redhead (1981). Briefly, tree seedlings were container-grown and planted in ploughed and harrowed land. Each investigation has a split-plot layout with food crops (maize or beans) and weeding treatments (spot-weeded or clean-weeded) forming the main plots, and tree spacings forming the sub-plots. The maize was planted at 75 cm x 30 cm, leaving a circle of 50 cm radius around each tree; fertilizer was applied at a rate of 400 kg/ha ammonium sulphate and 200 kg/ha triple superphosphate, in two applications. The beans were planted at 40 cm x 20 cm, leaving a circle of 20 cm radius around each tree seedling; fertilizer was applied at a rate of 200 kg/ha ammonium sulphate and 200 kg/ha triple ammonium phosphate, with the former applied when the beans were well established. Clean weeding was done by harrow, supplemented by hoeing; no fertilizer was applied in these plots. Spotweeding was done by hoe in a circle of 50 cm radius around each tree, as in normal Tanzanian forestry practice; no fertilizer was applied. This main-plot treatment was omitted from trials 2 and 4.
With regard to the spacing, trials 1 and 3 had sub-plots with trees at 3 x 3 m, 4 x 4 m, and 5 x 5 m, respectively. In trial 2, the 3 x 3 m spacing was replaced by a 6 x 6 m spacing in order to maximize fodder production. Similarly, in trial 4, the Leucaena leucocephala was planted in rows 3, 4, 5, and 6 m apart to maximize fodder production. Trees within rows were only one metre apart. In all four trials sub-plots without trees were planted as controls.
Long term soil studies are in progress so that soil under Eucalyptus, Acacia, and Leucaena can be compared with soil under fallow and under pure food crops. The nitrogenfixing activity of the Acacia and Leucaena is being measured with a portable gas-liquid chromatograph.
The trees in all investigations were too young to have had a marked effect on the maize and bean yields during the first cropping season apart from the space they occupied. Differences are expected to show up from the second year onwards, when the canopy starts to close. Unfortunately, the yield of maize and beans was normal in only one set of plots combining Leucaena. In this case, the mean yield of maize was 1,645 kg/ha, which compares favourably with yields obtained on the university farm and is over twice the national average in Tanzania of 670 kg/ha (Acland 1971). The mean yield of beans was 401 kg/ha, an average yield by peasant standards (Acland 1971).
In the other investigations the planting time and tasselling time of maize coincided with a severe drought, and growth yields were very poor and uneven. The drought caused great variation in the interaction between the food and tree crops and masked the significance of the results.
In contrast with the little effect the trees had on the food crops, the maize in particular has had a marked effect on the trees because it is a tall crop casting considerable shade (figs. 3-6). In all investigations, at the time of maize harvest, the trees among maize were taller than those in other treatments-24 per cent taller in both stands of Leucaena and 20 per cent taller in the Eucalyptus The height differences were statistically highly significant except in the Acacia plots, where the maize-grown trees were only 13 per cent taller. After harvest, the clean-weeded trees grew best, and in all cases their height surpassed that of trees in other treatments by the end of the year. The spot-weeded treatment has proved much inferior to the other treatments, as the trees are only two-thirds of the height of clean-weeded trees and approximately 20 per cent less in height than trees intercropped with either maize or beans. These growth trends are emphasized even more by the differences in root-collar diameter (fig. 7).
FIG. 3. Mean Height of One Year-Old Eucalyptus camaldulensis Intercropped with Maize and Beans Compared with E. camaldulensis Grown CleanWeeded and Spot-Weeded, Morogoro, Tanzania. The vertical bars indicate differences significant at P = 0.05; N.S. indicates differences not significant at P = 0.05.
FIG. 4 Mean Height of One-Year-Old Acacia albida Intercropped with Maize and Beans Compared with Clean-Weeded A. albida. The vertical bar indicates a difference significant at P = 0.05; N.S. indicates differences not significant.
FIG. 5. Mean Height of One-Year-Old Leucaena leucocephala Grown for Fuel and Intercropped with Maize and Beans Compared with That of L. Ieucocephala Grown Clean-Weeded and SpotWeeded. The vertical bars indicate differences significant at P = 0.05.
FIG. 6. Mean Height of One-Year-Old Leucaena leucocephala Grown for Fodder and Intercropped with Maize and Beans Compared with That of L. Ieucocephala Grown Clean-Weeded. The vertical bars indicate differences significant at P = 0.05; N.S. indicates differences not significant.
FIG. 7. Mean Root-Collar Area of One-Year-Old Trees Intercropped with Maize and Beans Compared with That of Clean-Weeded and Spot-Weeded Trees, Morogoro, Tanzania. The vertical bars indicate differences significant at P = 0.05; N.S. indicates differences not significant.
The results demonstrate that good food yields can be obtained, at least during the first year of intercropping with trees. The trees benefit more from the weeding associated with growing food crops than they do from the Tanzanian standard practice of spot weeding. Moreover, spot weeding allows a dense growth of grass to grow in the intervening spaces, and this is a serious fire hazard. Clean weeding gives the best growth, but it is not realistic to expect this in community afforestation projects.
It will be interesting to monitor the subsequent yields of food crops at the wider tree spacings. If a valuable pole and fuel crop is nearing harvest by the time food yields drop seriously, agro-forestry in this form could become a standard practice in community afforestation projects. Should tree intercropping prove beneficial in improving soil fertility, this will have great significance for areas where a shortage of land precludes a long natural fallow.
It is suggested that trials of similar design be established for a range of tree and food crops over varied ecological sites in tropical Africa. These will serve as valuable demonstrations and at the same time yield useful data for afforestation.
Selection of leguminous trees for agro-forestry in Cameroon
Anthony Mapri Maimo
Institute of Agronomic Research, Yaounde, United Republic of Cameroon
From 1977 to 1981 the Institute of Agronomic Research (lRA), through its Centre for Forestry Research in Edea, Cameroon, conducted a project with the objectives of: identifying the c/imatic and ecological factors of Edea (from literature, meteorological services, and soil analyses); studying the farming systems and the utilization of forest products in the dense humid forest zone (through farmer surveys and observations); and selecting leguminous forest species of agronomic and forestry interest (through elimination trials). The soil analysis confirmed that the soil is nutrient-poor. The study of farming systems showed that, in Edea, the peasant farmers cultivate 0.5-4.2 ha and that collective farming is atypical. Furthermore, the farming system is essentially a shifting agriculture involving a fallow of three to seven years. Mixed cropping is common, and the farming practices involve very simple technology (cutlasses, hoes, and fire). Land used by the community is often subject to erosion. Finally, the peasant population thrives principally on cassava, taro (Colocasia esculenta), macabo (Xanthosoma sagittifolium), yams, sweet potatoes, bananas, and plantains. Also planted are maize, groundnuts, and beans. The forest trees associated with traditional farming systems were found, as in the case of the humid forest zone of Western Nigeria (Getabun 1980), to be Leucaena leucocephala, which is planted on cocoa and coffee farms as a shade tree, and Treculia africana, Irvingia gabonensis, Dacryodes edulis, Samanea (Pithecolobium) saman, Cassia siamea, and Pterocarpus soyauxii. Tree elimination trials led to a preliminary selection (made on the basis of mean height growth attained after 18 months), in order of descending importance, of Albizia falcataria, Samanea saman, Albizia lebbeck, Leucaena leucocephala, and Pterocarpus soyauxii as suitable for use in subsequent experiments in Edea because of their potential for yam-supports, shade trees, improvement of soil, and pulp production - a matter of prime importance to CELLUCAM, the pulp manufacturing company established in Edea.
The importance of agro-forestry research as a tool to improve agricultural production, and therefore the standard of living of the people, in the humid tropical zone was brought home to the research sector (the National Office for Scientific and Technical Research at the time) in the United Republic of Cameroon in 1976 by the International Development Research Center (IDRC). This importance was stated to lie in the possibility that research could formulate agro-forestry systems to replace the shifting agriculture that is prevalent in many humid tropical zones. In particular, agro-forestry systems promised that the use of trees-especially leguminous forest trees- to replace the bush fallows could ensure the improvement of the soil and, at the same time, provide forest produce that could yield revenue to the peasant farmers.
An agro-forestry project was, thus, conceived and agreed upon by IDRC and the United Republic of Cameroon in 1976. The first phase of the project was to last three years (i.e., until April 1980), but in fact it did not end until March 1981. This was due, in large part, to the delay in purchasing the project vehicles.
The objectives defined for the project were: to identify the climatic and ecological factors of Edea, which is located in the dense humid forest of Littoral Province; to study the farming systems and the utilization of forest products in this zone; to select leguminous forest species that would be of agronomic and silvicultural interest in the zone being studied and to determine the propagation techniques for such species; and to establish experimental plantations with the object of studying the effects of leguminous species on soil impoverished by shifting cultivation.
The study of climatic and ecological factors was carried out partly through a review of existing literature, partly through contact with the meteorological services at Edea, and partly through analysis, in the soils laboratory at Ekona, of soil samples taken from the principal soil types of Mangombe, Edea. With regard to the latter, six samples of upland, sandy soil (30 cores were bulked to yield a sample) from Mangombe (under forest, under food crops, and under fallow) were taken and analysed in the laboratory at Ekona. Again, in May 1978, 15 samples were taken at different depths under natural secondary forest, food crops, five-year old fallows, and leguminous forest species (Afzelia pachyloba, Piptadeniastrum africanum).* The soil studies of May 1978 were repeated in May 1979 to cover the upland as well as the other two subtypes of Mangombe, i.e., gravelly and hydromorphic soils.
The study of farming systems was essentially an exercise in enumeration. Randomly selected peasant farmers were asked to address themselves to standard questionnaires on their farming activities. These data were supplemented by the inspection and measurement of the farms owned by the farmers. Difficulties were encountered by the lack of a suitable researcher to undertake a detailed socio-economic study of Edea and its environs. In the study of the utilization of forest products in traditional farming, staff observed and recorded the species of the shade trees planted in traditional cocoa and coffee farms and the multipurpose trees found in the permanent compound farms.
The selection of leguminous forest species of interest for agro-forestry was made in elimination trials that were aimed at identifying local or exotic species that would thrive in the ecological conditions of Edea. A search was undertaken for mature leguminous seed bearers at Edea and at Kumba, while seeds of exotic leguminous species were acquired from various individuals and forestry research institutions. The availability of seeds and problems in germinating certain species severely limited the scope of the elimination trials.
The first elimination trial was planted in August 1978 and covered 2 ha. It involved leguminous species from only eight seed lots; five of the seed lots were planted according to a statistical design (complete randomized blocks, five treatments, four replications) at a spacing of 4 x 4 m in plots measuring 40 x 25 m (66-70 plants/plot). The remaining three seed lots, because of seed shortage, were simply planted in unreplicated plots. The seed lots planted in the 1978 trial were: Pterocarpus soyauxii, P. osun, Afzelia pachyloba, Acacia sp., Tetrapleura tetraptera, and three lots of Leucaena leucocephala.
The 1979 elimination trial involved 23 seed lots that were sown between February and April 1979 and planted out in July 1979. The plantings were made in an area slightly less than 2 ha. As a result of the low germination rates for many of the seed lots, planting was not carried out according to a strict statistical design. Still the principle of replication was maintained, with 23 plots in each block corresponding to the different seed lots. The spacing was 3 x 2 m, but, as a consequence of the limited planting material, two sizes were adopted for unit plots: small plots (each measuring 36 m²) and large plots (each measuring 72 m²). There were 21 plants per large plot, whereas the number of plants in each small plot ranged from one to nine. The 23 seed lots included one of the seed lots (Tetrapleura tetraptera) employed in the 1978 trials.
The site preparation for both the 1978 and 1979 trials consisted of felling the big trees, clearing the underbrush, burning, staking, and hole digging. The planting stock for the 1978 trial consisted of striplings, wildings, and stumps. For the 1979 trial, however, only seedlings raised in polythene pots were used.
Climatic and Ecological Factors
Mangombe (in Edea) is at latitude 4°00'N and longitude 10°15'E. Its altitude is 32 m above sea level. The mean annual rainfall is 2,600 mm. The rainfall distribution is generally regular, there being some rainfall even in the dry season (November-April). The temperatures are high; the mean monthly maximum and minimum temperatures were 27.8°C and 23.2°C, respectively, in 1978.
According to a study carried out by CTFT (1969), there are three distinct soil types in Mangombe:
Laboratory analysis showed that the upland (sandy) soils were acidic (pH 4) and varied little between the two depths 1020 cm, 40-60 cm). In general the physical properties were quite good. Soil water was well distributed except during heavy rainfall; clay particles were 35 40 per cent under forest, 45 per cent under food crops, and 37-40 per cent under fallow. There was a good distribution of the fine and heavy particles, and stones were absent.
Soil chemical properties were poor, probably because of heavy leaching. According to the 1977 analysis total nitrogen showed a tendency to increase with depth, from 0.09-0.14 per cent and 0.09-0.13 per cent, respectively, for soils under forest and under food crops. The sum of exchangeable bases was, on the whole, low, as was available phosphorus. In the 1978 study the content of organic matter decreased with depth-in sharp contrast to the trend exhibited in the 1977 samples. Nitrogen also decreased with depth in the 1978 samples (i.e., from 0.1 per cent at the surface to 0.06 per cent at 110 cm depth). The contrasting results on organic matter and nitrogen levels in 1977 as compared with 1978 are confusing unless the 1977 result is erroneous. This possibility is supported by results from the May 1979 samples.
Soils under leguminous forest species (Afzelia pachyloba and Piptadeniastrum africanum) exhibited slightly higher amounts of organic matter (1.51-1.22 per cent at 0-40 cm) than was found in soils under forest, food crops, and fallow.
The farmers in and around Edea have holdings of about 0.54.2 ha. Farming on a collective basis is rather rare, although there is one example, the Groupe des Agriculteurs Modernes supported by FONADER (Fonds National de Developpement Rural; often referred to as the farmers' bank).
The farming system is essentially a shifting agriculture involving a fallow period of three to seven years. Mixed cropping is common, and the cultural practices of field preparation are partial or complete clearing and burning, making such land subject to erosion.
The peasant population depends principally on cassava, taro (Co/ocasia esculenta), macabo (Xanthosoma sagittifolium), yams, sweet potatoes, bananas, and plantains. Maize, groundnuts, and beans are also planted. The two principal industrial crops of Littoral Province are robusta coffee and cocoa. The Sanaga-Maritime Division (Edea) is third among the divisions of Littoral Province for the production of both coffee and cocoa. Rubber and oil palms are also found as industrial crops in Sanaga-Maritime Division.
The forest trees associated with traditional farming systems were found, as in the case of the humid forest zone of western Nigeria (Getahun 1980), to be Leucaena leucocephala, which is planted in cocoa and coffee farms as a shade tree, and Treculia africana, Irvingia gabonensis, Dacryodes edulis, Samanea saman, Cassia siamea, and Pterocarpus soyeuxii, which are multipurpose trees present in permanent compound farms.
Selection of Leguminous Forest Species
Mean height was the only measurement taken after 18 months in the elimination trials. No statistical analysis was undertaken to determine whether or not the difference between the mean heights for any selected pair of seed lots was significant. Nevertheless, the results (table 1) seemed to justify some conclusions:
Conclusion and Recommendations
From the work carried out by the agro-forestry project, it seems that the leguminous species Albizia falcataria, Samanea saman, A. Iebbeck, Leucaena leucocephala, and Pterocarpus soyauxi; should be provisionally selected as trees that will thrive in the humid zone of Edea. It is immensely important to the future of the agro-forestry research project in Edea that the majority of the species selected from our elimination trials were found to exhibit a great natural capacity for nodulation. L. Ieucocephala is the only species likely to be used in our future work that did not appear to nodulate in our experiment either in the nursery or in the field. These aspects are worthly of further investigation. Given the potential of these selected leguminous species for soil improvement, use as shade trees, and pulp production, they should be given first consideration in the choice of species to be used for the establishment of experimental plantations.
In view of the great role that is likely to be played by CELLUCAM in the socio-economic development of Edea in the near future, the choice of species to be used for the establishment of the experimental planations should also include pulp species (e.g., Pinus caribaea, Eucalyptus urophylla, Gmelina arborea) which have been chosen for use in the next five-year reforestation programme of CELLUCAM.
The specific objectives for the experimental plantations should include:
Immense thanks are due Patrick Shiembo, the agro-forestry research assistant at Edea, for kindly accepting to carry out the final measurements of the 1979 elimination trial.
TABLE 1. List of Seedlots Sown in 1979: Their Germination, and Height (m) at 18 Months
|Species||Date Sown||Date of Germination||No. Sown||No. Germinated||Mean Ht. (m) at 18 months|
|Adenanthera pavonina||13/2/79||17/2/79||18||4 2||.48|
|C. mimosoides||13/2/79||17/2/79||600||10||(weed sp.)|
|Leucaena leucocephala (K6)||23/2/79||28/2/79||150||149||4.42|
|L. Ieucocephala (K8)||23/2/79||28/2/79||150||150||4.33|
|L. Ieucocephala (K67)||23/2/79||28/2/79||150||148||4.32|
|L. Ieucocephale (K132)||23/2/79||28/2/79||150||147||4.26|
|Leucaena sp. (Stat Cruz Porrillo)||14/2/79||21/2/79||22||22||2.0|
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