Selection of leguminous trees for agro-forestry in Cameroon

Anthony Mapri Maimo
Institute of Agronomic Research, Yaounde, United Republic of Cameroon

Abstract

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.

Introduction

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.

Objectives

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.

Methods

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.

Results

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:

• Sandy solis, situated on the uplands, which are deep, stone-free and suitable for plantations; these soils cover about 60 per cent of the area;
• Gravelly soils, situated on the slopes, with lateritic gravel or quartz or altered rock (mica schist or gneiss); these are unsuitable for plantations and cover about 30 per cent of the area; and
• Hydromorphic soils, situated on valley bottoms, often gravelly, frequently moisture-laden in the vicinity of water courses; these soils are also unsuitable for plantations and cover about 10 per cent of the area. The field work carried out by our forestry research centre confirmed these results.

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.

Farming Systems

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:

• Albizia falcataria (which recorded 9.1 m) grew significantly faster than any of the other species, with the next species in height performance (Samanea saman) attaining only 5.5 m;
• There seems to have been little difference among the Leucaena leucocephala provenances from Hawaii; they showed a height growth ranging from 4.2 m (for K132) to 4.4 m (for K6); and
• Piptadeniastrum africanum and Acacia eburnea exhibited the slowest growth rates (1.32 and 1.15 m respectively).

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:

• Study of the effect of tree plantations on the restoration of soil fertility in soils depleted by farming;
• Study of the effect of spacing on the production of wood and food crops; and
• A preliminary inventory of the tree-insect/fungus/ virus associations so that tree species which act as hosts to the pests of agricultural crops in the Edea region can be identified.

Acknowledgement

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