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View the documentSummary of discussion: Current agro-forestry activities

Intercropping tree and field crops

D. M. Osafo
Department of Crop Science, Faculty of Agriculture, University of Science and Technology, Kumasi, Ghana


This paper describes an experiment started in 1978 in the humid forest belt of Ghana that attempted to demonstrate to farmers how they can establish plantations of economic tree species and, at the same time, produce food crops by means of judicious intercropping. A population density of 625 plants/ha of a fast-growing tree species, Gmelina arborea, was established at four different spatial arrangements. The stands were interplanted with an important West African food crop, plantain (Muse cultivar), at various spacings (densities ranging from 536 to 1,588 plants/ha). The minimum plant-toplant distance was 2.0 m, and the maximum was 2.8 m. Both species are being assessed periodically by standard forestry and agricultural methods. The implications of the results so far are discussed along with the objectives of the experiment and projected future experiments.


One of the problems facing manufacturing industries that have been set up in developing countries has been the difficulty of obtaining raw materials and, quite often, the inadequacy of these raw materials. If the pulp and paper industry in Ghana is not to be bedeviled by this sort of problem, then attempts must be made to ensure sustained supplies, in requisite quantities, of tree species suitable for the industry, and local farmers must be encouraged to supplement the production from factory-owned plantations. In this regard, estimates of farmers' production must be reliable, based on an accurate enumeration by region or district of not only the farmers engaged in forest tree cropping but also the stands their farms carry.

Gmelina arborea is a medium to large fast-growing deciduous tree, capable of growing successfully in mixed forests of moist regions, such as the humid forest belt of Ghana. Usually grown in plantations at spacings of 2.0 x 2.0 m (Streets 1962), it is one of the species recommended for a proposed pulp and paper industry in Ghana.

The establishment by farmers of forest tree crops, although known in Ghana, has not been very popular, mainly because of the long time it takes for the farmers to realize any income from the operations. However, if this desirable practice were somehow combined properly with food cropping, it would probably gain popularity as a prospect for almost immediate and sustained income or savings on food costs.

On account of its ease of establishment, mode of growth, and time of maturity, as well as the demand for it as a staple item in local diets (Purseglove 1972), plantain (Muse cultivar) was selected as the food intercrop for the initial experiments with G. arborea.

The two species can be planted at a minimum distance of 2.0 m (1: 1 rectangularity). When interplanted, the rapid growth and development of effective canopies and roots, yield of leaf litter, and the number of suckers that plantain develops as it grows were all considered to be favourable for the maintenance of soil fertility (even if not its build-up), and for early protection against soil erosion, particularly that caused by splash and runoff.

It was the objective of the experiment described in this paper to establish 625 trees/ha of G. arborea. Plantain was interplanted at various spacings so that the minimum distance between and within rows (Gmelina-Gmelina; Gmelina-plantain; plantain-plantain) was 2.0 m, and the maximum distance was 2.8 m, distances at which either species can be planted in a 1:1 arrangement. In the first two years, maize (Zea mays) and cocoyam (Xanthosoma sagittifolium) were planted in the spaces between the young plants of Gmelina and plantain. Although they were not included in the study, they could provide additional sources of income and food for the farmers.

This experiment therefore served as both a quantitative scientific study, and a demonstration to farmers of the advantages of intercropping trees with food crops, both of which would have ready markets.

Materials and Methods

The South Fomangsu Forest Reserve was chosen for the study, and the research site, which is about 80 kilometres from Kumasi on the Kumasi-Nkawkaw road, is at the western end of the reserve. The land was cleared and burned during the 1977-1978 dry season, and it was ready for planting by April 1978. Meanwhile, seedlings of G. arborea had been raised for the experiment by early 1978, and bigfruited (Apantu) plantain suckers had been purchased from February to April.

A Gmelina tree population of 625/ha was maintained, and the spacings used in the four different treatments were 4.0 x 4.0m (1:1); 5.7 x 2.8m (2:1); 6.9 x 2.3m (3:1); and 8.0 x2.0 m (4:1).

Where the between-row or within-row distance from one Gmelina plant to the next was 4.0 m or more, a plantain sucker was planted so that the distance from any one plant (Gmelina or plantain) to the next measured at least 2.0 m but not more than 2.8 m. Thus, for every four Gmelina plants in the 4.0 x 4.0 m spacings, there were five plantain plants; for every four Gmelina plants at 5.7 x 2.8 m spacings, there were only two plantain plants; for every four Gmelina plants at 6.9 x 2.3 m spacings, there were also four plantain plants; and for every four Gmelina plants at 8.0 x 2.0 m, there were six plantain plants. Planting started in May 1978 and continued for about one month.

The layout for each treatment was as follows: (1) each plot measured 32.0 x 28.0 m, with nine rows of Gmelina plants along the 32.0 m axis and eight Gmelina plants within each row, interplanted with plantain so that there were nine rows of Gmelina/plantain alternating with eight rows of plantain, each only 2.0 m from the next; plantain plants numbered 183; (2) each plot was adjusted to 34.2 x 28.0 m, with seven rows of Gmelina plants along the longer axis, each with 11 plants, alternating with six rows of plantain, 2.8 m apart with 11 plants apiece (total 66 plantain plants); (3) each plot was adjusted slightly to measure 34.5 x 27.6 m, with six rows of Gmelina plants along the longer axis, each with 13 plants; in between every set of two adjacent rows of Gmelina were two rows of plantain plants, 2.3 m apart (i.e., ten rows of plantain plants, each with 13 plants); and (4) each plot measured exactly 32.0 x 28.0 m, with five rows of Gmelina along the longer axis, each with a population of 15 plants; between the two adjacent rows of Gmelina were three rows of plantain plants, a total of twelve such rows, 2.0 m apart, each with 15 plants.

Thus, for a Gmelina population of 75±3 trees per plot, the plantain population varied from a low of 66 in the second spacing to a high of 183 plants in the first spacing. The four treatments were replicated six times in a randomized complete block design, with 24 plots in all. Any plants that died in the first few months were replaced; weeding and removal of fallen branches were carried out regularly.

Data collected for assessment of the Gmelina at the end of the first year (1979) and in 1980 were from 25 plants chosen from the middle of each plot in order to avoid border effects. The data collected were:

  • Height from the soil surface to the terminal bud;
  • Girth at breast height;
  • Height to the main fork where applicable;
  • Approximate length of the first major branch as an index of the size of the branch, with qualification as to where it was small, medium, or large;
  • Size of the crown (depth and diameter);
  • Crown exposure scoring from l to 5, with 1 being for the lower understorey, in which the plant is completely suppressed, with no direct light from above or from the sides; 2 for upper understorey, in which the plant is exposed to some direct light from the sides but not from above; 3 for lower canopy, in which the plant receives direct light from above but light from the sides is impeded; 4 for upper canopy, in which the plant receives much light from above and direct side light is only slightIy impeded; and 5 for emergent, with the crown completely exposed to light from all angles; and
  • Canopy-forming agents (whether shading of plants is caused by Gmelina, plantain, or weeds, a term used to describe any plant, e.g., oil palm, other than the two species being studied).

TABLE 1. Assessment of Gmelins arborea in the Mixed Cropping (Gmelia and Plantain) Experiment South Fomangsu Forest Reserve, Ghana


Treatment 1 (4 x 4 m)

Treatment 2 (5.7 x 2.8 m)

Treatment 3 (6.9 x 2.3 m)

Treatment 4 (8 x 2 m)

Mean height to terminal
bud (m)
Mean girth at breast height (cm) Mean depth of crown (m) Mean diameter (horizontal) of crown (m) Mean height to terminal bud (m) Mean girth at breast height (cm) Mean depth of crown (m) Mean diameter (horizontal) of crown (m) Mean height to terminal bud (m) Mean girth at breast height (cm) Mean depth of crown (m) of crown (m) Mean diameter (horizontal) Mean heighan to terminal bud (m) Mean girth at breast height (cm) Mean depth of crow (m) Mean diameter (horizontal) of crown (m)
Block I 10.00 44.1 4.80 5.16 7.04 39.2 3.12 4.46 10.12 42.4 3.77 5.0 7.64 30.8 4.07 5.45
Block II 9 09 44.7 4 09 5.22 8.70 45.0 4.23 5.36 8.99 36.5 5.47 6.36 9.37 39 4 4 47 5.16
Block III 754 41.5 3.50 5.29 9.85 40.8 6.11 6.92 6.35 34.8 3.22 4.88 9.70 41.3 3.93 5.0
Block IV 8.98 33.7 4.20 4.59 9.38 42.5 4.17 5.44 8.25 38.0 2.73 4.14 6.88 32.8 2.66 3.94
Block V 10.69 42.8 5.16 4.50 4.88 25.6 1.92 3.84 8.68 39 6 4.92 5.20 10.27 42.3 4.52 5.44
Block VI 5.74 29.6 3.03 3.97 7.92 38.6 5.08 5.08 10.76 42 1 5.14 4.79 10.14 39 3 4.90 4.15
Means 8.67 39.4 4.13 4.78 7.96 38.6 4.10 5 03 8.86 38.9 4.20 5.06 9.00 37.6 4.09 4.85

Assessment of the plantains (all the plants except those in the outer rows) involved height measurements; grouping of plants into small, medium, and large; enumeration of the number of functional leaves per plant; determination of the lengths and the widest breadths of the oldest functional and newest (but completely unrolled) leaves; enumeration of the number of suckers produced per plant; and yield and components,-i.e., mean number of hands per bunch, mean number of fingers per hand, and mean weight of individual fingers.

Results and Discussion

The first and second years' assessment data of G. arborea are available, but only those for the second year, taken at the end of 1980, are presented in table 1. Although the data have not been statistically analysed, to date there is little evidence that one treatment is markedly superior to another in the parameters evaluated. It is possible that two years of growth is too short a period for treatment differences to be shown. Streets (1962), in fact, suggests that a minimum of four years is required for the manifestation of differences as a result of interference between trees and crops. Even if no differences are detected in the near future, this work will have shown the ability of Gmelina trees to successfully withstand competition from other trees and plantain plants at the spacings used.

Because the trees are destined for the paper and pulp industry, an important consideration is the production of those parts useful as raw material under the conditions of the experiment. Also, the claim of plentiful natural regeneration is to be carefully examined from about the fifth or sixth year onwards.

In May 1979, when the Gmelina and plantain were assessed, only a few of the plantains in each plot had produced mature bunches. Four plants per plot were harvestable so their yields were determined.

An analysis of variance of the bunch yield did not show any significant treatment effects on yield. Yet, when the yields were resolved into their components-the mean number of hands per bunch, the mean number of fingers per hand, and the mean individual finger weight-it was found that the block effect on the mean individual finger weight was significant (P = 0.05). Thus, the consistently higher yields obtained in one block can only be due to this higher component. This point has been stressed because the mean weight per finger is an important component of yield in plantains and must be studied in future data analysis. Mean weight per finger is dependent on the number and size of functional leaves and their exposure to sunlight during the period of fruit filling (Osafo, unpublished).

At this early stage and on the basis of only four plantains per plot, not much can be expected with respect to treatment differences. However, within a year of the planting date, some food was obtained from the operation as well as sizeable quantities of maize and cocoyams, and this fact can be used by extensionists to canvass farmers. If, as indicated by the preliminary yield data for 1980, nearly all the plantain stands have produced mature bunches after a year, then farmers may be convinced to take up this sort of agro-forestry practice.

It is thought that, after four years or so, plantain suckers will be produced in abundance, the Gmelina trees will be producing seeds, and natural regeneration will be noticeable. Then it may be necessary to thin the plantain stands to promote both growth and yield. This possibility assumes that under the conditions of the experiment, the Gmelina trees will grow to heights of 15 m or more and form dense canopies that will interfere with light interception by the plantains. The competitive effects would need careful assessment on the basis of crown exposure scoring and enumeration of canopyforming agents.

To date, it has not been possible to follow up the soil aspects of this study because of financial difficulties and scarcity of competent technical staff. However, the plans to assess soil effects have not been abandoned.

The final choice of an agro-forestry system should be made by the farmer, and this will depend on what combination of trees and crops leads to the most enduring benefits in terms of food production, wood output, and maintenance of soil fertility. Answers with regard to combinations of Gmelina arborea with plaintain may not be forthcoming until the experiment has run, and been assessed yearly, for a minimum of eight years.


I would like to thank S.P.K. Britwum of the Forest Products Research Institute in Ghana and the project leader of the agro-forestry team for his suggestions on the forestry aspects of this paper. My sincere thanks also go to J.J. Afuakwa, K. Ohene-Yankyera, and C.F. Yamoah for their assistance in the field.