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Food crop yield under gmelina plantations in southern Nigeria

O. O. Agbede and G. O. A. Ojo
Forestry Research Institute of Nigeria, Ibadan, Nigeria


Experimental taungya plots were established at six different locations in southern Nigeria to investigate the productivity and competitive relations of intercropping agricultural crops (yams, maize, and cassava) with trees (Gmelina arborea). Experiments were set up in 1978 and 1979 at Gambari, Ore, Sapoba, Ukpom-Bende, Ikom, and Awi-Calabar. With the exception of Gambari, the locations are in the tropical rainforest zone of Nigeria. Results of the experiments showed that cassava depresses Gmelina arborea, especially when planted at close intervals. This effect tends to diminish at age 12 months, when the trees usually attain canopy closure. Trees under yams and maize tend to perform better than those planted a/one or with cassava.

Varying the space between Gmelina arborea plants markedly influenced the trees, while the different spacings of agricultural crops had no effect Likewise for crops, the space between trees was not important but the spacing between crops was significant. Yams planted at 1.5x 1.5 m were found to be the most profitable.


A recent trend in Nigeria is towards the production of sufficient pulpwood, fuel, poles, and timber. Hence, state forestry services have set up annual planting targets of 20,000 and 6,000 ha for Gmelina arborea and teak, respectively. Similarly, the recent food crisis in Nigeria has become a major concern in official circles. In 1977, the value of Nigerian food imports was approximately N270 million (Enabor 1978). Adequate food supply for the increasing Nigerian population demands the harnessing of both the productivity of Nigerian agriculture and the contribution of forestry through the management of forest lands for the simultaneous production of wood and food.

Taungya is a form of agrisilviculture, or farm forestry, whereby food crops are interplanted with tree crops at the time of establishment (or regeneration) of the tree crop, and in which the forestry agency collaborates with peasant farmers (King 1968). The farmers are responsible for clearing, burning, and packing operations within their allocated plots. The Forestry Department may assist with the felling of large trees. In return for their labour, farmers are per misted to grow food crops until the tree crop canopy closes, which is generally for one to three years, depending on the species planted and the spacing. Gmelina arborea, for example, which produces pulpwood in 10 years and timber in 20 years, closes its canopy within 12 to 15 months when planted at 2.5 x 2.5 m. Taungya is a land management system used extensively by state forestry services to reduce the cost of plantation establishment and maintenance.

Usually, peasant farmers involved in taungya are allowed to cultivate the food crops of their choice, with some control as to how and when to plant the crops, especially in the case of plantains and cassava. The crops commonly raised are yams, maize, cassava, and rice, usually intermixed with melon, okra, pepper, tomatoes, beans, and other vegetables. In Cross River State, where the forestry staff are directly involved in food crop production, greater attention is paid to maize and cassava than to labour-intensive crops such as yams. The Forestry Research Institute of Nigeria is currently growing yams, maize, and cassava in its taungya farms at six research locations in southern Nigeria.

The taungya farms established by the Forestry Research Institute of Nigeria are located at Gambari (Oyo State), Ore (Ondo State), Sapoba (Bender State), Enugu-Ngwo (Anambra State), Ukpom-Bende (Imo State), and Ikom and Awi (Cross River State). Since the participation of local farmers has resulted in variations in and damage to the tree crop during the agricultural cropping, the work is now undertaken by the Institute staff. In this way they can ensure that the crops are spaced, treated, and harvested according to plan, and thereby validate the results. Both land preparation (clearing, felling, burning, and packing) and cultivation for arable cropping, especially yams, are carried out by the staff of the Institute. The harvesting and marketing of the food crops are also undertaken by the Institute.

The purpose of this study was to determine quantitatively the yields of food and tree crops and their competitive relations under taungya in selected locations in Nigeria.

Materials and Methods

Experimental taungya farms were established at six different locations in southern Nigeria where taungya is widely practiced. With the exception of Gambari, the locations are in the tropical rainforest zone of Nigeria.

In 1978, 0.80 ha of Gmelina arborea was established in combination with three agricultural crops-yams, maize, and cassava. These crops were selected because they are the most widely grown crops in all the states where the experiments were located. Other important arable crops such as rice, cocoyams, plantains, and vegetables, and tree crops such as Tectona grandis, Nauclea spp., and Terminalia spp., are proposed for future experimental work. For 1978, each experimental farm was divided into five blocks, of four plots each. Each plot was 20 x 20 m and contained 64 trees at 2.5 x 2.5 m. A randomized complete block design was used. The treatments and the control were Gmelina arborea (control), Gmelina arborea + yam, Gmelina arborea + maize, and Gmelina arborea + cassava. Yam (Dioscorea rotundata), Nigerian selection no. 1 maize, and a local cassava variety were grown on all the sites. All the arable crops were planted at 1.2 x 1.2 m. Maize and yams were planted during MarchApril, whereas cassava and stumps of Gmelina arborea were planted in June-July of the same year.

In 1979, separate areas were used for each arable crop (yam, maize, cassava), and G. arborea was used as the tree crop. The experiment was to study the interaction of both agricultural and tree crops when interplanted at different densities. The design was a 4 x 4 x 3 factorial, i.e., three planting distances plus a control for Gmelina arborea and each agricultural crop. G. arborea was planted at spacings of 2.0 x 2.0 m, 2.4 x 2.4 m, and 3.0 x 3.0 m, while yam, maize, and cassava were planted at spacings of 1.0 x 1.0 m, 1.5 x 1.5 m, and 2.0 x 2.0 m. There were 16 treatment combinations replicated three times for each arable crop. The experimental area for each arable crop was 50 x 160 m divided into three blocks of 48 x 48 m: each block was further divided into 16 small plots (12.0 x 12.0 m). Crop varieties and time of planting were the same as in the 1978 experiments.

All farm operations, including land preparation, planting, tending of arable crops, weeding, harvesting, and marketing, were carried out by departmental labour so as to minimize mishandling of the tree crop and to have firm control over the farms and the crop yields.

At maturity (i.e., 3 months, 8 months, and 12 months for maize, yams, and cassava respectively!, the agricultural crops were harvested, weighed, and marketed fresh. The tree crop was assessed for height, girth, and percentage of survival 6 and 12 months after planting.

Results and Discussion

Growth assessment of G. arborea interplanted with yam, maize, and cassava in 1978 indicated that the growth of the trees under agricultural crops was superior to trees grown alone. Nevertheless, the performance of trees under cassava was slightly less acceptable than trees intercropped with yam and maize (table 1). Observations indicated some competition for space between the trees and cassava as shown from the whiplike, weak, and deformed stems of the G. arborea trees.

The trees that were planted at 2.4 x 2.4 m had attained canopy closure at 12 months in all the sites. Thus the growth of any crop under the Gmelina had become impracticable. The depressing effect of cassava on the survival and girth of Gmelina trees observed during the assessment 6 months after planting was still evident at 12 months though in a diminishing magnitude. This finding shows that, though Gmelina is a fast-growing species, it takes some time before the tree can overcome the effects of being intercropped early with cassava. The good performance of trees with agricultural crops (table 2) was possibly due to the initial cultivation and regular cleaning of the yam and maize plots in the taungya farm.

TABLE 1. Survival and Heiaht of Gmelina arborea Six Months after Plantino in Taungya Farm, 1978

Treatment Survival % Mean height (cm)
Gambari Sapoba Bende Awi Ikom Gambari Sapoba Bende Awi Ikom
G.arborea(control) 58a 99a 66a 95a 85a 87a 193a 131a 107a 225a
G. arborea and yam 68 b 95 b 78 b 90 b 97 b 84 a 205 a 168 a 123 a 227 a
G. arborea and maize 75 c 94 b 66 a 92 b 93 c 80 b 173 b 200 c 130 c 223 a
G.arboresandcassava 67b 98a 69a 93b 81d 91 c 216a 159d 126b 161 b
Standard error 3.12 1.06 1.68 1.02 1.91 2.08 8.2 3.37 2.24 4.00

The figures represent the means of five replicates.
The means in anv given column with the same letters in common are not significantly different at P = 0.05.

At each of the three yam spacings, the height growth of Gmelina tended to decrease as planting distance between the trees increased (table 2). This supports observations that the denser the tree population the more they compete for space and light. The performance of Gmelina under maize and cassava followed the same trend (table 3).

Data have been collected on the yield and revenue accruing from each of the agricultural crops, and these have been converted to a per hectare basis (table 4). Actual measurements of yam, maize, and cassava crops were made, and their values obtained from organized sales made at the various sites where the experiments were carried out. The higher yield of yams in 1979 than in 1978 is a reflection of better weather conditions and improved management practices (e.g., earlier land preparation and planting time). The yields compare favourably with those of peasant taungya farmers in the state forestry services (Ball 1977).

The influence of different densities of agricultural and forest crops on the yields of agricultural crops is exemplifled by the values obtained for yams: the yields per hectare increased with increasing agricultural crop density at each level of the tree crop spacings. However, there was a slight decrease in the yields of yams, as the planting density of G. arborea was increased. In plots where no G. arborea was planted, the yield of yams was 17.8 tonnes ha; this yield dropped to 13.08, 12.68, and 12.86 tonnes ha at Gmelina spacings of 3.0 3.0m, 2.4x2.4 m,and2.0 x2.0 m respectively. The implication of the above observations is that, though agricultural crops have positive effects on the establishment of trees, the latter tend to have a slight negative effect on the yields of agricultural crops.

TABLE 2. Survival and Mean Height of Gmelina arbores at Three Different Sites after Six Months of Interplanting with Yam

Treatment levels Survival % Mean height (cm)
Y-levels G-levels Sapoba Awi Ikom Sapoba Awi Ikom
Y0 G0 - - - - - -
G 1 82 90 96 127 134 334
G2 85 63 96 117 147 325
G3 81 81 99 109 170 329
S.E. 1.2 3.0 1.1 2.5 3.5 1.7
Y1 G0 - - - - - -
G1 74 93 96 95 161 350
G2 93 94 95 116 116 316
G3 95 85 99 102 150 298
S.E. 3.9 1.8 1.2 2.7 4.0 4.2
Y2 G0 - - - - - -
G1 87 90 94 117 136 333
G2 89 81 100 107 139 349
G3 77 87 97 110 1 35 315
S.E. 2.1 1.8 1.4 1.8 1.7 3.4
Y3 G0 - - - - - -
G1 88 88 96 105 132 348
G2 84 94 99 92 167 332
G3 80 86 98 95 175 332
S.E. 1.6 1.7 1.1 2.1 3.9 2.5

Key to tables 2 and 3
G0 = No Gmelina arbores planted;
G1 = G. arbores planted at 2.0 x 2.0 m;
G2 = G. arbores planted at 2.4 x 2.4 m;
G3 = G. arbores planted at 3.0 x 3.0 m;
Y0 = No yams planted;
Y1 = Yams planted at 1.0 x 1.0 m;
Y2 = Yams planted at 1.5 x 1.5 m;
Y3 = Yams planted at 2.0 x 2.0 m. The same spacings used for yams were used for maize (M0, M1, M2, and M3) and cassava (C0, C1, C2, and C3).

TABLE 3. Effect of Different Spacings of Yams, Maize, and Cassava on the Survival and Height of Gmelina arborea Six Months after Planting

Factor Levels Survival % Height (cm)
Y0 83 118
Y1 87 104
Y2 84 111
Y3 84 97
Standard error +1.2 +6.4
M0 79 58
M1 89 73
M2 86 64
M3 84 64
Standard error +3.0 +4.4
C0 75 113
C1 77 142
C2 84 136
C3 85 161
Standard error +3.5 +14.0

For key see Table 2.

The most profitable yam spacing in the experiments was 1.5 x 1.5 m. At this spacing, the yield of yams was almost equal to those at 1.0 x 1.0 m, whereas the inputs were about half.

Practical Implications

Agricultural cultivation during the establishment of G. arborea was found to be desirable. In all cases of taungya farming, the growth of G. arborea usually improves with the additional advantage of achieving reasonable agricultural yields. The results presented in this paper tend to support earlier observations made by Jaiyesimi (1966), King (19681, and Agbede and Ojo (1978) that the combination of agricultural crops with tree crops during the establishment phase of the latter is not detrimental.

On plots of Gmelina grown alone or interplanted with food crops, the trees reached a mean height of 5.80 m and 6.50 m respectively in 12 months; the equivalent girths were 29 cm and 33 cm. G. arborea interplanted with yam and maize performed better than those planted alone or with cassava. The provision of shade for the newly-planted tree crop by the sprouting maize and the improved soil conditions in both the maize and yam plots positively affected the growth of G. arborea.

TABLE 4. Yields and Revenue Obtained From Agricultural Crops in Taungya Research Plot. 1978


Yield (fresh weight in tonnes/ha)

Revenue (naira*/ha)

Food Crop Gambari Ore Ukpom-Bende Gambari Ore Ukpom-Bende
Yam 7.7 3.3 6.4 1,540 660 1,070
Maize 4.5 2.0 1.0 410 185 93
Cassava 6.6 4.5 12.4 130 90 250

* In 19781 naira = US$1.60


Summary of discussion: Taungya systems from biological and production viewpoints

From the papers presented, and subsequent discussion, it was clear that taungya systems have met with widely varying success in different countries and-at least in the case of Nigeria-in different sections of the same country. In large part this has not been due to technical or even economic problems, but more to basic political decisions and social acceptance. In particular, lack of continuity was cited as a problem. In Togo taungya has generally failed because the government has claimed forests as its property. This has led to widespread discontent and the destruction of timber trees in favour of fruit trees, which provide regular income after a relatively short period of establishment. In Kenya the taungya system is being phased out because labour costs are relatively high, there is a problem with the labourers or their relatives trying to settle in the forest, and very large areas need to be planted as industrial plantations.

With regard to the experiments on taungya in Sierra Leone, it was clarified that the plantations are used for food crops for the first one or two years, and then the farmers must stay out of the area until the final thinning is carried out. The farmers are then allowed to return and plant tree crops such as cola, citrus, coffee, and cocoa. It is expected that the timber crop will mature and the tree crops will decline in yield at about the same time (about 30-35 years after planting the timber crop), and the area will be clearfelled in order to begin a new cycle. It was noted that in Nigeria a similar system had been tried but had failed because the farmers began to think the forest reserve belonged to them and acted accordingly. The importance of crown shape and density on the yields of the tree crops was raised, but long-term data were not available.

The long-term effect of tree plantations was also discussed, although there was, again, a lack of decisive evidence. It was noted that fertilization may well be necessary when establishing second or third generation tree crops, depending on the inherent soil fertility, and that an initial dose of fertilizer at an early stage of growth could be far more effective than fertilization at establishment. The type and frequency of the products removed are also important. The export of timber, for example, might remove large quantities of calcium, while palm oil is almost pure carbohydrate. In most cases the harvesting and removal of forest products would remove far fewer nutrients than represented by the harvesting of food crops on an annual basis.

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