Agroforestry in Burkina Faso: a Careful Balance

If néré (Parkia biglobosa) and karité (Vitellaria paradoxa) trees inhibit the growth of sorghum in the Sahel, why do Burkinabe farmers leave them in their fields? According to a recent study, the benefits of leaving the trees in the fields outweigh the costs. Sorghum grain yields under the karité and the néré are reduced by an average of 50% and 70% respectively. However, the fruit the trees produce is worth more than the sorghum yield loss.

The study area in Burkina Faso is 150 km west of the capital Ouagadougou, within the territory of the village Oula in the Mouhoun Province. The region is sparsely populated. The natural vegetation is a mixed shrub-tree savanna. About 10% of the village territory is cultivated by farmers who rarely use animal traction or fertilizers. Tree canopies on cropland, largely dominated by karité and néré, cover 10 to 15% of the area.

A Tale of Two Trees

The karité or shea butter tree is tall (10 to 15m), with a round canopy reaching to 150 m2. The average density of the karité is 5 to 10 trees per hectare in the study area. Kernels from the tree are used in making shea butter and soap and are exported.

The néré, or locust bean tree, is a taller (20 m), low-branched tree with a broad canopy cover reaching reaching 200 m2. It has an average density of two trees per hectare at the study site. The néré fruit is eaten and the kernels are fermented to make "soumbala" (a condiment used in Burkinabe cuisine).

Most karité and néré trees grow naturally and their use is not restricted by government officials. Both trees mature slowly, producing fruit 10 to 20 years after initial growth. The roots of both trees are largely long, shallow and lateral. Both trees drop leaves in the dry season. Néré is a leguminous plant, but is not known to fix nitrogen.

Researchers examined tree characteristics, crop growth and production and site characteristics to determine the effect of trees on crop production in two sites: one in a valley and one on a plateau. They analyzed the extent to which light, water and nutrients are responsible for the effects observed.

Eighteen trees were selected for the study. These trees were relatively isolated, associated with sorghum, had relatively homogenous site conditions and were in fields whose owners were willing to collaborate. Tree height and trunk diameter at breast height were measured, as well as canopy projection and estimated cover and height of the lowest branches.

Growth of sorghum plants was monitored in July and September 1989, in every plot as well as in two control plots far from each tree. Soil samples were taken just after the first rains at two different soil depths (0 to 5 and 5 to 20 cm), from six trees: two karité and two néré trees in the valley and two karité trees on the plateau. The samples were analyzed for chemical and textural properties. Light intensity was measured by tracing shade contours around the trees several times during the growing season.

The research methodology was designed to maximize the inclusion of farmers in the process. More sophisticated research methodologies might have alienated the farmers, an important information source.

Limiting Factors

It was found that under néré, sorghum growth is significantly lower from July on. Sorghum growth under karité is reduced from September on. Yields beyond the canopy of trees in the valley were still 10 to 25% lower than those of control plots.

The harvest index (ratio of grains to vegetative plant matter) increased as the distance between planted sorghum and the tree increased, from 0.19 to 0.23 in the valley and from 0.23 to 0.40 in the plateau. This suggests that the tree canopy limits grain production more than vegetative matter.

Sorghum plants under the tree canopy mature more slowly than in the open field. Farmers participating in the study frequently mentioned this as a disadvantage of trees. Farmers also cited the higher incidence of disease in sorghum plants under the tree canopy. Farmers claimed the disease resulted from higher humidity under tree canopies.

The study results showed that soil fertility was not the major limiting factor to plant growth. Generally, soil fertility is higher under tree canopies than in the open field, due to increased plant-litter-soil recycling. The highest soil fertility was found under the large néré trees, probably partly due to limited crop growth under these trees. Trees also have a stabilizing effect on the micro-climate under their canopy, possibly leading to higher soil moisture availability. During the study, rainfall was high and soil moisture was not a limiting factor. Nutrient competition between trees and crops may have some inhibiting effect on crop growth, but it is not the main limiting factor.

Reduced light intensity is the primary cause of lower sorghum yields under tree canopies. Néré suppress crop production much more than the karité. Light intensity is reduced by up to 80% under néré canopies. Karité trees are smaller and most sorghum plants under their canopies receive morning or afternoon light. The variation in shading, and therefore yield reduction depends mainly on tree size. Light intensity measurements taken from one pruned néré show that pruning reduces the negative impacts of the tree canopy on sorghum yields. Pruning, although a common local practice, is not greatly developed. In much of the Sahel, laws forbid cutting or pruning of woody plants.

Another limiting factor to sorghum growth under néré and karité is increased weeds in the shade at the beginning of the rainy season. This means that initial crop growth is poor under canopies that are not weeded, particularly under large néré trees. Farmers do not weed under trees, since sorghum yields will be relatively low regardless.

Income for Women

Despite their effect on grain yields, néré and karité trees are important assets to farmers. With an average density of eight large karité and two large néré trees per hectare, and an estimated sorghum grain yield of 500 kg/ha, the yield reduction due to shading by the trees would be about 30 kg/ha. This amounts to roughly an 1,500-3,000 fCFA/ha (8-12$US, 250 fCFA calculated @1$US) reduction in farmer profit from sorghum. The karité tree produces about 5 kg of fruit; the fruit of eight trees, when processed into karité butter, converts to about 3,000 fCFA. One néré produces at least 25 kg of fruit per year. The fruit of two trees, processed into "soumbala," yields an annual profit of about 5,000 fCFA. Thus the average benefit from néré is about 5,000 fCFA per hectare. This benefit accrues mostly to women who pick, process and sell the tree products.

Based on this analysis one would think that farmers would forget about sorghum and grow only trees. However, most of the tree products are used for home consumption. An increase in production would result in a glut in the domestic market as supply would exceed demand. By growing both sorghum and tree products, farmers diversify their risk.

Ecological Benefits

Néré and karité trees have ecological as well as economic benefits. They lose their leaves in the dry season, the leaves are incorporated into the soil, increasing soil organic matter and improving moisture infiltration. The trees act as windbreaks. The low number of trees per area is beneficial, because more trees would greatly reduce crop growth.

J.J. Kessler. 1992. The Influence of Karité (Vitellaria paradoxa) and Néré (Parkia biglobosa) Trees on Sorghum Production in Burkina Faso. Agroforestry Systems, 17, 97-118.


J.J. Kessler
Dept. of Forestry
Agricultural University
P.O. Box 342, 6700 AH Wageningen
The Netherlands
Fax: 31-837083542