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.
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.
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.
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.
Dept. of Forestry
P.O. Box 342, 6700 AH Wageningen