For example, before planting woody species to improve crop production in the Sahelian and Sudanian zones of West Africa, a careful analysis of site conditions is needed to determine which trees would be most suitable and under what circumstances. By considering specific site factors that might nullify the benefits of trees, perhaps inappropriate agroforestry systems can be avoided.
The main factors affecting primary production in the Sahelian and Sudanian zones are water and nutrient availability. The following critical analysis looks at how these factors are influenced by woody species, paying attention to the synergistic effect between trees and the rangeland herb layer or annual crops. Table 1 shows the vegetation and climate characteristics of the zones.
In the presence of trees, moisture availability for plant growth in any zone can be affected in various ways. For example, greater evaporative demands of woody species can lead to a faster depletion of water resources. Also, the greater leaf area index of trees causes a favorable transpiration/evaporation ratio, which could allow trees to outcompete the herb layer when water is scarce.
Without inputs, N and P availability depend on the amount of soil wetting and the prevalence of organic matter in the soil. The largest amount of absorbable nutrients, 10kg N/ha and 1kg P/ha, comes from wetting the first 50cm of the soil profile, due to the mineralization of organic matter. Another 5kg N/ha and 0.5kg P/ha become available in wetting the next 150cm. Beyond 200cm the risk of leaching increases.
When woody species are present, they absorb the limited available nutrients. For example in the south Sahelian zone, woody species at 15 percent cover do not affect the N content of the herb layer. However, for every 5 percent increase in cover above this level, there is about 1.3 kg/ha less of available N for herbage production. Thus a 30 percent cover could decrease the N content of the herb layer by about 4 kg/ha.
On a macro scale, the higher soil fertility commonly found under tree canopies in semiarid climates can often be explained as a redistribution of nutrients within the agro-ecosystem, rather than a real increase in nutrient availability. Some of the factors involved in redistribution include the following:
Lateral roots. Studies from West Africa show that the lateral roots of woody species can stretch out 10m and longer. The fine roots concentrate in the topsoil where most of the water and nutrients are. Thus, nutrients concentrated under tree canopies are pulled in by lateral roots, leaving nutrient depletion at a distance.
Runoff. Nutrient-rich surface flow from higher elevations collects in depressions or other catchments along with organic matter, allowing the establishment of woody species and at least a doubling of available N for herbage.
Animals. Tree shade draws birds, herbivores, and livestock which deposit excrement. Where livestock are kept overnight on croplands in the Sahelian zone, N deposition through manure is 4 to 10 times higher than N deposition on grazing lands.
Other factors influencing nutrient availability include the following:
Leaching. Deep-rooting woody species may do a better job at recapturing leached nutrients, however, deep rooting is not common in the region. Deep rooting is often limited by shallow soils, hardpans, and scarce deep water supplies.
Enrichment. N-fixation by leguminous plants depends on water availability and significant P supplies. But P deficiency is common in the region, made worse by acidification from continuous cropping. Tree legumes common to West Africa are not expected to have much potential for N-fixation; few data exist on their root nodules.
In the Sudanian zone, using trees to improve dry season pastures would increase nutrient competition between woody species and the herb layer. Therefore, developing silva-pastoral systems without negative impacts would require careful design in either zone.
In some cases, deep-rooting woody species may be used to check leaching where annual crops are produced, providing local soil conditions are favorable.
Windbreaks are generally not useful in the Sudanian zone because water is not limiting to plant growth, sandy soils prone to wind erosion are not common, and the risk of abrasion caused by hot dry winds is low during the growing season. In the Sahelian zone, windbreaks are difficult to establish on the shallow soils and may cause crop yields to decrease.
In summary, the deterrents to agroforestry in the Sahelian and Sudanian zones can be great and deserve careful consideration before deciding where woody species would be beneficial to crop production.
For more information:
J.J. Kessler
Department of Nature Conservation
Agricultural University of Wageningen
PO Box 8080,6700 DD Wageningen
The Netherlands