Restoring the Soil - Canadian Foodgrains Bank

40Decision Tree Guideleast a week or two. This happens because with the reduced temperature underthe trees, both the rate of evaporation and the rate of transpiration are reduced.Therefore, the soil dries out slower allowing crops to grow longer. Soil nitrogenalso burns off more slowly. These differences often mean that farmers can, forinstance, grow maize instead of sorghum.The benefit to crops from shade trees provides a win-win situation. In additionto improving crop yields, the trees can produce fruit, timber, firewood, naturalpesticides, and/or soil fertility. All these advantages come with the simple cost ofplanting the trees, protecting them for one or two years from animals (includingtermites), and pruning them once a year (or twice a year if there are two rainyseasons). Even the pruning offers a benefit to families, because the firewood producedin this way usually requires less labor than cutting and carrying firewoodfrom distant forests.Most dispersed shade systems consist of trees planted 8 to 12 m apart in eachdirection, creating a population of about 64 to 150 trees/ha. If only bushes areused (for example, to supply shade to low-growing vegetables), the spacing mightbe only 1.5 to 2.5 m between the bushes. Gm/ccs used in this way include pigeonpeas (Cajanus cajan) and tephrosia (either Tephrosia vogelii or T. candida).Dispersed tree systems will also provide significant protection against the effectsof global warming. As the weather gets hotter, the trees can be pruned a little less,thereby creating the same favorable temperatures for the crops below them. Go to#24.24. Native trees or exotic species? Several rapid growing non-native tree speciescan improve crop yields, simplify management and provide multiple benefits forfarmers. These are mother of cacao (Gliricidia sepium), leucaena (Leucaena leucocephalaor L. diversifolia) and pigeon pea (Cajanus cajan). Mother of cacao growsthree to four times faster under Sahelian conditions than does Faidherbia albida,one of the best native Sahelian species for use as dispersed trees. The mother ofcacao has edible flowers, no thorns and bark that is a natural pesticide. If exotictree species are desired, go to #26.However, native species are usually more resistant to local pests and diseases, ecologicallymore desirable, and better known by the local farmers. Sometimes fieldsalready contain trees of these species, or still harbor root systems that grow everyyear but are often cut off at ground level by farmers. Advantages of local treesinclude the fact that they do not need to be propagated, and their already welldevelopedroot systems will provide good growth and drought resistance from thebeginning. The system of allowing tree roots to grow into trees in farmers’ fieldsis now widely called “Farmer Managed Natural Regeneration,” or FMNR. It hasbeen successfully used to dramatically increase the number of trees in major partsof Niger and Mali.