EFFECT OF FIRE ON SOIL, RICE, WEEDS AND FOREST REGROWTH IN A RAIN-FOREST ZONE (COTE-DIVOIRE)

In south-western Cote d'Ivoire mature forest is felled and burned and one crop of upland rice is grown. Forest is then allowed to regenerate for 15 or more years before being cropped again. Soils are very gravely, clayey, acid, thoroughly leached and chemically poor. Forest, soil, rice, weeds and regrowth were studied in the fields of local farmers in permanent plots during 2-5 years. The fields (16) covered all currently cultivated soil types and forest types. Burning leads to an instant mineralisation of organic matter and the ashes produce a marked decrease in acidity (before burning pH 4.4-5.2, after burning pH 5.4-6.2). The nutrient availability is further stimulated by the reduction of Al activity in the soil solution. Fire is a decisive factor in cultivation: slightly burnt or unburnt areas are wholly unproductive. The rice thrives on the nutrients contained in the ash of the burnt forest rather than on the nutrients of the mineral soil: the nature of the soil is of relatively little importance, as long as large quantities of biomass are burnt. A fire of normal intensity destroys enough buried seeds and vegetative parts of forest plants to avoid weed stress, yet allows the survival of a sufficient number of forest plants for forest regrowth after the rice crop. Excessive burning kills sprouts and buried seeds. Regeneration comes then from seeds dispersed into the field after the fire, mainly grasses and forbs. Grassy shrub land or thickets are thus formed.