Run-on contribution to a sahelian two-phase mosaic system : soil water regime and vegetation life cycles

An experiment was carried out from 1992 to 1995, in South-Western Niger on a banded vegetation pattern which dominates on a laterite-capped plateau in the region. We quantified the changes in infiltration and vegetation in a thicket from which run-on from the upslope bare soil zone was artificially divested. A concret wall (40 m long, 60 cm high, 20 cm thick, with a foundation 25 cm deep) was constructed at its upslope boundary. Infiltration was measured to a depth of 5.4 m by a neutron probe, and densities of annual plants were monitored along transects crossing perpendicularly a control thicket and the thicket deprived of run-on. Phenological phases and leaf water potential of the two dominant shrub species were recorded from stratified sampling according to their preferred location along the water resource gradient. Results indicated that run-on contributed the most to infiltration in the central zone, but the water content available to the annual plants (layer 0-10 cm) between zones (upslope and central), and between thickets after the wall was built. However, in the thicket deprived of run-on, life cycle and physiology of the shrubs were severely disturbed upslope, while much smaller effects were observed in the centre. Surprisingly, within the study interval, run-on contribution was not found to be as essential to shrubs' life cycle at the location where it contributed the most to the infiltration. (Résumé d'auteur