SPATIAL PARTITIONING OF THE SOIL-WATER RESOURCE BETWEEN GRASS AND SHRUB COMPONENTS IN A WEST-AFRICAN HUMID SAVANNA

Most savanna water balance models assume water partitioning between gr asses and shrubs in a two-layer hypothesis, but this hypothesis has no t been tested for humid savanna environments. Spatial partitioning of soil water between grasses and shrubs was investigated in a West Afric an humid savanna by comparing the isotopic composition (oxygen-18 and deuterium) of soil water and plant stem water during rainy and dry con ditions. Both grass and shrub species acquire most of their water from the top soil layer during both rainy and dry periods. A shift of wate r uptake pattern towards deeper horizons was observed only at the end of the dry season after shrub defoliation. The mean depth of water upt ake, as determined by the isotopic signature of stem water, was consis tent with grass and shrub root profiles and with changes in soil water content profiles as surveyed by a neutron probe. This provides eviden ce for potentially strong competition between shrubs and grasses for s oil water in these humid savannas. Limited nutrient availability may e xplain these competitive interactions. These results enhance our under standing of shrub-grass interactions, and will contribute to models of ecosystem functioning in humid savannas.