SEASONAL-VARIATIONS IN SOIL, GRASS AND SHRUB WATER STATUS IN A WEST-AFRICAN HUMID SAVANNA

Despite their overwhelming influence on (1) variations in the physiolo gical activity of vegetation (2) plant phenology, and (3) potential co mpetitive interactions between coexisting species, comprehensive infor mation on water relations and soil water uptake patterns is still lack ing for grasses and shrubs in African humid savannas. Over 2 years, se asonal variations in plant water status were measured with the pressur e bomb technique for two deciduous shrub species (Cussonia barteri and Crossopteryx febrifuga) and a perennial grass species (Hyparrhenia di plandra) in a humid savanna. Concurrently, soil moisture was sun eyed with the neutron probe technique ill grassy areas and under shrub clum ps. Absence of transpiration at dawn was assessed by surveys of the le af water isotopic signal, and plant water relations were characterized during rainy and dry periods with the pressure-volume curve technique , The seasonal changes in predawn and minimum shoot water potentials w ere pronounced for the grass species and the shrub C. febrifuga, but w eak for the shrub C. barteri Relationships bel ts:cen plant and soil w ater status showed that both the grass species and the shrub C. febrif uga did not maintain high water potentials when soil moisture in the u pper soil lavers decreased. In contrast, the shrub C. barteri exhibite d a predawn water potential around -0.5 MPa when the 0 to 60-cm soil l ayer was at the permanent wilting point. During the early dry season, leaf dehydration was observed for C. febrifuga, but not for C. barteri . We conclude that the grass H. diplandra and the shrub C. febrifuga h ad very pool and limited access to deep soil layers (below 60 cm), res pectively while the shrub C. barteri was able to access this water res ource, The relatively low osmotic potential and tissue elasticity meas ured for C. febrifuga as compared to C. barteri were Consistent with t he use of water from upper soil layers by this species, since these ch aracteristics could help water withdrawal and promote turgor maintenan ce at lower shoot water potentials. Different water uptake patterns co uld explain the contrasted leaf shedding patterns reported for the two shrub species, These results emphasize that the water economy of deci duous shrub species of African humid savannas call differ significantl y and that both grasses and some shrub species acquire water from the upper soil layers even during dry spells. These findings will aid unde rstanding of interspecific competition and modelling ecosystem functio n in this kind of humid savanna.