Seasonal variations in soil, grass and shrub water status in a West African humid savanna.

Abstract

Despite their overwhelming influence on (1) variations in the physiological activity of vegetation (2) plant phenology, and (3) potential competitive interactions between coexisting species, comprehensive information on water relations and soil water uptake patterns is still lacking for grasses and shrubs in African humid savannas. Over 2 years, seasonal variations in plant water status were measured with the pressure bomb technique for two deciduous shrub species (Cussonia barteri and Crossopteryx febrifuga) and a perennial grass species (Hyparrhenia diplandra) in a humid savanna. Concurrently, soil moisture was surveyed with the neutron probe technique in grassy areas and under shrub clumps. Absence of transpiration at dawn was assessed by surveys of the leaf 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 were pronounced for the grass species and the shrub C. febrifuga, but weak for the shrub C. barteri. Relationships between plant and soil water status showed that both the grass species and the shrub C. febrifuga did not maintain high water potentials when soil moisture in the upper soil layers decreased. In contrast, the shrub C. barteri exhibited a predawn water potential around -0.5 MPa when the 0 to 60-cm soil layer 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 had very poor and limited access to deep soil layers (below 60 cm), respectively while the shrub C. barteri was able to access this water resource. The relatively low osmotic potential and tissue elasticity measured for C. febrifuga as compared to C. barteri were consistent with the use of water from upper soil layers by this species, since these characteristics could help water withdrawal and promote turgor maintenance at lower shoot water potentials. Different water uptake patterns could explain the contrasted leaf shedding patterns reported for the two shrub species. These results emphasize that the water economy of deciduous shrub species of African humid savannas can differ significantly and that both grasses and some shrub species acquire water from the upper soil layers even during dry spells. These findings will aid understanding of interspecific competition and modelling ecosystem function in this kind of humid savanna.