WATER RELATIONS BETWEEN SHRUBS AND GRASSES IN SEMIARID ARGENTINA

The two-layer hypothesis argues that in shrublands shrubs take up most of the water from the lower layers of the soil whereas grasses take up most of the water from the upper layers of the soil. We tested two predictions from this hypothesis in a shrubland of central semi-arid Argentina: (1) the correlation between plant water potential and soil water potential in the upper layers is stronger for grasses than for shrubs and (2) rooting depth is greater for shrubs than for grasses. The apparent pattern of water use by the deciduous shrub (Prosopis caldenia) and the two perennial grasses (Stipa tenuis and Piptochaetium napostaense) under study was consistent with the two-layer hypothesis whereas the evergreen shrub (Condalia microphylla) did not support this model. Our results suggest that the pattern of water use by grasses and shrubs in shrublands may be dependent upon strategies to counteract water stress and the phenological patterns developed by species within each life form. © 1994 Academic Press Limited.