Partitioning of soil water among tree species in a Brazilian Cerrado ecosystem

Source water used by woody perennials in a Brazilian savanna (Cerrado) was determined by comparing the stable hydrogen isotope composition (delta D) o f xylem sap and soil water at different depths during two consecutive dry s easons (1995 and 1996). Plant water status and rates of water use were also determined and compared with xylem water delta D values. Overall, soil wat er delta D decreased with increasing depth in the soil profile. Mean delta D values were -35 parts per thousand for the upper 170 cm of soil and -55 p arts per thousand between 230 and 400 cm depth at the end of the 1995 dry s eason. Soil water content increased with depth, from 18% near the surface t o about 28% at 400 cm. A similar pattern of decreasing soil water delta D w ith increasing depth was observed at the end of the 1996 dry season. Patter ns consistent with hydraulic lift were observed in soil profiles sampled in 1995 and 1997. Concurrent analyses of xylem and soil water delta D values indicated a distinct partitioning of water resources among 10 representativ e woody species (five deciduous and five evergreen). Among these species, f our evergreen and one deciduous species acquired water primarily in the upp er soil layers (above 200 cm), whereas three deciduous and one evergreen sp ecies tapped deep sources of soil water (below 200 cm). One deciduous speci es exhibited intermediate behavior. Total daily sap flow was negatively cor related with xylem sap FD values indicating that species with higher rates of water use during the dry season tended to rely on deeper soil water sour ces. Among evergreen species,minimum leaf water potentials were also negati vely correlated with xylem water delta D values, suggesting that access to more readily available water at greater depth permitted maintenance of a mo re favorable plant water status. No significant relationship between xylem water delta D and plant size was observed in two evergreen species, suggest ing a strong selective pressure for small plants to rapidly develop a deep root system. The degree of variation in soil water partitioning, leaf pheno logy and leaf longevity was consistent with the high diversity of woody spe cies in the Cerrado.