Comparative water use by the riparian trees Melaleuca argentea and Corymbia bella in the wet-dry tropics of northern Australia.

We examined sources of water and daily and seasonal water use patterns in two riparian tree species occupying contrasting niches within riparian zones throughout the wet-dry tropics of northern Australia: Corymbia bella Hill and Johnson is found along the top of the levee banks and Melaleuca argentea W. Fitzg. is restricted to riversides. Patterns of tree water use (sap flow) and leaf water potential were examined in four trees of each species at three locations along the Daly River in the Northern Territory. Predawn leaf water potential was higher than -0.5 MPa throughout the dry season in both species, but was lower at the end of the dry season than at the beginning of the dry season. Contrary to expectations, predawn leaf water potential was lower in M. argentea trees along the river than in C. bella trees along the levees. In contrast, midday leaf water potential was lower in the C. bella trees than in M. argentea trees. There were no seasonal differences in tree water use in either species. Daily water use was lower in M. argentea trees than in C. bella trees. Whole-tree hydraulic conductance, estimated from the slope of the relationship between leaf water potential and sap flow, did not differ between species. Xylem deuterium concentrations indicated that M. argentea trees along the riverbank were principally reliant on river water or shallow groundwater, whereas C. bella trees along the levee were reliant solely on soil water reserves. This study demonstrated strong gradients of tree water use within tropical riparian communities, with implications for estimating riparian water use requirements and for the management of groundwater resources.