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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. [ABSTRACT FROM PUBLISHER]

Summary 1. Patterns of leaf attributes were compared at regional and global scales in relation to the seasonal availability of water. 2. Light-saturated assimilation rate (A mass ), leaf mass per area (LMA), leaf density, thickness, life span, saturated water content, chlorophyll, nitrogen and phosphorus content were determined during the wet season for 21 tree species in four contrasting habitats in northern Australia. Rainfall in this area is extremely seasonal. 3. A mass and foliar chlorophyll, N and P contents were positively correlated with each other, and were all negatively correlated with LMA, leaf thickness, density and life span. 4. Deciduous species had smaller LMA and leaf life span, and larger foliar N and P contents than did evergreen species. 5. The eight Myrtaceous species had smaller A mass , foliar chlorophyll, N and P contents, and larger LMA, leaf thickness and leaf life span than did the non-Myrtaceous species. 6. Leaves from the closed canopy dry monsoon forest had significantly larger A mass , chlorophyll and P contents than did leaves from the three open canopy habitats (eucalypt open forest, mixed woodland and Melaleuca swamp). This reflected the relatively low proportions of evergreen and Myrtaceous species in the dry monsoon forest. There were also significant intraspecific differences among habitats. 7. Leaf thickness, density and LMA were lower than predicted from globally derived relationships with temperature and precipitation. Tropical seasonally dry biomes are under-represented in such global analyses. [ABSTRACT FROM AUTHOR]

Daily and seasonal patterns of transpiration were measured in evergreen eucalypt trees growing at a wet (Darwin), intermediate (Katherine) and dry site (Newcastle Waters) along a steep rainfall gradient in a north Australian savanna. Relationships between tree size and tree water use were also determined.Diameter at breast height (DBH) was an excellent predictor of sapwood area in the five eucalypt species sampled along the rainfall gradient. A single relationship existed for all species at all sites. Mean daily water use was also correlated to DBH in both wet and dry seasons. There were no significant differences in the relationship between DBH and tree water use at Darwin or Katherine. Among the sites, tree water use was lowest at Newcastle Waters at all DBHs.The relationship between DBH and tree leaf area was similar between species and locations, but the slope of the relationship was less at the end of the dry season than at the end of the wet season at all locations. There was a strong relationship between sapwood area and leaf area that was similar at all sites along the gradient. Transpiration rates were significantly lower in trees at the driest site than at the other sites, but there were no significant differences in transpiration rates between trees growing at Darwin and Katherine.Transpiration rates did not vary significantly between seasons at any site. At all sites, there was only a 10% decline in water use per tree between the wet and dry seasons. A monthly aridity index (pan evaporation/rainfall) and predawn leaf water potential showed strong seasonal patterns. It is proposed that dry season conditions exert control on tree water use during the wet season, possibly through an effect on xylem structure. [ABSTRACT FROM AUTHOR]

Australian savannas exhibit marked seasonality in precipitation, with more than 90% of the annual total falling between October and May. The dry season is characterized by declining soil water availability and high vapor pressure deficits (up to 2.5 kPa). We used heat pulse technology to measure whole-tree transpiration rates on a daily and seasonal basis for the two dominant eucalypts at a site near Darwin, Australia. Contrary to expectations, transpiration rates were higher during the dry season than during the wet season, largely because of increased evaporative demand and the exploitation of groundwater reserves by the trees. Transpiration rates exhibited a marked hysteresis in relation to vapor pressure deficit, which was more marked in the dry season than in the wet season. This result may be attributable to low soil hydraulic conductivity, or the use of stored stem water, or both. Tree water use was strongly correlated with leaf area and diameter at breast height and there were no differences in transpiration between the species studied. These results are discussed in relation to scaling tree water use to stand water use. [ABSTRACT FROM AUTHOR]