Transpiration increases during the dry season: patterns of tree water use in eucalypt open-forests of northern Australia

Australian savannas exhibit marked seasonality in precipitation, with more than 90% of the annual total falling between October and May. The dry seaso n is characterized by declining soil water availability and high vapor pres sure deficits (up to 2.5 kPa). We used heat pulse technology to measure who le-tree transpiration rates on a daily and seasonal basis for the two domin ant eucalypts at a site near Darwin, Australia. Contrary to expectations, t ranspiration rates were higher during the dry season than during the wet se ason, largely because of increased evaporative demand and the exploitation of groundwater reserves by the trees. Transpiration rates exhibited a marke d hysteresis in relation to vapor pressure deficit, which was more marked i n 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 brea st height and there were no differences in transpiration between the specie s studied. These results are discussed in relation to scaling tree water us e to stand water use.