Transpiration and groundwater uptake from farm forest plots of Casuarina glauca and Eucalyptus camaldulensis in saline areas of southeast Queensland, Australia

Plantings of salt-tolerant tree species are commonly used to manage shallow saline water tables in agricultural lands in Australia. Eucalyptus camaldulensis is often used for this purpose due to its salt tolerance and ability to use groundwater. Salt tolerance studies suggest that Casuarina glauca would also have groundwater management potential, however, little is known about its transpiration and groundwater uptake properties. A combination of stable isotope ( 2 H and 18 O ) techniques and sapflow measurements were used to determine groundwater discharge by both species at three saline sites. This included the use of a three-layer model for determining the proportions of water utilised from various soil depths. At one site, groundwater was the dominant water source for C. glauca in over 70% of sampling times. However, at a higher rainfall site, groundwater was the major water source for this species in only 40% of sampling times. E. camaldulensis relied less on groundwater than C. glauca, utilising water from mid-soil layers to a greater extent. Groundwater was the dominant water source for this species in 50% and 25% of sampling times at each of the two sites, with lower use of groundwater at the higher rainfall site. Low water use rates were observed in both the species; 1.5 to 3 mm day −1 in C. glauca and 1 to 3 mm day −1 in E. camaldulensis. Due to the low water use rates, groundwater discharge rates seldom exceeded 2 mm day −1, with the maximum discharge rate of 2.6 mm day −1 being recorded for C. glauca. As C. glauca relied more heavily on groundwater than did E. camaldulensis, and both species exhibited conservative water use rates, under the conditions of this study, C. glauca showed greater potential than E. camaldulensis to discharge saline groundwater.