Water Use Strategies of Two Co-occurring Tree Species in an Australian Semi-arid Karst Environment

Regional communities are often reliant on groundwater to sustain social, economic and environmental uses, with concern arising over substantial water level decline in many aquifers. In recognition of this, water use strategies of Eucalyptus diversifolia and Allocasuarina verticillata were investigated using a water balance, in conjunction with the stable isotopes of water, leaf and soil water potentials to test whether 1) groundwater recharge occurs in the presence of these species, and 2) groundwater is being used by the trees to sustain transpiration during drier months.

Overall, actual evapotranspiration (ET) exceeded rainfall (RF) received (E diversifolia ET = 560 and 504 mm year^-1; A. verticillata ET = 501 and 560 mm year^-1 vs. RF = 461 and 422 mm year^-1), but was similar to the estimated long term average rainfall (502 mm year^-1), suggesting convergence between evapotranspiration and water supply during typical years. Subsurface soil water was a better isotopic match to twig water than either surface soil or groundwater, and contained regions of low bulk density (1.02 g cm^-3) and substantial water holding capacity (0.38 g g^-1). However, geological variability makes it difficult to extrapolate this to the landscape level. Transpiration was greater for E. diversifolia than A. verticillata over the study period (341 mm vs. 184 mm), corresponding with higher midday leaf water potentials for E. diversifolia (average – 2.06MPa) than for A. verticillata (average –  2.86MPa), suggesting extraction at different locations within the soil profile.

These findings suggest it is unlikely groundwater resources are being used to sustain transpiration, despite the water balance indicating losses from the system during the study period. We conclude that the water holding capacity of the porous karst substrate plays an important role in moderating the effect of rainfall variability, and should be considered when assessing ecohydrological characteristics of karst systems.