The potential for deep groundwater use by Acacia papyrocarpa (Western myall) in a water‐limited environment

Knowledge regarding the use of groundwater by plants has implications for successful mine rehabilitation and revegetation programs in water-limited environments. In this study, we combined several approaches to investigate water sources used by Acacia papyrocarpa (Western myall) in the far west of South Australia, including stable isotopes, water potential, groundwater and soil chemistry, and root mapping techniques. Plant δ¹⁸O signatures and water potentials were compared against a range of possible sources: rainwater, surface soil water (≤1 m depth), and deep groundwater (>20 m depth). Our aim was to determine whether groundwater contributed to the mix of waters used by A. papyrocarpa.

Overall, we found that trees did not source surface soil water (≤1 m), and probably sourced deep soil water (i.e. >1 m) rather than deep groundwater. Groundwater, however, could not be dismissed as a potential source, as root mapping showed tree roots were capable of reaching groundwater at depths >20 m, and isotope results indicated a potential contribution by groundwater to tree water use. However, low osmotic potentials and/or high acidity levels were shown to pose likely barriers to groundwater uptake, at least at the time of sampling. We conclude that because groundwater salinity and acidity are spatially variable in this region, plants with extensive root systems may be able to utilize zones of groundwater with lower salinity and pH levels. Overall, this study contributes to our limited understanding of groundwater use by trees occurring in water-limited environments where groundwater is extremely deep (>20 m depth).