The Potential for Deep Groundwater Use by Acacia Papyrocarpa (Western Myall) in a Water‐Limited Environment

Received: 16 May 2016 Revised: 25 August 2016 Accepted: 2 September 2016 DOI 10.1002/eco.1791 RESEARCH ARTICLE The potential for deep groundwater use by Acacia papyrocarpa (Western myall) in a water‐limited environment Emma K. Steggles1 | Kate L. Holland2 | David J. Chittleborough1 | Samantha L. Doudle3 | Laurence J. Clarke4 | Jennifer R. Watling1,5 | José M. Facelli1 1 Biological Sciences, University of Adelaide, Adelaide, South Australia, Australia Abstract 2 CSIRO Land and Water, Glen Osmond, South Knowledge regarding the use of groundwater by plants has implications for successful mine reha- Australia, Australia bilitation and revegetation programs in water‐limited environments. In this study, we combined 3 Iluka Resources Ltd., Kent Town, South several approaches to investigate water sources used by Acacia papyrocarpa (Western myall) in Australia, Australia the far west of South Australia, including stable isotopes, water potential, groundwater and soil 4 Australian Centre for Ancient DNA, chemistry, and root mapping techniques. Plant δ18O signatures and water potentials were com- University of Adelaide, Adelaide, South pared against a range of possible sources: rainwater, surface soil water (≤1 m depth), and deep Australia, Australia groundwater (>20 m depth). Our aim was to determine whether groundwater contributed to 5 Manchester Metropolitan University, Manchester, UK the mix of waters used by A. papyrocarpa. Correspondence Overall, we found that trees did not source surface soil water (≤1 m), and probably sourced deep Emma K. Steggles, Biological Sciences, soil water (i.e. >1 m) rather than deep groundwater. Groundwater, however, could not be University of Adelaide, Adelaide, SA 5005, dismissed as a potential source, as root mapping showed tree roots were capable of reaching Australia. Email: [email protected] 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 occur- ring in water‐limited environments where groundwater is extremely deep (>20 m depth). KEYWORDS rehabilitation, stable isotopes, tree water sources, water potential 1 | INTRODUCTION reestablishing sustainable plant populations (Wang, Mu, Zhang, & Zhang, 2013). Mine sites in dry and remote regions of Australia are often established Plant water use strategies, including seasonal shifts in groundwa- in areas considered high in conservation value. Some of the immediate ter dependency, have been studied in a range of ecosystems including impacts of mining include vegetation clearance and modifications to montane coniferous forests (Xu et al., 2011); karst systems (Swaffer, soil physical, chemical, and biological properties (Jasper, Robson, & Holland, Doody, Li, & Hutson, 2013); and riparian systems (Holland, Abbott, 1987; Rokich, Meney, Dixon, & Sivasithamparam, 2001), as Tyerman, Mensforth, & Walker, 2006; Mensforth, Thorburn, Tyerman, well as changes to groundwater chemistry from tailings storage facili- & Walker, 1994; Thorburn, Hatton, & Walker, 1993a; Wang et al., ties (Wang, Harbottle, Liu, & Xu, 2014). In general, there are legislative 2013). Most research in Australia has focused on semiarid riparian eco- requirements in place for mining companies to manage their environ- systems where groundwater is relatively shallow, <5 m depth, and few mental impacts during extraction processes and to rehabilitate areas studies have investigated water use by trees in regions where ground- for the reestablishment of self‐sustaining native ecosystems. The water is more than 10 m deep. One exception is the study by Zencich, long‐term success of revegetation programs requires an understand- Froend, Turner, and Gailitis (2002), who used stable isotope tech- ing of plant water use strategies in undisturbed areas, so that plant– niques (deuterium δ2H) to identify potential water sources for two soil–water relations can, as far as possible, be optimized for species of Banksia growing over groundwater that ranged in depth Ecohydrology 2017; 10: e1791; wileyonlinelibrary.com/journal/eco Copyright © 2016 John Wiley & Sons, Ltd. 1of10 DOI 10.1002/eco.1791 2of10 STEGGLES ET AL. from 2.5 to 30 m. Both species were shown to use groundwater at highlights a discrepancy between the root‐zone depth in undisturbed shallow depths but not at its deepest, and the authors suggest that this areas and the much shallower depth of overburden soils (6–8m) pattern of water use was a function of moisture availability in replaced on top of tailings in post‐mine rehabilitation sites. It raises shallower soil horizons, root distribution, and maximum rooting depth. questions about potential groundwater use, with groundwater fre- The stable isotope, oxygen‐18 (18O), is also used to identify poten- quently present at depths ranging between 20 and 50 m, and also tial water sources used by plants. Two studies characterized δ18O about how altered plant–soil–water relations may affect the long‐term in deep soils of temperate semiarid regions in Australia. Allison and survival of this species in rehabilitation sites. Shallow soil profile, due Hughes (1983) and Allison, Barnes, Hughes, and Leaney (1984) sam- to insufficient overburden volumes, is a widespread issue for mine pled soil water below 0.5 m at intervals down to 15 and 7 m depths, rehabilitation across arid regions in Australia and elsewhere (Huang, respectively. They found δ18O signatures were relatively constant at Baumgartl, & Mulligan, 2012). For many species, roots are required depths below 3 m and ranged between −2.0 and −4.0‰ (relative to to grow in mine tailings (fine‐grained waste material), which need to standard mean ocean water, SMOW). In contrast, soil water above be physically and hydro‐geochemically stable for plant growth. It is 0.5 m depth was found to have positive δ18O values, most likely necessary to restore physical structures and hydraulic functions across reflecting rainwater infiltration and enrichment from evaporation. the whole rooting zone, and the complexity of this challenge often No significant fractionation of 18O has been observed during plant results in short‐lived remediation success as soil structure and function uptake of soil water (Barbour, 2007), and thus, the isotopic composi- fails to develop, leading to poor plant survival and low recruitment tion of xylem water should match that of water sources (Mensforth levels (Huang et al., 2012). et al., 1994). However, as plants with large root systems generally In this study, we analyzed δ18O from xylem of twigs, trunks, source water from a range of soil locations, the resulting composition opposing lateral roots, and taproots of A. papyrocarpa, as deep‐rooted of twig water is a complex mix of isotope signatures. Consequently, species with dimorphic root architecture are likely to access water multisource mass balance analyses, such as IsoSource™ (United States from a variety of sources. Xylem δ18O signatures and shoot Ψ were Environmental Protection Agency), are used to estimate proportional compared against a range of possible sources: rainwater, surface soil contributions for each possible water source and have been used in water at four depths ≤1 m and deep groundwater >20 m below the several studies (e.g., Fan, Li, Li, & Zhu, 2013, Wang et al., 2013, and surface. The overall aim of our research was to determine whether Swaffer et al., 2013). The IsoSource™ model examines all possible com- groundwater contributed to the mix of waters used by A. papyrocarpa. binations of each source contribution (0–100%) in small increments (e. g., 1–2%), and combinations that sum to the observed isotopic mixture ‰ within a small tolerance (e.g., <0.1 ) are considered to be feasible 2 | METHODS solutions (Phillips & Gregg, 2003). In addition to δ18O measurements, water potentials (Ψ) are also 2.1 | Study site used to infer the accessibility of water to plants. Soil Ψ helps to identify depths in the soil profile from which roots are physically capable The study site (30°50′17.99″S and 132°12′10.37″E) was located at of extracting water. It represents the sum of soil moisture (matric the Jacinth‐Ambrosia (JA) mine site in Yellabinna Regional Reserve, potential), soil salinity (osmotic potential), and gravity. Only soil regions approximately 200 km northwest of Ceduna in South Australia with higher Ψ than shoot Ψ are available to a tree at any given time (Figure 1). The nearest long term weather station to our study site (Holland et al., 2006). Shoot Ψ can be used as an indication of overall was located at Tarcoola, which is 220 km to the east and in similar veg- plant Ψ because water flow from roots to leaves is proportional to etation to that found at the study site. Mean monthly minimum and the root–leaf Ψ difference and to root–leaf hydraulic conductance (Cook & O’Grady, 2006). Water potentials from the saturated zone, where matric potential approximates 0 MPa (i.e., groundwater), can also be compared using osmotic potentials calculated from the chloride concentration of the water (Holland et al., 2006). The survivorship of some plant species in arid ecosystems depends on their ability to access groundwater, which can be located at great depths (e.g. >20 m). Some tree species in arid regions are known to have roots that extend more than 50 m below the surface, for example, Boscia albitrunca and Acacia erioloba (Jennings, 1974) and Prosopis juliflora (Phillips, 1963). A number of forest trees have also been reported to have roots that extend below 20 m depth (Stone & Kalisz, 1991). In this paper, we examine water use in the long‐lived (250+ years) tree, Acacia papyrocarpa (Western myall), which has extensive lateral and vertical root systems.

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