Estimation of Water Sources of Invasive Tree Species in Arid Environments by Oxygen Stable Isotope Analysis Tadaomi SAITO* 1) , Mayu TSUKUMO 2) , Mohamed A. M. Abd ELBASIT 3), 9) , Hiroshi YASUDA 4) Takayuki KAWAI 5) , Naoko MATSUO 6) , Koji INOSAKO 1) , Kumud ACHARYA 7) , Amir E. BABIKER 3) Abubakr A. HAMD 3) and Hiroshi NAWATA 8) Abstract: The objective of this study is to estimate water resources of native and invasive tree species in arid environments by oxygen stable isotope analysis. The target trees were Prosopis juliflora (invasive) in Sudan and Tamarix ramosissima (invasive) and Prosopis pubescens (native) in USA. Three sampling sites were established in Khartoum, Sudan and one site was established in Nevada, USA in 2012. Stems of target trees, soils at various depths, groundwater and rainwater were collected in each site. Meteorological conditions and soil-stem water content have been monitored in these sites. For the plant and soil samples, water was extracted using a vacuum distillation system. The į 18 O content of the groundwater, rainfall, stem water and soil water were measured using an isotope ratio mass spectrometer. The results from oxygen stable isotope analysis showed that Prosopis juliflora in Sudan did not use only groundwater but also used the soil water in the rainy season, although this species is generally thought to use groundwater through deep tap roots. Similar results were shown from stem water content monitoring in the sites. Both Tamarix ramosissima and Prosopis pubescens in USA used shallow saline groundwater, suggesting that not only Tamarix but also Prosopis pubescens are salt tolerant species. Key Words: Invasive species, Mesquite, Oxygen stable isotope analysis, Tamarix , Xylem water 1. Introduction Trees in arid and semi-arid regions have several mechanisms to make efficient use of limited water resources to survive harsh conditions. Recently, approaches using stable isotopes have been developed as a powerful tool for investigating processes in plant-water relations such as recognizing plant water use and response to different types of water sources, better understanding water utilization processes, water use efficiency, and the pattern, mechanism, and ability to adopt to arid environments (Yang et al ., 2010). In nature, there exist two stable isotopes of hydrogen ( 1 H - protium and 2 H - deuterium) and three stable isotopes of oxygen ( 16 O, 17 O, 18 O). The concentrations of 2 H and 18 O in meteoric waters exhibit a broad range of variations, both in time and space. The isotopic composition of water within the water cycle varies during phase changes (evaporation and condensation). For example, 18 O has two extra neutrons and is a bit heavier than 16 O, which results in a preference for evaporating the lighter 16 O containing water and leaving more of the 18 O water behind in the liquid state (called isotopic fractionation). However, water is not isotopically fractionated when taken up by the plant (Thorburn and Walker, 1994). As a result, water in plant tissues carries the same isotopic signal as the source water ( e.g. rainwater, groundwater, soil water, etc.) until it reaches the sites of evaporation or transpiration, generally in the leaves. Therefore, the isotope ratio of stem water can be used as a measure of the isotopic signature of the water being utilized (Craig, 1966). The objective of this study is to estimate water sources of native and invasive tree species in arid environments by oxygen stable isotope analysis. The target trees were Prosopis juliflora (invasive) in Sudan and Tamarix ramosissima (invasive) and Prosopis pubescens (native) in US. 2. Materials and Methods 2.1. Target trees Prosopis (mesquite) is a genus of “multi-purpose” tree, native to South and Central America and the Caribbean, which has been introduced widely into arid and semi-arid regions of the world (Gallaher and Merlin, 2010; Pasiecznik et al ., 2001). Nilsen et al . (1983) reported that Prosopis * Corresponding Author: tadaomi@muses.tottori-u.ac.jp Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori, 680-8553, Japan 1) Faculty of Agriculture, Tottori University 6) Graduate School of Bioresources, Mie University 2) Graduate School of Agriculture, Tottori University 7) Center for Environmental Remediation and Monitoring, Desert Research Institute, USA 3) Environment and Natural Resources and Desertification Research Institute, Sudan 8) Research Institute for Humanity and Nature, Japan 4) Arid Land Research Center, Tottori University 9) Faculty of Science, University of Johannesburg 5) Research Institute for Natural Hazards and Disaster Recovery, Nigata University Received, November 10th, 2013; Accepted, February 1st, 2014 － DT11 Refereed Paper － 沙漠研究 24-1, 29-32 (2014) Journal of Arid Land Studies