Populus euphratica Oliv. is a highly salt tolerant tree species, and this study represents the first comprehensive investigation of salt tolerance mechanisms of mature trees of P. euphratica in the field. We measured NaCl concentration in xylem sap, NaCl accumulation in leaves, the effect of NaCl on leaf physiological parameters and osmotic adjustment and the allocation and distribution of NaCl between different plant organs on a whole plant level in trees exposed to mild saline groundwater (around 30 mM) in China. Populus euphratica showed three key mechanisms of salt tolerance. The primary mechanism had a strong control over Na(+) and Cl(-) uptake with effective exclusion mechanisms for Cl(-) with up to 99% of the external NaCl being excluded from the xylem. Secondly, the trees allocated large proportions of NaCl into the leaves, which served as a salt elimination mechanism as the leaves are ultimately shed at the end of the growing season. Thirdly, the trees tolerated high foliar Na(+) concentrations through a combination of osmotic adjustment using sucrose and probable sequestering of Na(+) in the apoplast. Our results indicate that the control of Na(+) and Cl(-) uptake and the regulation of Na(+) and Cl(-) delivery to the shoot are key to salt tolerance of P. euphratica in the field with tolerance of high Na(+) concentrations in leaves being a critical component.