Objective  Root is the key organ for Populus euphratica to colonize and regenerate in the field. In this study, we investigated the responses of root growth to soil conditions and biological factors to provide support for further study on adaptive strategy of Populus euphratica seedlings and provide a theoretical basis for protection management of Populus euphratica forests in northwestern arid area of China.

  Method  Here, a controlled pot experiment was conducted to study the responses of root morphological features of annual Populus euphratica seedlings to interspecific competitor under 3 levels of water and salinity and their interactive effects.

  Result  (1) Under various levels of water and salinity, roots mainly for absorption (diameter under 5 mm) and for anchoring (above 5 mm) maintained stable proportions, indicating a conservative root morphology of annual Populus euphratica seedlings. But interspecific competition with Tamarix ramosissima changed this property. (2) Annual Populus euphratica seedlings had larger special root length and special root area while growing with Tamarix ramosissima seedlings, which meant higher absorbing ability in mixed planting pattern. However, total root volume, root surface area and basal diameter significantly dropped compared with those grew with intraspecific individuals, demonstrating neighboring Tamarix ramosissima seedlings could intensively restrict root growth of annual Populus euphratica seedlings. (3) Water condition and neighbor had more significant effects on root growth of Populus euphratica seedlings instead of salinity condition. Neighboring Tamarix ramosissima seedlings could change root plastic responses of Populus euphratica seedlings to water and salinity, making Populus euphratica seedlings more vulnerable to salinity stress.

  Conclusion  In summary, root morphology of annual Populus euphratica seedlings shows adaptive strategy while competing with neighboring Tamarix ramosissima seedlings, but still interspecific competition leads to harmful outcomes for growth of Populus euphratica. For seedlings growing on floodplain, interspecific competition may also be a major threat to their survival.