Spatial distribution characteristics of fine roots of Populus euphratica Oliv. under different groundwater depths in arid regions

The spatial distribution of fine roots can well reflect the utilization degree and adaptability of plants to the environment, which is essential for the evaluation of adaptation to adverse circumstances. We investigated the spatial distribution of Populus euphratica Oliv. fine roots (D ≤ 2 mm) and the relationship with groundwater depth and soil water content. Fine roots and soil samples were collected by the artificial trench profile method. Results showed that:(1) In the horizontal direction (range of 550 cm), fine root length density (RLD), fine root surface area density (SAD), and fine root mass density (RMD) of< P. euphratica changed insignificantly with distance from the tree trunk. (2) In the vertical direction, there were almost no fine roots in the top layer of the soil profile. With the increase in soil depth, both RLD and RMD exhibited increasing trends at first, and then showed decreasing trends. We found high fine specific root length (SRL) and specific root area (SRA) of< P. euphratica at the 280 cm soil depth. (3) RLD and RMD showed positive correlations with soil moisture content of the upper soil (0-180 cm), but exhibited spatial heterogeneities with soil moisture at deeper layers. These results revealed that P. euphratica roots were restricted by soil water conditions in the upper soil, and the fine roots in deeper soil were mainly affected by groundwater. The P. euphratica root system not only demonstrated the ability to develop into the deep moist soil, but also showed a strong horizontal expansion capability to cope with drought environments. This study provides a reference for understanding the adaptation mechanism of P. euphratica to extremely arid environments.