【Objective】 Forest productivity is closely related to fine root growth, and in this study, we simulated field concentrations of phenolic acids to examine the morphological responses of poplar seedling roots to phenolic acids. The objective was to provide in-depth insights into the rhizosphere effects of tree roots. 【Method】 Using an improved Hoagland solution, we generated phenolic acid environments designed to reflect the contents of phenolic acids in the soils of a successive rotation poplar plantation. All roots of poplar seedlings were harvested and 50% of the fine roots (diameter < 2 mm) were sampled and grouped according to order. A WINRHIZO root system analyzer and associated software were used to determine the morphological traits, including root length and diameter, of each fine root order (orders 1-5). Permanent paraffin cross-sections of fine roots of each order were prepared to observe anatomical traits, such as cross-section diameter and vascular cylinder (stele) diameter. Finally, several important parameters related to fine root morphology, including specific root length (SRL), root tissue density (RTD), and the ratio of vascular cylinder to cross-section area were calculated. Origin Pro 8.0 software was employed for data analysis and MS Excel was used to analyze the relationship between root morphology and the cross-section structures of the different fine root orders. 【Result】 We found that poplar roots biomass (dry weight) was significantly reduced after phenolic acid treatment. Although the biomass of fine root orders 1 to 5 showed no significant difference between the control check (CK) and phenolic acid treatments, the ratio of fine roots to total roots was significantly higher in seedlings receiving phenolic acid treatment than that of CK seedlings. Phenolic acids inhibited the elongation growth of fine roots, with the lengths of fine root orders 1 to 3 being significantly reduced under phenolic acid treatment. Furthermore, seedlings treated with phenolic acids showed an increase in fine root diameter, whereas the surface areas of fine root orders 1 to 5 were smaller under phenolic acid treatment than that of CK seedlings. Phenolic acids also affected the SRL and RTD, with the former being reduced and the latter increased in response to treatment. The anatomical traits of poplar roots were significantly altered under phenolic acid treatment, and the ratios of vascular cylinder to cross-section diameter of the roots of each order were increased, thereby indicating significant changes in the transport tissues of fine roots.【Conclusion】 Phenolic acids were found to have significant inhibitory effects on the fine root growth and development of poplar cuttings. The changes in fine root morphology revealed the variability in roots function under phenolic acid treatment, which would affect the absorptive function of fine roots and further inhibit the above-ground biomass growth of poplar. Furthermore, we characterized the strategies of tree root development and growth investment in response to phenolic acids with respect to differences in fine root morphology among different root orders.