The increasing frequency of extreme drought events due to global climate change has resulted in reduced productivity and heightened mortality risks in plantation forests on the Loess Plateau. However,it remains unclear whether adjustments in the physiological traits of typical tree species in response to extreme drought conditions can enhance their drought tolerance. This study selected four typical tree species-Robinia pseudoacacia,Syringa reticulata,Quercus liaotungensis,and Pinus tabulaeformis-widely distributed across the Loess Plateau to investigate how their root systems change during extreme drought conditions. We investigated the hydraulic characteristics,non-structural carbohydrate (NSC) content,variations in the pit membrane pore structure,and their interrelations throughout the drought process. The results showed that: (1) After 20 days of drought stress,the root xylem hydraulic conductivity of R. pseudoacacia,S. syringa,Q. liaotungensis,and P. tabulaeformis dropped to 0.08 kg m^-1 s^-1 MPa^-1,0.22 kg m^-1 s^-1 MPa^-1,0.09 kg m^-1 s^-1 MPa^-1,and 0.71 kg m^-1 s^-1 MPa^-1,respectively,while the embolism of each species roots were 97.2%,43.2%,63.1%,and 22.7%. (2) With decreasing hydraulic conductivity,the NSC content in the root xylem of R. pseudoacacia,Q. liaotungensis,and P. tabulaeformis initially increases and subsequently decreases. The content of NSC in the xylem of R. pseudoacacia and Q. liaotungensis roots significantly increased by 57.9% and 85.5% compared to the initial state after 30 days of drought stress,but increased by 23.5% and 47.4% compared to the initial state after 50 days of drought stress,respectively. The content of NSC in the root xylem of P. tabulaeformis increased significantly by 41.2% compared to the initial state after 50 days of drought stress,and decreased significantly by 8.2% compared to the initial state after 70 days of drought stress. As the hydraulic conductivity declined,the content of NSC in S. syringa root's xylem decreased by 20.2% and 15.5% after 30 and 50 days of drought stress,respectively. (3) Throughout the extreme drought process,the pore membrane porosity in various plants initially contracted and then expanded in response to alterations in their water microenvironment and osmotic potential. The pit membrane porosity of the four tree species contracted to 0.04-0.60 times the initial state after 50 days of drought stress,but increased to 0.42-1.38 times the initial state after 70 days. This study suggests that when extreme drought stress occurs,the xylem of R. pseudoacacia roots is first at risk of hydraulic failure,while the xylem of S. syringa roots may face the risk of carbon starvation,and P. tabulaeformis roots can survive the longest. The study clarifies the response mechanisms of plant roots to severe drought stress,providing a scientific foundation for the management and nurturing of artificial forests.