Running bamboos are a group of clonal plants exhibiting rapid and widespread expansion. They possess two root subsystems, arising from different shoot parts, culm roots and rhizome roots. To date, it remains unclear what the functional differences are between the two root subsystems and what their relation is to the remarkably competitive ability of running bamboos. A typical running bamboo, Phyllostachys glauca, which dominates in a highly heterogenous habitat of limestone hills, was sampled to compare the morphology, architecture, anatomy, chemical composition, mechanics, and nutrient acquisition of the two root subsystems to explore their differences in functional traits. Compared with rhizome roots, culm roots possessed greater tensile strength, modulus of elasticity, stele diameter to root diameter ratio, root tissue density, root length density, rooting depth, basal diameter of shoot-borne roots, and more lignified cells. Culm roots also had a higher phosphorus (P) concentration (>0.4 mg g^-1), which was nearly twice that of rhizome roots. By contrast, rhizome roots exhibited a greater specific root area, specific root length, cortex thickness to root radius ratio, total branch order, and greater P-uptake capacity than culm roots. Our findings revealed distinct differences in functional traits between culm roots and rhizome roots in P. glauca. Culm roots are considered to primarily provide anchorage, but also function in resource acquisition and nutrient storage, while rhizome roots mainly play a role in resource acquisition. Except for the anchorage and resource acquisition of culm roots, the extra resource acquisition by rhizome roots and the high concentration of a limiting nutrient (P) in culm roots enhance the performance in a highly heterogenous habitat which offers a physiological explanation for P. glauca dominating on limestone hills. The functional differentiation between culm roots and rhizome roots provides insight into the mechanism underpinning the remarkable expansion of running bamboos and offers a new perspective to explain the strong competitive ability of clonal plants with dimorphic roots.