Moso bamboo(Phyllostachys edulis), an economically important plant, is often cultivated not only for its delicious shoots and versatile culms, but also as an important biomass resource in southern China. However, with its robust growth and strong rhizomes, Moso bamboo populations have been expanding rapidly into adjacent forests. Root plasticity may enable the bamboo to efficiently forage for nutrients in heterogeneous soils. The ability of a Moso bamboo plant to respond to competition through root plasticity may be an important trait for the species, but little information is available on how root systems respond to belowground competition during colony expansion. To provide baseline information on the morphological plasticity of the response of Moso bamboo roots during population expansion, we continuously sampled three forest types. These were 1) Moso bamboo forest; 2) mixed bamboo, needle-and broad-leaved forest(or transition forest); and 3) mixed needle-and broad-leaved forest. This study was conducted in a typical transition zone in the TianMu Mountain Nature Reserve, Zhejiang Province, China, where there is ongoing bamboo expansion. Rhizomes and roots from Moso bamboo, and roots from mixed needle-and broad-leaved trees were collected by the soil core method to analyze root morphological parameters, including root biomass density, special root length, and internodes length of lateral roots of the same order. The results showed that(1) the root/rhizome system biomass of the three forest types could be ranked as Moso bamboo forest(Moso bamboo root/rhizome system biomass=1630.46 g/m²) > Moso bamboo in the transition forest(902.76 g/m²) > the mixed needle-and broad-leaved forest(the needle-and broad-leaved trees root system biomass=412.76 g/m²) > the needle-and broad-leaved trees root system biomass in the transition forest(272.36 g/m²). As Moso bamboo expanded into a mixed needle-and broad-leaved forest, Moso bamboo culm density increased gradually, with a corresponding increase in root biomass.(2) In a vertical direction, the root system biomass of Moso bamboo in the 0-10 cm soil layer increased from 44% of all root biomass in the Moso bamboo forest to 56% in the transition forest. Simultaneously, the root biomass of needle-and broad-leaved trees shifted from the upper to lower soil layers. The root system of Moso bamboo managed to place more biomass in upper soil layers when it encountered other trees, allowing it to gain a competitive advantage.(3) In the horizontal direction, the root biomass of Moso bamboo did not significantly decline with an increase in distance from the sample tree, while the root biomasses of needle-and broad-leaved trees decreased with increasing distance from the sample tree.(4) The special root length of Moso bamboo in the transition forest increased by an average of 15% compared to the Moso bamboo forest, while the internodes length of first and second lateral roots decreased. The number of bamboo lateral roots also increased. Moso bamboo, with its ability to increase the special root length and decrease the length of the lateral root internodes, would outcompete trees in the belowground environment. These findings indicate that the morphological plasticity of Moso bamboo, shown by its root biomass, special root length, and internode length of the same order lateral roots, easily results in the expansion of Moso bamboo populations into surrounding forest.