1. The competitive advantage of lianas over trees has been widely documented in studies of their leaf functional traits across diverse habitats; however, the relative contribution of root functional traits to the competitive superiority of lianas over trees has not yet been evaluated. The aim of this study was to explore the root functional traits, phylogenetic structure of these traits and root trait dimensions of lianas to clarify why lianas can outperform trees. 2. We sampled 69 liana species from tropical and temperate forests in China and measured nine key functional traits of first-order roots of each species, including morphological, architectural, anatomical and chemical traits, as well as the percentage of mycorrhizal colonization. Data on these traits were then compared with similar data of 127 tree species from the same biome obtained from the Global Root Traits (GRooT) database and our previous studies. 3. Liana roots had lower construction costs and could acquire resources more rapidly compared with tree roots. Significant differences were observed in most tree root traits between tropical and temperate sites. However, no significant differences were observed in any of the liana root traits between tropical and temperate sites, apart from the root branching ratio. 4. Lianas showed much weaker phylogenetic conservatism in their root traits than trees when species were pooled across sites. Phylogenetic constraint was lower for nearly all root traits of both temperate lianas and trees compared with those of tropical lianas and trees. 5. The root economics space of lianas and trees had two orthogonal dimensions with 'conservation' and 'collaboration' axes. However, lianas occupied the trait space with higher root nitrogen concentration and greater specific root length, showing 'fast' resource acquisition strategy, while trees placed opposite space and exhibited relatively 'slow' strategy. 6. Synthesis. The ability of lianas to outcompete trees in harsh environments might be explained by their faster resource acquisition strategy and the lower phylogenetic constraint in root traits. Generally, lianas might play an increasingl important role in the structure and function of forest ecosystems in the future with ongoing habitat fragmentation and climate change.