Although fine roots are essential for the water and nutrient uptake of plants, there is limited understanding of root trait variation and the underlying mechanism. Here, six first‐order root morphological and chemical traits were measured for 181 species from eight subtropical and boreal forests to test the hypothesis of different phylogenetic and environmental regulations of root morphological and nutrient traits result in the multidimensions of root traits. Two independent root trait dimensions between root thickness and nutrient traits were detected at both species and community levels. At the species level, diameter‐related traits were mainly restricted by phylogenetic structure and showed little plasticity to the changing environments, whereas the variation in woody root nutrient was influenced significantly by soil variables. For community‐level traits, the diameter‐related axis scores of principal component analysis were mainly driven by mean annual temperature through shifting species composition, whereas the root nutrient‐related axis scores were strongly influenced by soil P availability. From both species and community levels, our study confirms, that the root‐thickness‐related dimension and root nutrient dimension represent new support for the multidimensionality of root traits which are driven by different selection pressure. This study also underlines that the community‐aggregated traits might serve as a promising avenue to improve our understanding of community assemblage processes, allowing us to predict changes of vegetation distributions in a changing climate. A plain language summary is available for this article.

Leaves and absorptive roots (i.e., first‐order root) are above‐ and belowground plant organs related to resource acquisition; however, it is controversy over whether these two sets of functional traits vary in a coordinated manner. Here, we examined the relationships between analogous above‐ and belowground traits, including chemical (tissue C and N concentrations) and morphological traits (thickness and diameter, specific leaf area and root length, and tissue density) of 154 species sampling from eight subtropical and temperate forests. Our results showed that N concentrations of leaves and absorptive roots were positively correlated independent of phylogeny and plant growth forms, whereas morphological traits between above‐ and belowground organs varied independently. These results indicate that, different from plant economics spectrum theory, there is a complex integration of diverse adaptive strategies of plant species to above‐ and belowground environments, with convergent adaptation in nutrient traits but divergence in morphological traits across plant organs. Our results offer a new perspective for understanding the resource capture strategies of plants in adaptation to heterogeneous environments, and stress the importance of phylogenetic consideration in the discussion of cross‐species trait relationships.