Intraspecific adaptation to bioclimatic origins: Intricate interplay of above‐ and below‐ground traits in a cosmopolitan grass species

Abstract Research on the factors driving below‐ground traits at broad scales is crucial for understanding plant adaptation to diverse environments and the function of the plant economics spectrum. However, studies on intraspecific trait variation (ITV) remain scarce, and even fewer address the integration of both above‐ and below‐ground traits. By cultivating 74 globally sourced genotypes of Phragmites australis , a cosmopolitan grass species, in a controlled environment, we measured both above‐ and below‐ground traits. Our results revealed significant latitudinal variation, with plant height, shoot diameter and rhizome diameter decreasing, while shoot number, leaf allocation and specific root length (SRL) increased with absolute latitude. Bioclimatic heterogeneity had a stronger influence on ITV than geographical isolation. Above‐ground traits responded primarily to radiation and temperature, while below‐ground traits were more influenced by precipitation. Our results highlighted the presence of two distinct yet interconnected trait modules for above‐ground and below‐ground traits. Synthesis . This coordination enhances resource use efficiency and ecological success across latitudinal gradients, enabling plants to thrive in diverse environments. The study establishes a comprehensive framework for deciphering the intricate interplay of environmental factors that drive plant adaptive strategies. By revealing how ITV responds to bioclimatic heterogeneity, our research offers insights into the resilience of widespread plants under global climate change.