The root economics spectrum (RES), a common hypothesis postulating a tradeoff between resource acquisition and conservation traits, is being challenged by conflicting relationships between root diameter, tissue density (RTD) and root nitrogen concentration (RN). Here, we analyze a global trait dataset of absorptive roots for over 800 plant species. For woody species (but not for non-woody species), we find nonlinear relationships between root diameter and RTD and RN, which stem from the allometric relationship between stele and cortical tissues. These nonlinear relationships explain how sampling bias from different ends of the nonlinear curves can result in conflicting trait relationships. Further, the shape of the relationships varies depending on evolutionary context and mycorrhizal affiliation. Importantly, the observed nonlinear trait relationships do not support the RES predictions. Allometry-based nonlinearity of root trait relationships improves our understanding of the ecology, physiology and evolution of absorptive roots.

Invasion by exotic species is one of the serious socio-economic, environmental and ecological problems currently faced by mankind. Biological invasions have changed the species composition, structure and function of ecosystems, and are seriously threatening global biodiversity, economy and human health ( Iqbal et al. 2021; Wang et al. 2020; Yang et al. 2021; Zhao et al. 2020; Zheng et al. 2015). Biological invasions have resulted in an economic loss of at least US$ 1.288 trillion over the past few decades worldwide ( Diagne et al. 2021). As a consequence of these far-reaching impacts, biological invasions have become a hot research topic in modern ecology, and attract major attention from international organizations, governments and scientists all over the world.