Revisiting the root economics space—its applications, extensions and nuances advance our understanding of fine-root functioning

Background and aims: Fine roots and their traits determine resource uptake from the soil, thus being fundamental for plant and ecosystem functioning. It has been five years since the concept of the root economics space (RES) has been developed to describe multidimensional fine-root trait coordination. The RES proposed a novel fungal collaboration gradient in addition to the established fast-slow gradient of resource conservation. This review addresses both researchers already using the RES and those newly introduced to the concept. Our objective is to evaluate the empirical support for the concept, explore trait extensions and implications for ecosystem functioning, and examine future prospects of the RES. Scope/Results: We conducted a literature review of 134 papers working with the RES to quantitatively assess support for the concept and its two trait gradients. The RES, particularly the collaboration gradient, is widely supported across organizational levels, habitats and study designs. Multidimensionality in the trait space appears to be a universal pattern. We further map traits that have been added to the RES concept and discuss the special role of legumes and ecto- versus arbuscular mycorrhizal fungi. Conclusions: We conclude that the RES is a powerful concept to understand fine-root functional variation. Moving forward, we emphasize the need to integrate additional traits to develop a more comprehensive framework for understanding plant and ecosystem functioning.