Future direction of searching for root economics spectrum: focusing on the fibrous roots “absorptive unit”

As one of the trait-based approaches, leaf economics spectrum (LES) has led to significant advances in plant ecology; however, hypothetical root economics spectrum (RES) which is even more important than the LES aboveground is still less well understood. By analogy with the LES, hypothetical RES may emerge from the coordinated functional traits of fine roots. However, lots of previous studies revealed inconsistent evidence of an RES across the woody species which dominate the terrestrial ecosystem. By discussing the sampling problems in previous studies, we realize that the main reason of failing to search the RES across woody species might be the lack of a standardized sampling protocol. Using inconsistent and inaccurate basic samples of absorptive roots in previous studies impeded the search for an RES. Mycorrhizal interactions and soil matrix constraints do have influences on fine-root traits; however, there is no adequate evidence to support that these factors can offset or impede selection for an RES. In order to make sure the comparability across species and between studies, we come up with a concept absorptive unit of fibrous roots. The absorptive unit, which integrates all the absorptive fibrous root orders interconnected in a distal nonwoody cluster as a whole, is a parallel to a leaf. Excluding all the pioneer roots and treating absorptive unit as a basic sample can make sure the comparability across species and between studies. We believe it will offer a promising way to find consistent evidence that support the RES.