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

Abstract

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.

Root exudation as a major competitive fine‐root functional trait of 18 coexisting species in a subtropical forest

“…This is because root diameter varies largely among species (Ma et al ., 2018), while root‐order definition provides a more precise cutoff between absorptive and transport roots (Pregitzer et al ., 2002; Xia et al ., 2010; Iversen, 2014; McCormack et al ., 2015). The change from the diameter definition to the order definition also favours interspecies comparison such that fine roots of similar function are compared (Sun et al ., 2017a; Shao et al ., 2019). Both first order and first three orders cut‐offs have been commonly used in recent studies (Freschet et al ., 2017b).…”

Topography as a factor driving small‐scale variation in tree fine root traits and root functional diversity in a species‐rich tropical montane forest

“…One intact fine root strand of at least 10 cm length was sampled from each tree by tracing coarse roots from the stem until fine roots (diameter ≤ 2 mm) were reached. Even though the arbitrarily chosen diameter threshold of 2 mm has recently been criticised for not adequately capturing the most active absorptive finest root segments (McCormack et al ., 2015; Shao et al ., 2019), Freschet & Roumet (2017) pointed out that measurements on roots ≤ 2 mm produce only marginally different results to measurements on the first three root orders. An analysis based on root orders is much more labour intensive, and according to Freschet & Roumet (2017), the 2 mm category is appropriate when the focus is on nutrient acquisition strategies.…”

“…One intact fine root strand of at least 10cm length was sampled from each tree by tracing coarse roots from the stem until fine roots (diameter ≤2 mm) were reached. Even though the arbitrarily chosen diameter threshold of 2 mm has recently been criticized for not adequately capturing the most active absorptive finest root segments (McCormack et al, 2015;Shao et al, 2019), Freschet & Roumet…”

Elevational trends of tree fine root traits in species‐rich tropical Andean forests

“…The roots were then washed, dead and damaged material was removed, and the samples separated into absorptive and transportive fine roots on the basis of morphology. Such a functional approach has been suggested by McCormack et al (2015) and Shao et al (2019), based on the notion that both order‐based and diameter‐based fine root definitions can be arbitrary (Freschet and Roumet 2017). The classification of fine roots of woody plants into primarily absorptive and transportive roots is based on the analysis of the anatomy of fine roots in cross sections, and it is widely agreed upon that the most distal two to four root orders maintain a mainly absorptive function (Kong et al 2016, Trocha et al 2017, Wang et al 2019).…”