Towards a multidimensional root trait framework: a tree root review

Abstract

Summary The search for a root economics spectrum (RES) has been sparked by recent interest in trait‐based plant ecology. By analogy with the one‐dimensional leaf economics spectrum (LES), fine‐root traits are hypothesised to match leaf traits which are coordinated along one axis from resource acquisitive to conservative traits. However, our literature review and meta‐level analysis reveal no consistent evidence of an RES mirroring an LES. Instead the RES appears to be multidimensional. We discuss three fundamental differences contributing to the discrepancy between these spectra. First, root traits are simultaneously constrained by various environmental drivers not necessarily related to resource uptake. Second, above‐ and belowground traits cannot be considered analogues, because they function differently and might not be related to resource uptake in a similar manner. Third, mycorrhizal interactions may offset selection for an RES. Understanding and explaining the belowground mechanisms and trade‐offs that drive variation in root traits, resource acquisition and plant performance across species, thus requires a fundamentally different approach than applied aboveground. We therefore call for studies that can functionally incorporate the root traits involved in resource uptake, the complex soil environment and the various soil resource uptake mechanisms – particularly the mycorrhizal pathway – in a multidimensional root trait framework.

Complex trait relationships between leaves and absorptive roots: Coordination in tissue N concentration but divergence in morphology

“…One possible explanation is that the complexity of the soil environment, including the heterogeneity in soil nutrient availability and mycorrhizal fungi, presents a variety of constraints to root trait variation (Bardgett et al., 2014; Valverde‐Barrantes, Horning, Smemo, & Blackwood, 2016; Weemstra et al., 2016). In addition, the different above‐ and belowground physiological processes may weaken the relationship between leaf N and absorptive root N. In leaves, tissue N concentration is integral to the proteins of photosynthetic machinery, especially Rubisco.…”

“…Weemstra et al. (2016) recently proposed that, in contrast to a single acquisition–conservation axis in leaves, a multidimensional root trait framework may better accommodate and explain the variation in root traits observed across species.…”

“…On the other hand, evidence is accumulating in support of no ecological or complex linkages between leaves and root traits (Freschet et al., 2013; Valverde‐Barrantes et al., 2015; Weemstra et al., 2016), challenging a single spectrum of worldwide plant economics spectrum. For example, data from temperate grassland plant communities and tree species reported that there are complex or multidimensional relationships between corresponding leaf and root traits (Freschet et al., 2013; Kembel & Cahill, 2011).…”

“…For example, data from temperate grassland plant communities and tree species reported that there are complex or multidimensional relationships between corresponding leaf and root traits (Freschet et al., 2013; Kembel & Cahill, 2011). The complex soil environment as well as mycorrhizal interactions resulted in the functional difference between leaves and roots; thus, the resource economics syndromes that have been widely observed in leaves cannot be directly extrapolated to roots (Mommer & Weemstra, 2012; Weemstra et al., 2016). …”

Intraspecific trait variation can weaken interspecific trait correlations when assessing the whole‐plant economic spectrum

“…Many studies have also demonstrated correlations between economic traits of leaves and stems (Brodribb & Feild, 2000; Markesteijn, Poorter, Paz, Sack, & Bongers, 2011) and between leaves and fine roots (Craine, Froehle, Tilman, Wedin, & Chapin, 2001; Reich et al., 2008; Tjoelker, Craine, Wedin, Reich, & Tilman, 2005). On the other hand, several other studies have reported weak correlations among leaf, stem, and root traits, or no correlation at all (Baraloto et al., 2010; Fortunel, Fine, & Baraloto, 2012; Jager, Richardson, Bellingham, Clearwater, & Laughlin, 2015; Pietsch et al., 2014; Weemstra et al., 2016; Wright et al., 2007), indicating that interspecific correlations across organs are not consistently observed. Moreover, while substantial volumes of theory and data support relationships between morphological functional traits and RGRs (Hunt & Cornelissen, 1997; Reich, Tjoelker, Walters, Vanderklein, & Buschena, 1998; Shipley, 2006), correlations observed in the field are sometimes weak or nonexistent (Paine et al., 2015; Poorter et al., 2008; Wright et al., 2010).…”