Root structure–function relationships in 74 species: evidence of a root economics spectrum related to carbon economy

Abstract

SummaryAlthough fine roots are important components of the global carbon cycle, there is limited understanding of root structure-function relationships among species. We determined whether root respiration rate and decomposability, two key processes driving carbon cycling but always studied separately, varied with root morphological and chemical traits, in a coordinated way that would demonstrate the existence of a root economics spectrum (RES).Twelve traits were measured on fine roots (diameter ≤ 2 mm) of 74 species (31 graminoids and 43 herbaceous and dwarf shrub eudicots) collected in three biomes.The findings of this study support the existence of a RES representing an axis of trait variation in which root respiration was positively correlated to nitrogen concentration and specific root length and negatively correlated to the root dry matter content, lignin : nitrogen ratio and the remaining mass after decomposition. This pattern of traits was highly consistent within graminoids but less consistent within eudicots, as a result of an uncoupling between decomposability and morphology, and of heterogeneity of individual roots of eudicots within the fine-root pool.The positive relationship found between root respiration and decomposability is essential for a better understanding of vegetation-soil feedbacks and for improving terrestrial biosphere models predicting the consequences of plant community changes for carbon cycling.

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

“…Therefore, the first‐order root of herbs or grasses may not perform the same function as in woody species. Nevertheless, the methodology remains challenging for herbs that have very fine and breakable roots, and there is no consistent conclusion to define which orders in nonwoody roots are absorptive so far (Geng et al., 2014; Liu, He, Zeng, Lei, & Arndt, 2016; Roumet et al., 2016). …”

“…Thus, we predict that relationships between leaf and absorptive root morphological traits may depend on plant phylogeny, and this possible coordination would disappear after adjusting for phylogenetic relatedness. Also, the possible morphological linkage between above‐ and belowground organs may differ between woody and nonwoody plant species because of the difference in their root branch systems (Geng et al., 2014; Roumet et al., 2016). …”

Maintaining connectivity: understanding the role of root order and mycelial networks in fine root decomposition of woody plants

“…Since Silver and Miya's analysis, fine root decomposition studies have largely focused on the effects of litter quality on rate of decay and generally support the notion that litter quality regulates microbial activity and thus decomposition (Chen et al 2001;Lemma et al 2007). In general, decay rates of fine roots are positively correlated with initial concentrations of Ca, Mg, Mn, N and P and negatively correlated with C:N, lignin:N, cellulose, and phenolic compounds including tannins, and lignin (Berg et al 1998;John et al 2002;Jalota et al 2006;Wang et al 2010;Tong et al 2012;García-Palacios et al 2016;Guerrero-Ramírez et al 2016;Roumet et al 2016). However, exceptions have been reported in which initial root C:N, lignin: N and N content did not correlate with rate of decay (Poret et al 2007;Hobbie 2008;Sun et al 2013;Zhang and Wang 2015).…”

“…Fine root lifespan has been shown to increase with branching order, because higher order roots contain more secondary tissues and increased concentrations of suberin, protecting roots from pathogens and desiccation (Guo et al 2008a;Hishi 2007;Xia et al 2010;Adams et al 2013). These differences in structure, function, and rhizosphere associates of different orders of living fine roots should translate into predictable patterns of fine root decomposition following senescence, but so far the ability to make such predictions has eluded the scientific community (Freschet et al 2012;Roumet et al 2016).…”

“…Larger diameter or denser roots (i.e., those with lower SRL) may contain more recalcitrant, mycorrhizal associated compounds (e.g. low concentration of soluble carbohydrates, a high AUF fraction) (Langley and Hungate 2003;Sun et al 2013;Roumet et al 2016).…”