Intra‐ and inter‐specific variation in root mechanical traits for twelve herbaceous plants and their link with the root economics space

Plant root traits are diverse and variable, and the way in which they interact has consequences for fundamental functions such as anchorage, or services such as soil fixation. Here, we characterize mechanical traits related to anchorage (tensile strength, strain, stiffness and toughness) at both intra- and inter-specific levels and examine how they covary with other traits related to the root economics space. We grew twelve herbaceous species from contrasting taxonomical families in a common garden experiment. For each species, we excavated root systems and measured mechanical, morphological and chemical traits at two locations (proximal versus distal) for two root types (absorptive versus transport roots). At the intraspecific level, transport roots tended to be stronger and tougher than absorptive roots and could extend further before failure, but were as stiff as absorptiveroots. Where the root was sampled (proximalversus distal) had a limited effect on any root mechanical trait. The five monocots (Poaceae) had stronger and tougher root material than the seven dicots (Fabaceae, Plantaginaceae and Rosaceae), but there were no differences in stiffness. At the interspecific level, mechanical traits covaried positively and were strongly and positively correlated with specific root length (a trait related to the 'do-it-yourself' soil exploration strategy), and negatively with root diameter (a trait related to the 'outsourcing' soil exploration strategy) and root tissue density (a trait related to root lifespan). We demonstrate the important role of species' taxonomical subgroup (monocot versus dicot) and root type in governing mechanical trait variation at both intra- and inter-specific levels. Our results can be regarded as the first evidence of a link between root mechanical robustness and the root economics space, through a strong association with the 'do-it-yourself' soil exploration strategy.