Fine root morphology is phylogenetically structured, but nitrogen is related to the plant economics spectrum in temperate trees

Summary Plant functional traits have revealed trade‐offs related to life‐history adaptations, geographical distributions, and ecosystem processes. Fine roots are essential in plant resource acquisition and play an important role in soil carbon cycling. Nonetheless, root trait variation is still poorly quantified and rarely related to the rest of the plant. We examined chemical and morphological traits of 34 temperate arbuscular mycorrhizal tree species, representing three main angiosperm clades (super‐orders asterid, magnoliid and rosid). We tested to what extent fine root chemical and morphological traits were correlated similarly to the leaf economical spectrum (LES) or were structured by ancestral affiliations among species. Root traits did not display the same trade‐offs as leaves (e.g. specific root length was not correlated with root N, whereas specific leaf area was correlated with leaf N). Moreover, 75% of below‐ground traits were phylogenetically structured according to Pagel's λ and Abouheif's Cmean autocorrelation tests, as opposed to 28% of above‐ground traits. Magnoliids showed thicker, less branched roots than asterids or rosids, but rosid roots exhibited lower N and higher non‐acid‐hydrolysable (e.g. lignin) content than other species. In contrast, leaf traits did not differ significantly among super‐orders. At the whole‐tree level, chemical traits such as nitrogen tissue content and lignin content were correlated between above and below‐ground organs. The distribution of root traits in woody temperate trees was better explained by shared ancestry than by the nutrient content and structural trade‐offs expected by the LES hypothesis. Root chemistry and morphology differed substantially among species belonging to different super‐orders, suggesting deep divergences in resource acquisition strategies among major angiosperm groups. Although we found partial support for the idea of whole‐plant integration based on corresponding nitrogen content across all organs (i.e. a plant economics spectrum), our study stresses phylogenetic affiliation as the primary driver of root trait distributions among angiosperms, a pattern that could be easily overlooked based solely on above‐ground observations.