Fine roots are responsible for most nutrient and water acquisition in plants and drivers of belowground ecological processes such as soil carbon accrual, soil microbial dynamics and soil respiration. Attributes such as specific root length (SRL), diameter, root tissue density (RTD) and root nitrogen content (RNC) have been reported as key traits influencing ecosystem processes. Nevertheless, few hypotheses have been proposed to explain fine root trait diversity among seed plants (Spermatophyta). One common hypothesis is that root traits should be directly correlated with leaf traits as part of the resource-use economic plan of the entire plant (PES hypothesis). Then, leaf and roots should exhibit similar trait syndromes reflecting their resource acquisition strategies. Similarly, is expected that mycorrhizal associations would align with the PES gradient with ectomycorrhizal species falling in the conservative side of the spectrum. Additionally, it has been proposed that trait syndromes in roots are highly phylogenetically conserved (RTPC hypothesis), reflecting independent evolutionary strategies for resource acquisition among plant lineages. Here, compiling root, leaf, climatic ranges and mycorrhizal status of 600+ species from the literature, we describe for the first time global patterns in trait variation for four key functional root traits, testing the validity of both PES and RTPC hypotheses.
Results/Conclusions
Rather than the tight coupling with leaves, or mycorrhizal types, as expected from the PES hypothesis, we found strong trait association with phylogenetic affiliation and growth form, supporting the RTPC hypothesis. Surprisingly, mycorrhizal associations had little effect on traits, whereas temperature had a significant effect on root diameter and SRL and rain patterns on RTD. However, these climatic patterns lost significance after phylogenetic corrections, suggesting that traits were largely affected by the representation of plant clades across climatic gradients. Trait coordination between above and belowground traits varied depending on the phylogenetic groups considered, with clades dominated by non-woody plants showing stronger morphological integration, whereas woody-dominated clades had higher N integration between organs. Phylogenetic structuring explained 30-46% of trait variation for diameter, SRL and RNC, whereas RTD had little phylogenetic signal. Moreover, variation in SRL and diameter was substantially affected by super-order divisions, suggesting deep divergences in the segregation of these traits. In contrast, RNC and RTD seem more influenced by recent evolutionary events. Overall, the inclusion of phylogenetic information appears as reliable way to improve global models, enhancing our ability to predict how shifts in vegetation will affect belowground functional traits, nutrient and carbon fluxes.