Variation of first-order root traits across climatic gradients and evolutionary trends in geological time

Aim Plant roots are crucial for water and nutrient absorption, but large-scale patterns and underlying mechanisms of root trait variation and evolution are poorly understood. Here we quantify the degree of variation in functional traits for the first-order roots across large geographical scales and examine the potential mechanisms underlying these patterns. Location China. Methods We collected first-order roots (stream-based ordering system) and leaf samples of 65 tree species in six forests from subtropical to temperate zones and determined the key morphological, architectural and chemical traits. Results We found wider variation in root morphology in the subtropical zone, where species with very thick and very thin first-order roots coexisted, in contrast with narrower variation among species in the temperate zone. Inconsistent with the predictions of trait economics spectrum, root nitrogen concentration was uncorrelated with root morphology. Furthermore, average root diameter at the plant family level decreased markedly with their divergence time in both piecewise and phylogenetic independent contrast regression analyses. Main conclusions Higher variation in root morphology in the subtropical zone appears to result from the coexistence of tree species with thick and thin roots, probably because of a more favourable water supply. Patterns of root evolution towards thinner roots were driven by both phylogeny and possible adaptation of newly diverged species to drier habitats from mid to late Cretaceous. Our findings reveal contrasting selection pressures at the root and leaf level in different climatic zones during plant evolution, and suggest that a single vector of variation linking morphology to resource acquisition or life span (trait economics') remains elusive in roots.