Plant functional groups and root traits are linked to exudation rates of mature temperate trees

Abstract Root exudation affects soil biogeochemistry profoundly, yet it is rarely quantified in mature, field‐grown trees and its controls are poorly understood. We measured rates of carbon (C) exudation in 11 tree species that exhibit divergent root traits, including gymnosperms and angiosperms that associate with either arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi. Our goal was to explore how tree species, plant functional groups and root traits collectively influence exudation patterns. Species‐level differences were modest owing to substantial intraspecific variability (among individuals of the same species). However, three of the four highest exuding species were EcM gymnosperms, which exuded ca. two times more C than the other functional groups. Principle component analysis revealed that relationships between root trait organization and exudation were pronounced in EcM‐associated trees but weak or absent in AM‐associated trees. In EcM trees, exudation rates were negatively correlated with root tissue density (RTD) and positively correlated with specific root area (SRA) and root diameter, driven largely by gymnosperms. In contrast, exudation in AM trees showed only a weak association with specific root length. Consistent with these findings, mixed‐effect models also showed that exudation rates were best explained by a combination of tree‐mycorrhizal type, phylogenetic group and SRA, though a large portion of unexplained variation suggests an important role for contemporary environmental and local edaphic conditions. Collectively, our results demonstrate that root exudation is a complex physiological process shaped by interactions among mycorrhizal associations, evolutionary history and root traits, rather than by functional groups or root traits alone. These findings highlight the urgent need for more integrative frameworks and new experimental approaches to incorporate exudation dynamics into plant strategy theory and large‐scale ecosystem models. Read the free Plain Language Summary for this article on the Journal blog.