Influence of root morphology and mycorrhizal type on root exudation rates among 10 co-existing broadleaved tree species in a cool-temperate forest

Abstract Identification of the factors explaining variations in tree root exudates is crucial for understanding belowground carbon dynamics. The relative importance of plant functional types and root morphological and physiological traits in explaining differences of root exudation remains unclear. To elucidate the key factors influencing root exudation, we examined root exudation rates, morphological traits (specific root length, specific root area, tissue density, and diameter), and respiration rates across 10 co-existing tree species in the absence of significant environmental gradients. The selected species differed in life form (deciduous and evergreen) and mycorrhizal type (arbuscular (AM) and ectomycorrhizal (ECM)). No significant differences (F = 0.001, P = 0.97) in root exudation rates were observed between life forms, whereas significant differences were observed between mycorrhizal types (F = 10.94, P < 0.01), with AM species exhibiting significantly higher exudation rates than those of ECM species. Principal component analysis among the species revealed that root exudation rate was associated with resource acquisition traits characterized by a smaller diameter along the first axis. Along the second axis, it was more closely linked with lower tissue density reflecting mycorrhizal types. Crucially, the best model for explaining the root exudation rate included diameter and mycorrhizal type, while root respiration rate, as a proxy of physiology, was not selected. These findings suggest that mycorrhizal type is a more dominant factor than life form among the 10 species. Furthermore, they reveal that variations in root exudation cannot be explained by morphological and physiological traits alone, highlighting the importance of accounting for both mycorrhizal identity and root morphology among the 10 studied co-existing broadleaved species. This study underscores the importance of considering mycorrhizal-specific processes to better understand belowground carbon cycling.