Higher Root Exudation as a Key Nutrient Acquisition Strategy for Mature Forest Trees in Ultramafic Soils

Ultramafic soil is a stressful environment for plants, often limiting species richness and productivity. However, some species or populations employ adaptive strategies to inhabit these soils. Root nutrient acquisition, mediated by fine‐root traits, may contribute to survival and growth in ultramafic soils, yet this process is poorly understood. We compared a range of fine‐root traits that have proximal influences on nutrient acquisition, such as exudation rate and specific root length (SRL), in mature trees growing in ultramafic and adjacent brown forest soils in a cool temperate forest. Measurements were performed on two dominant canopy species, Clethra barbinervis and Quercus serrata , that occur in both soils but differ in mycorrhizal association types (arbuscular mycorrhizal and ectomycorrhizal fungi). Both species showed higher fine‐root exudation rates of carbon and low‐molecular‐mass organic acids in ultramafic soil, though root respiration rates were similar between the soils. Fine‐root tissue density was marginally lower in ultramafic soil for both tree species, potentially allowing trees to efficiently explore for soil nutrients. No significant interaction between species and soil type was detected for any traits, indicating that responses of fine‐root traits to ultramafic soil were similar between the two species. Higher root exudation was associated with higher SRL, lower root diameter, and lower tissue density, and was greater at a given SRL or root diameter in ultramafic soil. These results suggest that trees in ultramafic soil develop efficient nutrient‐acquisition strategies through alteration primarily in root exudation, thereby enabling nutrient uptake, growth, and survival in ultramafic soil.