Species-specific patterns in fine-root traits and their coordination in the multidimensional root economics space at the Mongolian forest-steppe ecotone.

In the bioclimatically sensitive forest-steppe ecotone of northern Mongolia, belowground strategies can be elucidated using an integrated approach that considers root functional traits and symbiotic associations. We elucidated how three dominant tree species, namely Larix sibirica, Pinus sylvestris, and Betula platyphylla, coordinate their morphological, chemical, and symbiotic (ectomycorrhizal [EM]) traits within a multidimensional root economics space (RES) to adapt to this harsh ecotone. Although the RES framework generally associates thicker roots with greater reliance on fungi, EM colonization was highest in the thinnest-rooted species, B. platyphylla, with the highest specific root length (SRL), and lowest in the thickest-rooted species, L. sibirica, with the lowest SRL. Principal component analysis revealed that the primary axis of variation represents a synergistic acquisition gradient, rather than a traditional trade-off between morphological exploration and symbiotic associations. Along this axis, SRL and EM colonization were positively coordinated, indicating that B. platyphylla maximized its resource acquisition capacity by integrating high soil exploration with intensive fungal collaboration. This synergy is likely driven by higher root branching intensity in thinner roots, which provides more infection sites for EM fungi. In contrast, the conifers exhibit different root-trait combinations: P. sylvestris optimizes structural and chemical conservation, whereas L. sibirica employs a decoupled strategy to minimize symbiotic investment and maintain high metabolic potential. Our study highlights that tree species coexist by employing divergent resource-acquisition pathways, ranging from integrated combinations to resource-conservative trait syndromes, under intensifying climatic stress in Central Asia.