Functional trait divergence explains the dominance of Betula ermanii over Picea jezoensis at the alpine treeline

Understanding functional trait differences among different treeline species is key to exploring their adaptive strategies under environmental stress and predicting subalpine forest dynamics. On Changbai Mountain (Mt), Betula ermanii dominates over 90% of the alpine treeline ecotone, while Picea jezoensis reaches its upper elevational limit, occupying over 70% of the adjacent subalpine forest below. While P. jezoensis dominates a treeline zone in many mountains, whether it can move up and coexist with B. ermanii in Changbai Mt remains unclear. To investigate this, we examined leaf functional traits, their intraspecific variation, and inter-trait relationships for both species along an elevational gradient. We found that with elevation increases, B. ermanii exhibited higher leaf dry matter content (LDMC), nitrogen (N), phosphorus (P) and stomatal conductance ( g s ), but lower water-use efficiency (WUE) and δ 18 O at higher elevations, with the greatest intraspecific variability in photosynthetic and hydraulic traits, and tighter linkages among traits. In contrast, P. jezoensis exhibited an increase in δ 13 C and a decrease in SLA with elevation, accompanied by the greatest intraspecific variability in photosynthetic traits and weaker correlations among traits. Overall, B. ermanii adopts a fast-acquisitive strategy characterized by tight trait coordination and liberal water use, allowing it to exploit short favorable periods and maintain dominance at the treeline. In contrast, P. jezoensis exhibits a more conservative strategy, with modular trait networks and stable trait performance, favoring persistence in shaded or relatively stable environments, which may reflect reduced competitiveness under harsher treeline conditions. These findings enhance our understanding of trait-based mechanisms underlying species dominance at the alpine treeline and provide insights into their potential responses to ongoing climate warming.