Treeline species Betula ermanii are more adaptable to alpine environments than non-treeline species Picea jezoensis: evidence from leaf functional traits

Abstract. Understanding functional trait differences between treeline and non-treeline species is key to exploring their adaptive strategies under environmental stress and predicting subalpine forest dynamics. On Changbai Mountain, Betula ermanii dominates over 90 % of the treeline zone, while Picea jezoensis accounts for over 70 % of the lower elevation zone. It remains unclear whether P. jezoensis, a treeline genus elsewhere, would eventually shift upward and replace B. ermanii. We thus investigated leaf functional traits, their intraspecific variation, and inter-trait relationships for both species along the elevational gradient. B. ermanii exhibited higher LDMC, N, P, and gs, but lower WUE and δ¹⁸O 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 δ¹³C and a decrease in SLA with elevation, accompanied by the greatest intraspecific variability in photosynthetic traits and weaker correlations among traits. Overall, B. ermanii employs a resource-acquisition strategy enabling it to occupy resources and space, while P. jezoensis adopts a resource-conserving strategy by emphasizing shade and drought-tolerance, resource conservation, and long-term adaptation at lower elevation, limiting its ability of upward range expansion. These findings enhance our understanding of their adaptive strategies and responses to elevational change, informing predictions of subalpine forest dynamics.