Xylogenesis Phenology of Pinus koraiensis Is More Sensitive to Elevation Increase than That of Betula platyphylla

The response of tree growth to environmental (climatic) changes has largely been analyzed through ring width–climate relationships, yet such analyses often lack the dynamic process of radial growth in response to environmental changes. Therefore, this study focuses on Korean pine (Pinus koraiensis Siebold & Zucc.) and white birch (Betula platyphylla Sukaczev) at three elevations (750 m, 950 m, and 1150 m) in the broadleaved Korean pine forest on the northern slope of Changbai Mountains, China. We systematically monitored cambial activity and the dynamics of xylem formation stages to analyze the different adaptation strategies of the two species in terms of phenology, cellular characteristics, growth rates, and climatic responses during cambial and xylem formation stages. The results showed that the phenological stages of xylem formation in Korean pine were more sensitive to elevation, while the phenological changes in birch were smaller, indicating greater growth stability. The seasonal dynamics of the number of xylem cell layers in both species followed a unimodal or sigmoid curve, but high elevations significantly inhibited the number of mature cell layers. Gompertz model fitting revealed that the maximum growth rate of Korean pine decreased significantly with increasing elevation, whereas no significant change was observed in birch. With increasing elevation, temperature emerged as the primary factor influencing cambial phenology and growth duration in both species, while precipitation dominated changes in growth rates. Xylem growth in Korean pine was co-regulated by growth rate (R2 = 0.62) and growth duration (R2 = 0.35), with tracheid diameter closely related to the duration of expansion (R2 = 0.36). The regulatory pattern of xylem growth in birch was similar to that in Korean pine but with weaker correlations. In summary, Korean pine, as a coniferous dominant species, is more sensitive to temperature changes induced by elevation and adapts to elevational variations by adjusting phenology and cell development. In contrast, birch, as a broadleaved pioneer species, exhibits a high buffering capacity in xylem formation in response to elevational changes, thereby maintaining growth stability. The divergent growth strategies of the two species reveal the potential response pathways of temperate forest tree species to environmental changes and provide important insights for predicting the dynamics of broadleaved Korean pine forests.