Xylogenesis of

Introduction: Tree cambial activity and xylem development are regulated by both external environment and internal factors, though their synergistic mechanisms remain unclear. Methods: This study was conducted during the 2024 growing season across three altitudinal gradients (750 m, 950 m, 1150 m) in Changbai Mountain. We selected Korean pine (Pinus koraiensis) individuals of different ages, sampled at intervals of one to two weeks, and monitored the dynamics of cambial activity and xylem formation processes. Results: The results showed that with increasing elevation, the effect of decreasing temperature on xylem growth of Korean pine was significant. For every 200 m increase in elevation, the spring onset of cambial activity in Korean pine was delayed by approximately 13 days, while the cessation of xylem growth in autumn advanced by 8-12 days. These changes shortened the growing season length by 12-19 days and further reduced the radial xylem growth of large and small trees by 278 μm and 54 μm, respectively. Compared with low elevation (750 m), the low-temperature environment at high elevation (1150 m) accelerated the earlywood to latewood transition, shortening the transition period by 14 days. Meanwhile, the expansion rates of earlywood and transition area tracheids were affected, decreasing by 1.69 μm/d and 1.63 μm/d, respectively, with corresponding reductions in tracheid diameter of 4.36 μm and 3.5 μm. To adapt to cold stress, Korean pine at high elevation enhanced the mechanical support capacity of cell walls, manifested as increased latewood cell wall thickness. Compared with large trees, small trees exhibited earlier cambial activity onset and later cessation, with a growing season duration 16-27 days longer, thereby producing greater radial xylem growth. Large trees, in contrast, allocated more resources to structural maintenance and stress resistance, as reflected by a 2.7% reduction in the proportion of time spent on earlywood tracheid expansion and a 3.07 μm decrease in tracheid diameter, indicating differences in growth strategies between age classes. Temperature was the key driver of these elevational patterns, and age-related differences in temperature response reflect contrasting adaptive strategies of small-age and large-age Korean pine across elevations. Discussion: This study elucidates divergent growth responses of Korean pine to climate and aids in predicting broad-leaved Korean pine forest dynamics.