Abstract: Abstract Aims The study focused on Pinus koraiensis and Quercus mongolica in the broadleaf Korean pine forest of Changbai Mountain, investigating the seasonal dynamics of radial growth in both species and their response to non-structural carbohydrates (NSC). Methods Seasonal sampling was conducted during the growing season (April to October), measuring the concentrations of soluble sugars, starch, and NSC in five organs: xylem, leaves, roots, wood, and phloem. Additionally, the micro-core sampling method was used to monitor the intra-annual growth dynamics of the xylem and investigate the potential relationship between NSC concentrations and xylem cell growth. Important findings The main results are as follows: 1) There are significant differences in NSC concentrations between species, with Pinus koraiensis having higher NSC concentrations than Quercus mongolica, and the range of variation in NSC concentrations being greater in Pinus koraiensis than in Quercus mongolica. NSC concentrations in Pinus koraiensis were higher from May to September compared to March and April, while in Quercus mongolica, NSC concentrations were higher from June to September compared to March to May. There were also notable differences in NSC concentrations between organs within each species, with the pattern being phloem > leaves > branches > roots > xylem. NSC concentration changes were most pronounced during periods of significant growth rate changes in Pinus koraiensis (April to June) and Quercus mongolica (April to July), and once growth rates stabilized, NSC concentrations also became stable. 2) The intra-annual xylem cell growth dynamics of both Quercus mongolica and Pinus koraiensis follow an "S" shape, with growth rate curves exhibiting an inverted "bell" shape. The maximum growth rate of Quercus mongolica is higher than that of Pinus koraiensis, and its radial growth begins earlier (DOY 112±2 vs. DOY 120±1), ends later (DOY 252±1 vs. DOY 241±7), and has a longer growth duration (140 days vs. 121 days). 3) The radial growth rate of xylem cells in Pinus koraiensis shows a positive correlation with the NSC concentration in its organs, while in Quercus mongolica, the correlation is negative. The dynamic changes in growth rate and organ NSC concentration during different growth periods are as follows: During the early growth period (April-May), the radial growth rate of both species increases as the NSC concentration decreases. During the peak growth period (June), the xylem growth rate of Pinus koraiensis increases along with the NSC concentration, while the xylem growth rate of Quercus mongolica increases as the NSC concentration decreases. After the peak growth period (July-September), NSC concentrations begin to accumulate again in both species. In conclusion, this study explored the NSC concentrations in different organs and the intra-annual xylem growth dynamics of two major tree species, Pinus koraiensis and Quercus mongolica, in the Changbai Mountain region, and found that NSC plays a key role in regulating xylem growth. The research revealed the coordinated relationship between NSC concentrations and xylem growth in both species, particularly the different carbon allocation patterns during the peak growth period. These findings provide new insights into the growth and carbon allocation mechanisms of trees in this region and offer important references for predicting the response of tree growth and carbon dynamics to future climate change.

Key words: non-structural carbohydrate, microcoring, radial growth