Light is often considered the primary factor leading to the regeneration failure of Korean pines (Pinus koraiensis) under the forest canopy. However, studies on the effect of light on Korean pines mainly focus on the use of an artificial sunshade net to control shade; field studies on the canopy are extremely scarce, and the current experimental results are contradictory. For a deeper understanding of the relationship between light conditions and understory Korean pine trees, the conditions of low, middle, high and full light (control) under the forest were tested at 18 years of age. The photosynthetic pigments, chlorophyll fluorescence, non-structural carbohydrate metabolism, antioxidant enzyme activity, and nutrient concentrations of current-year needles from Korean pine trees were measured. From June to September, light intensity and quality decreased under full light, but following leaf fall, understory light conditions improved slightly. As the light conditions improved, the photosynthetic pigments in the needles decreased, but Car/Chl were highest in the needles under full light. All light conditions had a positive correlation with glucose concentrations and Rubisco activity. Full-light needles had the highest APX activity, DPPH scavenging capacity, and proline concentration, as well as higher NPQ and lower Fv/Fm readings. This indicated that full-light Korean pine trees were stressed and inhibited photosynthesis to some extent, while the understory light environment may alleviate stress. The conservative strategy of storing more starch and using less glucose in understory Korean pine trees may be one of the reasons for the observed differences in growth rates among Korean pine trees under varying light conditions. Overall, this study implies that understory light during the growing season is not always unfavorable to 18-year-old Korean pine trees; this means that 18-year-old Korean pine trees still have shade tolerance to some extent and are capable of living under a canopy of deciduous trees.

Carbon is the fundamental element of plant life. Non-structural carbohydrates (NSC), synthesized using carbon dioxide through photosynthesis by plants, are essential for their growth and survival, which are also affected by light and temperature. However, few studies have investigated the effects of light conditions, season, and needle age together on field plants’ carbohydrates. We measured total carbon, glucose, sucrose, fructose, and starch concentrations in current and 1-year-old needles of Korean pine (Pinus koraiensis Sieb. et Zucc) growing under two distinct light conditions (dense canopy and full light) from early summer to cold winter. Total carbon, glucose, fructose, SS (soluble sugar, sucrose + glucose + fructose), NSC (soluble sugar + starch), starch, and SS/NSC all significantly (p < 0.05) related to the DOY (day of the year, 2021). Total carbon reached the maximum in September when the fresh needles were mature. Glucose, NSC, SS, and SS/NSC reached the maximum at the last sampling time, which provided protection for the cells in cold winter. The season showed a bigger impact on total carbon and NSC (all parameters except total carbon) than light and needle age. Two different-aged needles under two light conditions all had similar patterns of variation in total carbon and NSC, but twigs showed a more significant (p < 0.05) difference in NSC concentrations between two light conditions on 25 October (DOY 249). Needles of Korean pines stored soluble sugars (mainly glucose) and consumed starch in winter. Moreover, we found that needles had more glucose, while twigs had more sucrose, which may be a result of their different functions and may be helpful for future spring growth. Although lacking light during the growing season, understory Korean pines still had enough cold tolerance, similar to full-light ones, which indicated that low light during the growing season has little effect on cold tolerance.