Contrasting physiological strategies of Quercus acuta and Q. glauca half-sib families under differing winter thermal conditions

As global temperatures rise, subtropical species such as Quercus acuta and Q. glauca are expected to expand into cooler regions, where winter cold stress may limit seedling establishment and growth. We assessed survival, growth, and leaf physiological responses in 21 Q. acuta and 22 Q. glauca half-sib families grown in Naju, South Korea, under control and cover treatments during winter, with the cover treatment providing an additional non-woven fabric layer. The covering treatment raised minimum temperatures from −8 °C to −3.2 °C and reduced the number of frost days from 15 to 9. Under the control, average mortality exceeded 90% in both species, whereas the cover treatment reduced mortality to 22.2% in Q. acuta and 7.8% in Q. glauca. Among surviving seedlings under the cover, Q. glauca showed 17.6% higher root collar diameter (RCD) growth than Q. acuta , and its net photosynthetic rate ( A net ) was 32.4% higher. Seasonal carbohydrate measurements revealed contrasting responses to cold. From December to March under the control, Q. acuta converted starch (−10%) into soluble sugars, which provided short-term frost protection but depleted carbon reserves and was accompanied by 55% higher proline content and higher mortality than Q. glauca . Q. glauca showed the opposite pattern, with starch increasing by 35% and soluble sugars remaining stable, supporting both frost protection and carbon reserves for regrowth. Intraspecific variation in cold responses was also substantial and was larger in Q. acuta (family-level mortality 0–85.7%) than in Q. glauca (0–26.7%). These findings indicate that predicting the northward expansion of subtropical evergreen oaks under climate warming requires considering species-specific winter physiology and family-level variation in cold responses.