Responses of leaf traits to low temperature in an evergreen oak at its upper limit

Low temperature is a major driver of the upper distributional limits of subtropical evergreen broadleaved forests in East Asia. The eco-physiology of evergreen broadleaved trees at the upper limits could respond to both lower winter temperature and lower averaged annual temperature that covary with elevation. To analyze these responses, we assessed the variations in leaf chemical traits of Cyclobalanopsis gracilis with seasonality and along elevation in the ecotone from subtropical to temperate climate, and analyzed the relationships between traits of mature leaves and temperature. The study showed that the mass-based nonstructural carbohydrate (NSC) content reached the highest level across elevations during winter; however, the winter NSC content did not differ between elevations. For mature leaves, leaf dry mass per area (LMA) tended to be higher approaching the upper limit. The leaf carbon to phosphorus ratio (C/P) showed an increasing trend with decreasing temperature, as did leaf carbon to nitrogen ratio (C/N) to some extent. The mass-based P content showed a decreasing trend approaching the upper limit, whereas area-based P content showed no significant variation. Our main results indicate that leaf NSC content is highest at all elevations in winter, and individuals at high elevations have high LMA and high nutrient use efficiency. The results suggest that for C. gracilis, an increasing leaf NSC results in a baseline protection to cope with cold in winter, which, at higher elevations is supplemented with increasing LMA and nutrient use efficiency to cope with associated low-temperature stress.