Carbon Sink Limitation Determines the Formation of the Altitudinal Upper Limit of an Evergreen Oak in Eastern China

Temperature is a critical environmental factor determining the upper limits of evergreen broadleaved tree taxa. However, whether carbon source or carbon sink limitation shapes this limit is not yet fully understood. We studied a subtropical evergreen oak (Cyclobalanopsis gracilis) at the northern limit of its distributional range. Along an elevational/temperature gradient towards its upper limit, we surveyed the variations in non-structural carbohydrate (NSC) concentrations of C. gracilis adults for 3 years. Additionally, a carbon balance manipulation experiment of debudding and defoliation was done to C. gracilis seedlings close to the upper distributional limit, aiming at investigating the changes in NSC concentrations and growth rates in different treatment groups. Our results showed that increasing elevation or decreasing temperature did not affect the trends of NSC concentration in twigs, old branches, or trunks of adults, nor did carbon balance manipulations (debudding or defoliation) of seedlings have a significant effect on the growth, while defoliation decreased NSC concentration in twigs. These results suggest that carbon sink limitation is the key physiological mechanism underlying low temperature in the shaping of this dominant evergreen broadleaved tree species in eastern China. Therefore, the formation of upper limits in evergreen oaks is most likely the result of a direct low-temperature restriction on meristematic activity and tissue formation instead of the result of insufficient carbon supply. More studies with expanded sample sizes are needed on other evergreen broadleaved tree species growing at their upper limits to confirm the carbon sink limitation hypothesis and reveal the detailed mechanisms.