Abstract: In this study, we examined the regularity of phenological rhythmical change of plant water-soluble compound δ¹³C (δ¹³C_wsc) in spring for two typical tree species in the warm temperate zone of China, Pinus tabuliformis and Robinia pseudoacacia. The δ¹³C_wsc in each organ of those two species in the spring phenological period were measured to explore the relationship between δ¹³C_wsc and related environmental factors. The results showed that there were significant differences in δ¹³C_wsc values of each organ between P. tabuliformis and R. pseudoacacia, with higher δ¹³C_wsc(-25.03‰±0.01‰) in the new shoot of P. tabuliformis. The δ¹³C_wsc value in the non-photosynthetic organs were 0.83‰-1.8‰ higher than that in the photosynthetic organs, while the δ¹³C_wsc value in the aboveground part was generally lower than that in the underground part. As spring progressing, different carbon storage strategies were found between two species. When the terminal bud of P. tabuliformis opened, the carbon was obtained from the proximal old leaves. At the beginning of leaf development, photosynthetic products accumulated by old leaves could not meet the growth requirements for new leaves and roots, with 90% of which depending on the carbon reserve in branches and stems. When full leaf having developed, the photosynthetic function of both new and old leaves recovered and the carbon consumed by branches and stems was gradually replenished. For R. pseudoacacia, at the beginning of leaf bud opening and leaf spreading, branches were the main carbon source for new leaves and roots. When leaves were fully unfolded, mature leaves with high capacity of carbon sequestration became the primary carbon source. Results of principal component analysis showed that temperature during observation period, ≥10 ℃ accumulated temperature, sunshine duration and solar radiation were the main factors influencing δ¹³C_wsc, which could explained 86.3% of the total variation. The δ¹³C_wsc values of both species was negatively correlated with temperature and relative humidity, but positively correlated with the difference of saturated water pressure, ≥10 ℃ accumulated temperature and sunshine duration. The main environmental factors affecting plant δ¹³C_wsc varied during the phenological process. Our results could provide a reference for more accurate estimation of spring organ carbon distribution pattern of regional typical tree species, and also a theoretical basis for formulating scientific and reasonable forest management strategy.

Key words: stable carbon isotope, plant organ, water-soluble compounds, phenological period, environmental factor