Variation patterns in foliar δ13C and δ15N among plant life types along an altitudinal gradient on the Tibetan Plateau

Understanding how foliar δ13C and δ15N vary with the environment is crucial for elucidating the carbon and nitrogen cycle dynamics within ecosystems. Yet, there is limited knowledge about these variations among different plant life types along altitudinal gradients, particularly in subtropical forest ecosystems found in the southeastern Tibetan Plateau. Therefore, we collected leaves from 67 species, including 20 trees, 24 shrubs, and 23 herbs along an altitudinal gradient of 834 m to 3105 m in the subtropical forest of Medog, and investigated their patterns of variation along the altitudinal gradient. Our findings revealed a positive relationship between foliar δ13C and altitude, as well as a negative relationship between foliar δ13C and soil temperature. There was no significant correlation between foliar δ15N and altitude, but a hump-shaped relationship was observed between foliar δ15N and soil moisture. Among the different plant functional groups, the δ13C of shrubs is more sensitive and representative of environmental changes. In addition, we discovered no difference in the δ13C (δ15N) values of leaves among trees, shrubs, and herbs in this study. Still, there were significant differences when compared to the δ13C (δ15N) values of leaves in other typical climatic zones, indicating that climatic zones influenced the δ13C and δ15N values of leaves more than plant functional groups. In conclusion, our results provide new insights into the climatic drivers of the changes in δ13C and δ15N of the different plant life types along the altitudinal gradient in the southeastern Tibetan Plateau.