**5. Conclusions**

Many studies have stated that the end-season resource remobilization is strategic since it supports growth at the beginning of bud break in deciduous species [18,49,50]. Except for July-shoot NSC and July-leaf K, tissue concentrations of NSC, N, P, and K did not decrease with increasing elevation for both July and September. Our results partly supported our hypothesis that resources (i.e., mobile carbohydrates, N, P, and K) were reallocated from leaves to storage tissues at the end of growing season in deciduous *B. ermanii* trees. In an economic view, shoots should be an important storage tissue for resources to reduce the transport costs at both the end-season and early growing season. However, our study indicates that stems and roots of deciduous *B. ermanii* trees at high elevations are the most important storage tissues for resources over winter. Inconsistent with our initial hypothesis, we found trees had lower end-season reallocation efficiency of NSC, N, P, and K from leaves to woody tissues at the higher elevations above the alpine treeline compared to lower elevations (Table 5; Figure 5a,e,i,m), and this may result in a resource limitation in high-elevation trees and further limit tree growth at high elevations. Our results contribute to better understand the resource-related ecophysiological mechanisms for treeline formation, and vice versa, to better predict treeline dynamics in response to global warming. Our study provides resource-related ecophysiological knowledge for developing managemen<sup>t</sup> strategies for high elevation forests in a rapidly warming world. To more closely explore carbon and nutrient remobilization mechanisms, further studies are needed to estimate the pool size of resources remobilized (concentration × biomass, see Table 4) [51] and their stoichiometry [52]. Furthermore, stable isotope labeling experiments would be taken into account (e.g., 13C, 15N) to accurately evaluate carbon and N remobilization from senescing tissues, and to determine which tissues contribute to the most internal resource remobilization [13].

**Author Contributions:** Conceptualization, Y.C., M.-H.L. and H.-S.H.; methodology, M.-H.L. and H.-S.H.; software, Y.C.; investigation, K.L., Y.-C.D., H.-D.H.; data analysis, Y.C. and M.-H.L.; writing—original draft preparation, Y.C. and M.-H.L.; writing—review and editing, M.-H.L. and H.-S.H.; funding acquisition, M.-H.L. and H.-S.H.

**Funding:** This research was funded by the National Key Research and Development Program of China (No. 2017YFA0604403, 2016YFA0602301), the National Natural Science Foundation of China (No. 41371076, 41601052, 41501089).

**Acknowledgments:** We sincerely thank Danae Yu and two anonymous referees for valuable comments on our manuscript, Xinhua Zhou for her assistance in the laboratory and Samuel Allen for English proofreading to improve our manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
