**5. Conclusions**

We found that the forest-steppe migration generally followed Holocene climate change in the Tien Mountains. Specifically, the forest belt where *Picea schrenkiana* dominates has a very low pollen taxa diversity, characterized by high richness and low evenness, which plays a key role in the mountainous diversity. By detecting the diversity change of the deposition sites, we found that in the process of coping with warm and dry climate during the early Holocene from 12,000 to 8000 yr BP, the forest belt moved upward, causing a reduced forest belt influence on, or contribution to, the low-elevation deposition sites, as the observed diversities around the two lakes were high. In this case, the forest-steppe ecotone moved upward accordingly. During the warm and wet middle Holocene from 8000 to 4000 yr BP, the forest belt with low taxa evenness expanded and widened, as the observed diversities around the two lakes were very low. As such, the forest-steppe ecotone moved downward accordingly. As the late Holocene became cold and wet from 4000 yr BP onwards, the forest belt moved downward, resulting in a reduced forest belt influence on or contribution to the sites, leading to the high diversities observed. Thus the forest-steppe ecotone moved further down. Moisture loss may pose the greatest threat to the narrow forest-steppe ecotone, considering the climatic niche space and the limited living space for humidity-sensitive taxa. This study highlights that temperature and moisture co-influence the forest belt change, which further determines the migration of the forest-steppe ecotone.

**Author Contributions:** H.L. and Y.C. proposed the idea and designed the study. Y.C. analyzed the data and wrote the first draft. Y.C., H.L., H.W., Q.H., Y.H., K.D. and Z.D. discussed the results and contributed to improving the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, Grant Number 41790422, 41901092. It was also funded by the Fundamental Research Funds for the Central Universities of China, Grant Number GK202003069.

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