# Climate Change and Topography Drive the Expansion of Betula ermanii in the Alpine Treeline Ecotone of the Changbai Mountain

## TL;DR

This study shows that climate change and topography are driving the upward expansion of Betula ermanii trees in the alpine treeline of Changbai Mountain.

## Contribution

The study reveals that interactions between climate and topographic factors, rather than single factors, drive tree expansion in different mountain aspects.

## Key findings

- Betula ermanii's highest expansion rate was 2.38% per year from 2017 to 2021.
- Interactions between climate and topography, such as wetness index and precipitation, explain most of the expansion variability.
- Expansion patterns differ across mountain aspects, with southern slopes influenced most by soil moisture and temperature interactions.

## Abstract

Alpine treelines ecotones are critical ecological transition zones and are highly sensitive to global warming. However, the impact of climate on the distribution of treeline trees is not yet fully understood as this distribution may also be affected by other factors. Here, we used high‐resolution satellite images with climatic and topographic variables to study changes in treeline tree distribution in the alpine treeline ecotone of the Changbai Mountain for the years 2002, 2010, 2017, and 2021. This study employed the Geodetector method to analyze how interactions between climatic and topographic factors influence the expansion of Betula ermanii on different aspect slopes. Over the past 20 years, B. ermanii, the only tree species in the Changbai Mountain tundra zone, had its highest expansion rate from 2017 to 2021 across all the years studied, approaching 2.38% per year. In 2021, B. ermanii reached its uppermost elevations of 2224 m on the western aspects and 2223 m on the northern aspects, which are the predominant aspects it occupies. We also observed a notable increase in the distribution of B. ermanii on steeper slopes (> 15°) between 2002 and 2021. Moreover, we found that interactions between climate and topographic factors played a more significant role in B. ermanii's expansion than any single dominant factor. Our results suggest that the interaction between topographic wetness index and the coldest month precipitation (Pre1), contributing 91% of the observed variability, primarily drove the expansion on the southern aspect by maintaining soil moisture, providing snowpack thermal insulation which enhanced soil temperatures, decomposition, and nutrient release in harsh conditions. On the northern aspect, the interaction between elevation and mean temperature of the warmest month explained 80% of the expansion. Meanwhile, the interaction between Pre1 and mean temperature of the growing season explained 73% of the expansion on the western aspect. This study revealed that dominant factors driving treeline upward movement vary across different mountain aspects. Climate and topography play significant roles in determining tree distribution in the alpine treeline ecotone. This knowledge helps better understand and forecast treeline dynamics in response to global climate change.

Over the past 20 years, the expansion rate of Betula ermanii in the alpine ecotone of Changbai Mountain varied with mountain aspects. We found that interactions between climate and topographic factors played a more significant role in B. ermanii's expansion than any single dominant factor. Climate and topography play significant roles in determining tree distribution in the alpine treeline ecotone.