Seasonal snow cover patterns explain alpine treeline elevation better than temperature at regional scale

Unprecedented modern rates of warming are expected to advance alpine treelines to higher elevations, but global evidence suggests that current treeline dynamics are influenced by a variety of factors. Seasonal snow cover has an essential impact on tree recruitment and growth in alpine regions, which may in turn influence current treeline elevation; however, little research has been conducted on its role in regional treeline formation. Based on 11,804 treeline locations in the eastern Himalayas, we extracted elevation, climate, and topographic data for treeline and snowline. Specifically, we used linear and structural equation modelling to assess the relationship between these environmental factors and treeline elevation, and the climate-snow-treeline interaction mechanism. The results showed that the treeline elevation increased with summer temperature and permanent or seasonal snowline elevation, but decreased with snow cover days and spring temperature at the treeline positions (P ​< ​0.001). Importantly, spring snowline elevation (33.4%) and seasonal snow cover days (21.1%) contributed the most to treeline elevation, outperforming the permanent snowline, temperature, precipitation, and light. Our results support the assertion that the temperature-moisture interaction affects treeline elevation in the eastern Himalayas, but we also found that the effects were strongly mediated by seasonal snow cover patterns. The increasing tendency of snow cover governed by climate humidification observed in the eastern Himalayas, is likely to limit future treeline advancement and may even cause treeline decline due to the mortality of the remaining old trees. Together, our findings highlight the role of seasonal snow cover patterns in determining treeline elevation in the eastern Himalayas, which should be considered when assessing the potential for treeline ascent in snow-mediated alpine systems elsewhere.