Global distribution and climatic controls of natural mountain treelines

Mountain treelines are thought to be sensitive to climate change. However, how climate impacts mountain treelines is not yet fully understood as treelines may also be affected by other human activities. Here, we focus on closed-loop mountain treelines (CLMT) that completely encircle a mountain and are less likely to have been influenced by human land-use change. We detect a total length of similar to 916,425 km of CLMT across 243 mountain ranges globally and reveal a bimodal latitudinal distribution of treeline elevations with higher treeline elevations occurring at greater distances from the coast. Spatially, we find that temperature is the main climatic driver of treeline elevation in boreal and tropical regions, whereas precipitation drives CLMT position in temperate zones. Temporally, we show that 70% of CLMT have moved upward, with a mean shift rate of 1.2 m/year over the first decade of the 21st century. CLMT are shifting fastest in the tropics (mean of 3.1 m/year), but with greater variability. Our work provides a new mountain treeline database that isolates climate impacts from other anthropogenic pressures, and has important implications for biodiversity, natural resources, and ecosystem adaptation in a changing climate.

Automated summary

This study focused on closed-loop mountain treelines that surround mountains completely, and found a surprisingly large quantity of such treelines, with a total length of approximately 916,425 km. It revealed a bimodal latitudinal distribution of treeline elevations and identified temperature as the main climatic driver in polar and tropical regions, while precipitation influenced treeline position in temperate zones. The study also showed that 70% of closed-loop mountain treelines have shifted upward at an average rate of 1.2 m/year in the first decade of the 21st century. This research provides a new mountain treeline database that separates climate impacts from other human activities, and has significant implications for biodiversity, natural resources, and ecosystem adaptation in a changing climate.