Global Study Reveals Complex Shifts in Tree Lines Due to Climate Change

As global temperatures rise, tree lines—typically expected to shift upward—are exhibiting more complex behavior. Traditionally, warmer conditions allow trees to colonize higher elevations previously too cold for survival. This phenomenon is visible in protected areas such as Switzerland’s Swiss National Park in Graubünden, where tree lines are distinctly observable. Credit: Sabine Rumpf, University of Basel

Evidence of these shifts is documented in comparative imagery from Canada’s Waterton Lakes National Park, showing tree line progression between 1913 and 2007. Credit: Mountain Legacy Project

Similarly, Montana’s Glacier National Park illustrates this trend near Jackson Glacier, with images from 1912 and 2009 revealing glacier retreat and concurrent tree line elevation. Credit: MJ Elrod, U of M Library; L McKeon, USGS

New Research Challenges Simplistic Views of Tree Line Migration

A study published in the International Journal of Applied Earth Observation and Geoinformation presents a more nuanced picture. Between 2000 and 2020, 42% of tree lines shifted upward, aligning with climate warming expectations. However, 25% moved downward, defying conventional assumptions.

Sabine Rumpf, an ecologist at the University of Basel, highlighted the limitations of existing research. Most studies focus on North America, Europe, and the Himalayas due to funding and accessibility. “

Many studies of tree line shifts tend to be concentrated in limited geographic areas. A preponderance are based primarily on data from North America, Europe, and the Himalayas, where researchers are more likely to have funding to head to the field to take measurements themselves. But that also means that a large proportion of the surface of our planet is so understudied.

To address these gaps, the research team employed remote sensing technology, enabling a global analysis without extensive fieldwork. “

Remote sensing data [are] really amazing because you can get a truly global picture, even though there’s nobody, or too few people, observing things in the field.

Methodology: How Researchers Defined and Analyzed Tree Lines

The team utilized a 250-meter resolution world mountain map (2018) to assess tree line dynamics. They excluded regions with less than 10% high-mountain coverage—areas with too few trees to define a meaningful tree line—and cells with more than 95% forest cover, where tree lines are indistinct.

For this study, the “observed tree line” was defined as the upper limit of trees standing 3 meters or taller. Researchers then developed a model to estimate potential tree lines—where trees could theoretically grow under ideal conditions. This model accounted for human environmental impacts, which often restrict tree lines below their natural potential.

The model evaluated two critical factors for each grid cell: growing season length and mean growing season temperature. Cells with a growing season of 94 days or longer and an average temperature of 6.4°C or higher were deemed capable of supporting trees. Cells failing to meet both criteria were classified as above the potential tree line, indicating inhospitable conditions for forest growth.