Mountain treelines are shifting – and not always upward

High in mountainous regions there is a calm, distinct boundary where woodland abruptly gives way to open ground. This boundary is known as the treeline, and at first glance it can seem straightforward.

It is often assumed that, as the planet warms, this boundary should steadily creep upslope. New scientific evidence, however, points to a far more varied reality. Depending on the location, treelines may advance, retreat, or remain unchanged.

A worldwide investigation led by the University of Basel indicates that there is no universal rule. Instead, treelines react to an interplay of climate conditions, human land use, and natural disturbance.

Treelines move in both directions

A widely held view is that higher temperatures should enable trees to colonise higher elevations. The research supports this to a degree: between 2000 and 2020, roughly 42 percent of treelines shifted upwards.

At the same time, the study reports that about 25 percent moved downwards. Close to one-third showed no movement. In other words, treelines behave very differently from one region to another.

The researchers also found that changes are typically gradual. On average, treelines advanced by only around 0.12 metres per year (about 0.39 feet per year), far slower than many would expect given rising temperatures.

“The shift of treelines occurs slowly, it would take a lifetime to fully grasp the changes,” said Dr. Mathieu Gravey of the Austrian Academy of Sciences.

Treelines: a gap between climate and reality

To interpret these patterns, the team set two key concepts side by side. The first is the “observed treeline”, meaning the elevation where trees are actually found. The second is the “potential treeline”, indicating where trees could grow if climate alone determined their limits.

The analysis highlighted a mismatch between the two. In many areas, trees have not reached as high as the climate would allow, showing that temperature by itself does not fully dictate treeline position.

Indeed, by 2000 almost half of the world’s treelines had already reached their climatic ceiling. Elsewhere, treelines sat well below that threshold because of additional constraints.

Recognising this gap helps explain why treelines do not consistently track warming in a rapid, predictable way.

Climate is not only reason

Temperature still matters greatly, as trees require a minimum level of warmth to persist. The study found that treelines generally mark the point where, during the growing season, temperatures drop beneath a threshold that trees can tolerate.

Yet warmth is only part of the picture. Rainfall patterns, soil properties, and available sunlight also influence whether trees can establish and survive. In certain regions, very wet conditions can even contribute to a lower treeline.

“When you talk to people about climate change, there are usually two images that come to mind: glacier retreat and the shifting of treelines,” said Professor Sabine Rumpf from the University of Basel.

“Treelines are often attributed solely to climate change. But it’s not that simple. While climate change is clearly the cause of glacier retreat, the reasons behind treeline positions are complex.”

Together, these findings underline that treelines are shaped by multiple connected drivers rather than a single cause.

Human actions leave a strong impact

Human land use can strongly influence where treelines sit and how they move. Across many mountain landscapes, grazing and farming historically kept trees from spreading into higher terrain.

In some regions, those practices have since declined or ceased. Where land has been abandoned, trees can gradually re-establish and, in time, move upslope again.

“It’s not about whether the alpine region is being used, but how that land use is changing,” said Rumpf.

“The more alpine pastures are abandoned, the more trees grow back in places where they could have been all along.”

The study further suggests that historical disturbance continues to shape present-day trends: places that experienced heavier human impact in the past now tend to show slower treeline movement.

Fires push trees down

Wildfire is a major factor behind treelines shifting downslope. The study indicates that nearly 38 percent of downward movements are associated with fire events.

“Fires are an example of natural disturbances,” said Dr. Tianchen Liang from the University of Basel.

“But many wildfires, such as those in North America, can no longer be completely separated from human influences. Climate change and other human activities are increasing their frequency and scale.”

Fires can kill seedlings and saplings and degrade soils, making it more difficult for forests to recover and expand upwards again.

Other extremes-including droughts and cold waves-also constrain growth by reducing seed survival and slowing the pace at which forests can extend into higher elevations.

Nature does not respond instantly

Even when the environment becomes more favourable, treelines rarely shift quickly. A range of biological constraints can hold back change.

Seeds may fail to disperse into higher ground, young trees may not withstand harsh mountain conditions, and established vegetation can outcompete new growth for space and resources.

Because of these limits, treelines often lag behind the pace implied by climate trends. In some places, warming conditions suggest trees should climb rapidly, yet observed movement remains modest.

This lag highlights that ecosystems often need time to adjust to changing conditions.

A sign of human influence

Treelines are not only a topic for researchers; they provide a visible marker of how people influence the Earth.

“The shifting of treelines is one piece of a large puzzle for understanding the impact of climate change,” noted Dr. Gravey.

“But their significance extends beyond science,” said Rumpf. “Treelines are a striking example of how we, as humans, are changing our environment, directly through land use and indirectly through the consequences of human-induced climate change.”

Compared with many environmental trends, treeline change can be easy to observe. Past and present photographs can clearly document how mountain landscapes transform over time.

In this way, treelines function as a living record of both historical and ongoing pressures, showing how small choices repeated over years can reshape the world around us.