How cold air and water shape the limits of mountain trees

Mountain forests don’t keep rising endlessly. At a certain point, trees run out of room to grow. A new global study shows why: cold temperatures set the upper limit everywhere, while water availability determines which tree species can survive close to that edge.

This boundary, called the treeline, is where tall forests give way to smaller, scattered, and stunted growth.

Research over decades has found that treelines align with temperature – not with a fixed altitude. They typically appear where the growing season lasts about 90 days and the average temperature hovers around 6.4°C (43.5°F).

The latest study was led by Yuyang Xie in the Department of Biology at the University of North Carolina at Chapel Hill (UNC).

The researchers pulled together one of the largest datasets of its kind to test a straightforward idea: heat sets the treeline’s height, while moisture determines which tree species can survive there.

Cold sets mountain tree boundaries

Xie’s team compiled more than 2,000 treeline records for dozens of mountain regions. Across genera and continents, the uppermost trees appeared where local heat fell well below each species’ comfort zone.

“Heat deficits, averaging around 35 percent below genus and species level thermal optima, consistently restrict the presence of treeline,” said Christian Körner, a plant ecologist at the University of Basel.

Water availability then sorted which species could endure those margins. Sites with similar cold at the limit showed different species mixes depending on how wet or variable the climate was throughout the year.

Predicting treeline shifts

To move past one-size-fits-all models, the team introduced the Relative Distance to Optimum (RDO) index. It measures how far a species lives from the temperature and moisture it prefers.

RDO captures the slack or stress that a population carries at the limit, making projections of upslope or downslope shifts more precise for particular tree groups.

Rainfall steers the forest edge

Independent evidence backs the moisture filter. A Himalayan study found that treelines advanced faster on the wet eastern slopes but slowed or stalled on drier sites despite warming.

Spring precipitation emerged as a key control. The pattern reinforces the idea that water availability governs whether warming translates into actual upslope establishment.

Shifting trees impact ecosystems

Treelines often lag behind climate change because trees take years to reach stature that qualifies as a tree. Seedlings may appear far above today’s forest but remain shrubs for decades.

Local conditions can blunt simple warming signals. Windswept ridges that dry quickly can resist upward transitions, while lee slopes with late melting snow may shelter starts.

Better forecasts for forests

Land managers can think of treelines as following two simple rules. Cold determines how high trees can grow in a region, and water decides which species can survive near that limit.

Forecasts that include species-specific tolerances will be more accurate than those based on temperature alone. Using RDO-style metrics can highlight areas where moisture is the main barrier, pointing to hotspots where monitoring or targeted planting could have the greatest impact.

Map shows tree limits

The new map does not claim that water controls treeline height. Instead, it shows that cold fixes the height, while water determines which species can function there.

This framing explains why distant mountain ranges share similar thermal limits yet display very different tree lineups. It also explains why some places shift quickly while others hardly budge.

Climate research refines treelines

Improved climate data near the ground will help scientists better define the limits within which each species can survive.

Long-term monitoring across the treeline zone will show whether seedlings are able to grow into mature trees or if their growth stalls at a younger stage.

The biggest gains will come from pairing physiology with climate. Knowing when populations live far from their optima can guide restoration and risk assessment before the forest edge moves.

“Our findings highlight the synergic effects between heat and moisture in determining the taxonomic variation in treeline formation, offering insights for alpine treeline studies under climate change,” the researchers concluded.

The study is published in Proceedings of the National Academy of Sciences.

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