Why are some mountain trees moving downhill as the planet warms? Scientists may have an answer

Climate change is pushing many mountain tree species toward cooler elevations, but a new study suggests the story is far more complicated than a simple uphill migration. While some trees are steadily climbing mountain slopes as temperatures rise, others are expanding downhill into warmer and potentially drier environments.

The difference, researchers say, may be linked to something hidden beneath the bark: the way each species moves water through its wood.

A global analysis led by researchers from Hebei Agricultural University and Peking University found that trees with highly efficient water-transport systems were generally more likely to shift uphill. Species with traits that make them more resistant to drought, meanwhile, were more likely to extend their ranges toward lower elevations.


The findings, published in Nature Climate Change, challenge the assumption that warming will send all mountain forests in the same direction.

The same warming, very different responses

Mountain landscapes naturally change with elevation. Lower slopes may support broadleaf forests, while colder areas higher up are dominated by species such as fir and spruce. Eventually, trees reach the treeline, beyond which conditions become too harsh for forests to grow.

As global temperatures rise, scientists have generally expected these vegetation zones to move upward in search of cooler conditions. Many species are doing exactly that.

But the new research found that nearly one-third of the mountain tree species examined were shifting in the opposite direction.

The contrast can be dramatic. Species such as box elder, Italian cypress and white poplar were among the fastest climbers, moving upward by more than 5 metres, or about 16 feet, per year. Other trees, including kermes oak and silver wattle, shifted downhill at rates exceeding 3 metres, or roughly 9 feet, annually.

In other words, trees exposed to the same broad era of global warming have responded in strikingly different ways.

A tree's internal water system may shape where it goes

The researchers focused on the hydraulic traits of trees — essentially, how efficiently their internal wood structure moves water from roots to leaves.

Trees with wider or more efficient water-conducting tissues can transport larger volumes of water and may support faster growth when moisture is readily available. But there is a trade-off. Highly efficient systems can be more vulnerable when drought conditions intensify.

Other species have developed a tougher approach. Their wood and leaves may be better able to continue functioning under water stress, even if they do not move water as quickly.

That distinction appears to matter when tree species respond to a changing climate.

The study found that species with stronger drought resistance were more likely to expand downslope, where temperatures are generally higher and water stress can be greater. By contrast, species with highly efficient water transport were more likely to move toward cooler, higher elevations.

One hydraulic trait — the amount of water a tree could transport relative to its leaf area — explained roughly a quarter of the variation in how quickly different species shifted their ranges. No other trait examined accounted for as much.

Scientists examined more than a century of tree records

The research did not involve planting trees on mountain slopes and waiting to see where they moved. Instead, the team combined several major databases containing records collected over decades.

The analysis began with information on 102 mountain tree species whose ranges had shifted between elevations. For a closer look at growth patterns, researchers narrowed the sample to 45 species with extensive tree-ring records.

Those records were substantial: 121,743 trees from 3,057 sites across both hemispheres.

Tree rings provide a natural archive of changing environmental conditions. A wider ring generally indicates stronger growth during a favourable year, while a narrower ring can signal stress from drought or other factors.

The researchers combined those historical records with plant trait data and local climate information to investigate why species living in mountain regions were moving in opposite directions.

Faster warming did not necessarily mean faster migration

One of the study's more surprising findings was that the pace of local warming did not clearly explain how quickly tree species changed their elevation ranges.

The researchers examined temperature trends at sampling sites between 1951 and 1990. Although most areas warmed, trees growing in the fastest-warming locations did not consistently shift their ranges more rapidly than species elsewhere.

That suggests temperature alone is not enough to predict the future of mountain forests.

Drought sensitivity also produced a complex picture. In some species, tree growth became increasingly vulnerable to drought at higher elevations. In others, the opposite pattern appeared. For many trees, the relationship changed over time or showed no consistent connection with elevation at all.

The findings underline how difficult it can be to forecast ecological change using a single climate variable.

The trees moving uphill may eventually run out of mountain

For species escaping warmer conditions by moving uphill, there is another problem waiting at higher elevations: mountains get smaller toward their summits.

As a tree population moves upward, the amount of available land can shrink sharply. Less than 5% of the world's mountain area lies above 2,000 metres, according to the study.

That means species already shifting toward higher elevations could eventually find themselves competing for increasingly limited space.

High-altitude areas have often been viewed as climate refuges — cooler places where vulnerable species can survive as lower regions become hotter. But the researchers argue that such refuges cannot be considered a universal solution.

Drought-sensitive trees may continue moving upward only to become confined to smaller and smaller areas.

Moving downhill brings a different set of risks

The trees heading toward lower elevations may have more land available, but that does not necessarily make their future secure.

As species enter new areas, they can encounter unfamiliar competitors, pests and diseases. Soil conditions, air dryness and the structure of existing forests can also influence whether a species successfully establishes itself.

Human activity adds another obstacle. Logging, fragmented forests and altered landscapes can make it difficult for tree populations to migrate, regardless of whether their biology favours an uphill or downhill shift.

The researchers also stress that the study identifies associations rather than proving that hydraulic traits directly cause trees to move in a particular direction.

Another important limitation is time. Much of the underlying evidence ends around 1990, meaning the analysis does not fully capture the rapid climate changes of recent decades.

Even so, the study offers a new way of thinking about climate-driven forest change. The future of mountain trees may depend not simply on how fast the planet warms, but also on the biological tools different species possess to cope with heat, drought and an increasingly unpredictable environment.

Rather than one great uphill migration, the world's mountain forests may be heading in several directions at once.