Why Do Trees Stop Growing High on a Mountain?
― Photosynthesis Still Works, but Cold Halts the "Construction Work"
Climb a high mountain and, at a certain height, the forest suddenly disappears. You are left with low shrubs and wildflowers. This is called the treeline. Surprisingly, trees do not vanish because they can no longer photosynthesise. The sugar they need is there, but it is too cold for the "construction work" of building new tissue to go ahead. And the taller a tree is, the harder that cold hits it.
You are hiking in the Northern Alps of Japan in summer. At first you are inside a forest of tall conifers, and the leaves shade you from the sun.
As you climb, the trees get shorter. More and more of them have twisted trunks and branches growing on one side only. Then the view suddenly opens up. At your feet, low shrubs no higher than your knee spread out, creeping along the ground.
The sunshine is, if anything, stronger. It rains too. And yet there is no forest above this point. What is stopping the trees?
There are just two reasons
Leaves can keep photosynthesising at close to 0 degrees. But the work of using that sugar to make new cells and thicken trunks and roots stops because of the cold first. The materials are there, but they cannot be used.
Plants close to the ground are wrapped in heat from the sun-warmed soil. The twig tips of a tall tree are exposed to the wind and are almost as cold as the air. The tree's own leaf shade also chills the soil around its roots.
When these two combine, you get a height at which "low plants can live, but tall trees cannot finish growing." That is the treeline. Let's take them one at a time.
If photosynthesis works, why don't trees grow?
For a long time, people thought the treeline existed because "it is too cold for enough photosynthesis, so the trees run short of sugar." But when researchers examined trees at the treeline, they found that the sugar stored in trunks and branches was, if anything, in surplus.
For a plant to grow, making sugar is not enough. It must divide cells, build cell walls, and pile up wood in the trunk. This work is far more sensitive to cold than photosynthesis is. Below about 5 degrees, construction is thought to slow almost to a halt.
Think of a warehouse piled high with timber, but the carpenters are too cold to work. The Swiss botanist Christian Körner and colleagues measured soil temperature at tree roots at treelines around the world. They reported that the average during the growing season was about 6 to 7 degrees almost everywhere. Whether on a tall tropical mountain or a low northern one, it settles on the same number.
Look at the left side of Figure 1. The dotted line marking the upper limit of the forest sits at the height where the growing-season average reaches about 6 to 7 degrees.
Why is it warmer close to the ground?
On a sunny day, touch the ground and it can feel much hotter than the air. Ground warmed by the sun creates a thin layer of warm air just above it. Wind is weaker near the ground, so this layer is not easily blown away.
Alpine wildflowers and plants that grow in round, cushion-like clumps sit right inside this layer. On a sunny midday, their leaves are reported to be more than 10 degrees warmer than the air.
The twig tips of a tall tree, on the other hand, are at a height where the wind blows straight through. However much sun they get, the heat is quickly carried away. So the twig temperature is almost the same as the air temperature. On top of that, once trees grow thick, their own shade keeps sunlight off the soil at their roots, and the soil turns cold too.
In other words, even at the same height on the same mountain, the climate felt by a low plant and the climate felt by a tall tree are two different things. Only low plants survive above the treeline because they live in a different, slightly warmer climate close to the ground.
The Japanese stone pine (hai-matsu), which spreads above the treeline in Japan's high mountains, grows creeping along the ground, as its name suggests. In winter it is buried under snow, which is thought to protect its branches from harsh cold wind and dryness. Branches that stick up above the snow are easily damaged in winter and can die. On windy ridges, you can also see trees whose windward branches have died, leaving a flag-like shape.
On tropical mountains, the treeline is said to lie at roughly 3,500 to 4,000 metres. It gets lower the further north you go, and in the Arctic it finally comes down to level ground. The line where forest ends on flat land is known as the Arctic treeline. Because Japanese mountains get heavy snow and strong wind, their treelines are said to be somewhat lower than those of continental mountains at the same latitude.
Summary
Trees disappear on high mountains not because of too little light or because they cannot make sugar. It is because cold stops the work of building new cells, and because taller trees are more exposed to the cold wind. Everywhere in the world, the treeline follows the same temperature line: "a growing-season average of about 6 to 7 degrees."
What stops the trees is not light or sugar,
but "cold too severe for building" and "being tall."
Why the direction a slope faces changes the kinds of trees is covered in "Why do north-facing and south-facing slopes on the same mountain have different trees?", and how snow protects what lies beneath is covered in "Why does it stay at 0 degrees under the snow even when it is minus 20 outside?". What happens to the human body high up is in the article on "altitude sickness", and why mountain weather changes so quickly is in the article on "mountain weather". The way it gets colder higher up is itself calculated in "Why does autumn colour start at the top of the mountain and come down?".
- On a clear, breezy day, go to a lawn or grassy area. Prepare two thermometers.
- Put one among the grass at ground level and the other in shade at chest height. Wait a few minutes, then read them. Cover the thermometers loosely with white paper so the sun does not shine directly on them.
- If you get a chance to go up a high mountain or hill, watch the height and shape of the trees as you climb. Note the height where trees become shorter and branches start to lean to one side.
If the air near the ground is a few degrees warmer, that is the "low-level climate" in which alpine plants live. If you climb a mountain, take great care with the weather and your equipment.
For those who want to know more ― terms, formulas and links to textbooksWe label which level each part belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Biology / Basic Earth Science"
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity specialist courses not taught in high school (plant ecophysiology, microclimatology)
- ResearchTopics even universities do not teach as settled, which researchers are studying now
Junior highTerms: this phenomenon has names
- Treeline: the upper limit of height (or latitude) at which a continuous forest can exist. Above it, tall trees can no longer form a stand.
- Alpine zone: the area above the treeline. In Japan it is covered by low Japanese stone pine scrub, alpine meadows and rocky ground.
- Lapse rate: the rate at which air temperature falls as height increases. On average, it is said to drop by about 0.6 degrees for every 100 metres of climb.
Junior highHigh schoolChecking with a formula: how high will the treeline be?
If you know the growing-season average temperature at the foot of a mountain, you can calculate how many metres you must climb for it to fall to about 6.5 degrees. Here we assume an average temperature of 22 degrees at sea level.
| In symbols | h ＝ ( T0 － Tc ) ÷ Γ |
| In words | Treeline height ＝ (temperature at the foot － lowest temperature trees can grow in) ÷ how fast temperature falls with height |
| Where it comes from | It comes from the approximation that air temperature falls in proportion to height (the lapse rate). Rising air expands and cools, so temperature drops at a nearly constant rate with height. Γ is said to be about 0.6 degrees per 100 metres. |
| Growing-season average temperature at sea level (assumed) | 22 degrees |
| Growing-season average temperature common to treelines | 6.5 degrees |
| Temperature drop per 100 metres of climb | 0.6 degrees |
| T0 | Average temperature at sea level (in degrees) |
| Tc | Treeline temperature guide (in degrees) |
| h | Treeline height (in metres) |
| Γ | Lapse rate (temperature drop per 100 metres, in degrees) |
| Temperature that must be lost | 22 － 6.5 ＝ 15.5 |
| How many 100-metre steps to climb | 15.5 ÷ 0.6 ≒ 25.8 |
| Convert to metres | 25.8 × 100 ＝ 2580 |
That is about 2,600 metres. This roughly matches the treeline of around 2,500 metres said to apply to mountains in central Honshu. If you lower the temperature at the foot by 2 degrees to 20 degrees, the same calculation gives a drop to about 2,250 metres.
| In a land 2 degrees colder at the foot | The treeline is more than 300 metres lower |
| Conversely, if temperature rises by 1 degree | The treeline could rise by about 170 metres in calculation |
A difference of just a few degrees shifts the mountain landscape by hundreds of metres. In reality, though, seeds must be carried in and trees take decades to grow, so they do not catch up with a change in temperature straight away.
High schoolHigh school+Link to the vertical distribution of biomes
High schoolIn Basic Biology you learn about "vertical distribution", where vegetation changes with elevation from the hill zone to the montane, subalpine and alpine zones. The boundary between the subalpine and alpine zones is the treeline in this article. Temperature falls with height because rising air expands and cools.
High school+In Japan's mountains, especially those on the Sea of Japan side where winter monsoon winds are strong, the treeline can sit lower than temperature alone would predict. This is attributed to a combination of heavy snow, strong wind and winter drying of branches that stick out above the snow. Such places are sometimes called "pseudo-alpine zones".
UniversityNot "a lack of materials" but "a lack of use for them"
📖 For the derivation of the formula and beyond: Lapse rate (Wikipedia, in Japanese)
In plant ecophysiology, growth limitation is split into the "sugar-making side" (photosynthesis) and the "sugar-using side" (cell division, elongation and wall synthesis). Many studies have reported that trees at the treeline hold reserves of non-structural carbohydrates (starch and sugars) in trunks and branches that are often no lower than those of lowland trees. For this reason, the "growth limitation hypothesis", which says cold limiting the sugar-using side is the main cause, has become the leading view. The effect of height is explained by the thickness of the boundary layer around a plant and the strength of heat exchange by wind. Low plants sit inside the boundary layer at the ground surface, where heat exchange with the air is weak, so they warm up easily in sunlight.
ResearchWhat we still do not know
- How far will the treeline rise with warming? Although temperatures are rising, the treeline has climbed fast in some places and hardly moved in others. How seedling establishment, grazing by animals and frost damage matter is being studied around the world.
- Why do the values converge on 6 to 7 degrees? A link to the temperature at which cell construction stops is suspected, but which process is the deciding one is not fully settled.
- Which matters more: winter damage or poor summer growth? In some places, winter drying of branches above the snow and freezing damage to young seedlings are also thought to set the treeline.
In other words, what is in this article is also "an explanation within what is known today." The rise of the treeline is also drawing attention as a marker for watching climate change.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: structure and function of plants / weather | Photosynthesis, getting colder with height |
| High school | Basic Biology: vertical distribution of biomes | The boundary between subalpine and alpine zones |
| High school+ | Basic Earth Science: temperature structure of the atmosphere | Calculating height with the lapse rate, pseudo-alpine zones |
| University | Plant ecophysiology / microclimatology | Growth limitation hypothesis, boundary layer and plant temperature |
| Research | Alpine ecology / climate change research | Speed of treeline rise, reason for 6 to 7 degrees |
| ― | Links to daily life | Reading the scenery when hiking, a marker of warming's effects |
- Körner, C. & Paulsen, J. (2004) A world-wide study of high altitude treeline temperatures. Journal of Biogeography 31: 713–732.
- Körner, C. (2012) Alpine Treelines: Functional Ecology of the Global High Elevation Tree Limits. Springer.
- Körner, C. (2003) Alpine Plant Life: Functional Plant Ecology of High Mountain Ecosystems, 2nd ed. Springer.
- Holtmeier, F.-K. (2009) Mountain Timberlines: Ecology, Patchiness, and Dynamics, 2nd ed. Springer.
- Biodiversity Center of Japan, Nature Conservation Bureau, Ministry of the Environment (環境省 自然環境局 生物多様性センター)
*This article is a general-audience science explainer. The numbers given are guides and rough estimates to help you understand the mechanism. The height of the treeline varies greatly with a mountain's terrain, snow and wind.