The edge of the forest
If you climb a mountain in a forested region, trees gradually become shorter and sparser, and then they stop. In the far north, the same happens as you travel toward the Arctic: the dense boreal forest thins into scattered stunted trees and finally gives way to open tundra. The boundary zone is called the treeline. On a mountain, it is the alpine treeline (or timberline); in the north, it is the arctic or polar treeline.
A treeline is rarely a sharp line. More often it is a transition belt that can span from tens to hundreds of meters in elevation on a mountain, and a much wider zone in the north. Ecologists often distinguish the forest limit, where continuous forest ends, from the tree species limit, the highest point where individual upright trees can still grow. Between them lies a zone of patchy and dwarfed trees.
What limits tree growth
The main cause is cold during the growing season. Trees need enough warmth to build tissue, and the widely supported view is that the treeline tracks a minimum seasonal temperature, especially soil temperature, below which trees cannot form enough new growth to ripen their shoots and survive the winter. This explains why treelines in different parts of the world, despite very different climates, often occur at similar growing-season temperatures. The idea is a broad pattern rather than an exact rule.
Several other factors contribute:
- Growing season length. A very short season leaves little time to build wood and mature new shoots before winter.
- Cold soil. Roots work poorly in cold ground, limiting water and nutrient uptake. In the north, permafrost can add to the problem, as discussed in the article on permafrost thaw.
- Wind and ice. Strong winds can sandblast foliage with ice crystals and snow, remove protective snow cover and dry out needles when the ground is frozen and roots cannot resupply water. Evergreen needles above the snow are especially exposed.
- Snow. Deep, late-melting snow shortens the season for tree seedlings in hollows, while snow-free ridges expose them to wind and cold. Snow can also protect small trees, so the effects differ by site.
- Seedling establishment. Even if mature trees survive, seedlings may be unable to establish because of cold, drought, or lack of seed. Seed production is often poor near the limit.
- Browsing, fire, avalanches and human use. Local treelines are often lower than their climate would allow because of grazing, burning or logging in the past.
Altogether, trees are limited less by an inability to photosynthesize than by an inability to use the carbon they capture to grow. This is still an active area of research, and different studies emphasize different mechanisms.
What the edge looks like
Approaching the limit, trees show visible signs of hardship:
- They get shorter, with trunks that are often twisted or leaning.
- Branches may grow only on the sheltered side, forming flag-shaped trees.
- In the harshest spots, trees form low, matted growth called krummholz, from a German word meaning crooked wood. These mats stay below the winter snow line and the tallest shoots die back when they poke through.
- Some trees spread by layering, in which low branches touch the ground and root, forming clumps that may be very old even though they are small. A tree like this might be a centuries-old individual only a meter tall.
- Growth rings are narrow and sensitive to summer temperature, so trees at the limit make valuable climate records, as covered in the article on tree rings.
The species vary by region. Spruces, firs, larches, pines and birches are common treeline trees in the northern hemisphere, while southern beeches are important in parts of the Southern Hemisphere. A single species often forms the treeline in a given area.
Mountain versus northern treelines
The two types share a cause, but differ in character. On a mountain, the climate changes over a short distance, so the transition can be seen in a day’s walk. Slope, aspect, rock and snow patterns create a complex mosaic, and conditions can differ greatly between neighboring slopes. A mountain treeline usually lies higher in warmer regions, such as lower latitudes, than near the poles.
Arctic treelines stretch over long distances across flat or rolling land, and they are shaped by the sea, permafrost, and patterns of fire and wetland. The transition zone from forest to tundra can be wide, and in some areas it is formed by a mosaic of forest patches and open ground.
Do treelines move?
Warming temperatures raise the question of whether treelines are advancing upslope and northward. Evidence from many regions shows that in some places trees are growing taller, becoming denser, or establishing above the old limit. In others, the treeline has barely changed, even where temperatures have risen, because seedlings are limited by factors such as drought, soil, seed supply, grazing or snow conditions.
Treelines are slow to move. Trees live long, and new seedlings must survive harsh conditions for years, so shifts often lag behind climate. Responses are uneven, which is why careful long-term monitoring using repeat photographs, field plots and satellite data is useful. This links to the general question of how species ranges respond to a changing climate.
Why it matters
Treelines mark the edge of a large habitat. Beyond them, alpine meadows and tundra host specialized plants and animals that would be crowded out if trees advanced. In mountains, forests above villages also help hold snow and soil in place, so their upper limit can matter for avalanches and slope stability. In the Arctic, a change from open tundra to forest could alter how much sunlight is reflected, how snow accumulates and how much carbon is stored, although the net effect is debated and likely varies by region.
Understanding treelines is therefore a way of seeing climate effects directly on the landscape, while remembering that the response of any particular treeline depends on local conditions and cannot be assumed from the global pattern.