Tree line

The tree line is the edge of a habitat at which trees can grow and beyond which they cannot. It is found at high elevations and high latitudes. Beyond the tree line, trees cannot tolerate the environmental conditions, usually because of low temperatures, heavy snow, or lack of moisture.

At the tree line, trees often grow in a sparse, stunted, and crooked way because of the wind and cold. This special kind of growth is sometimes called krummholz (which means "crooked wood" in German).

The tree line does not always appear as a sharp border. Instead, it can be a gradual change where trees become shorter and fewer as they get closer to the limit where they cannot grow at all. Generally, for a given area, the tree line lies about 300 to 1000 meters below the permanent snow line and runs roughly parallel to it.

Causes

Trees grow tall and are more affected by cold weather than smaller plants that stay close to the ground. During summer, warmth decides where trees can grow. Even though some trees, like conifers, can handle cold most of the year, they can be hurt by just a little frost in the middle of summer. For example, warm summers in the 1940s allowed more spruce trees to grow above the old tree line near Fairbanks, Alaska. For trees to survive, they need enough new growth each year. Wind can also hurt trees by damaging their leaves and branches, especially when the leaves stick up above the snow.

Variation

The height where trees can grow, called the tree line, depends mostly on temperature. Trees need a seasonal mean temperature of about 6 °C or 43 °F, and a growing season of about 94 days above 0.9 °C (33.6 °F) to thrive.

Because of climate change, the tree line in North Cascades National Park has moved upward by more than 400 feet (120 m) in just 50 years. Local factors like the direction a slope faces (aspect) and areas protected from rain (rain shadow) can also change where the tree line is. For example, north-facing slopes in the northern hemisphere have lower tree lines because snow stays longer, shortening the growing season. On coasts and isolated mountains, tree lines are often lower due to the Massenerhebung effect, where larger mountain ranges keep more heat and reduce wind, helping trees grow at lower heights.

Types

There are different kinds of tree lines, which are places where trees stop growing because of tough conditions. These are studied in ecology and geography.

The alpine tree line is found on mountains. It marks the highest place where trees can grow. Above this line, it is too cold or there is too much snow for trees to survive. The area above the tree line is called the alpine climate, and the area where trees can grow is the alpine zone.

The line where trees stop growing is not sharp. Instead, there is a zone where trees become shorter and smaller, turning into bushes called krummholz. This happens because the air gets colder as you go higher up the mountain. Snow can also affect trees. Too much snow can stop trees from growing, but the right amount can help them by keeping them warm and giving them water.

The arctic tree line is found far north. It is the northernmost place where trees can grow. North of this line, it is too cold for trees to live. The cold can freeze the sap inside trees, and the ground, called permafrost, can be too hard for tree roots to grow deep enough.

The area near the arctic tree line changes from forest to tundra, a flat, open land with few plants. In some places, there are small patches of short trees mixed with tundra plants. Further south, there are more trees and fewer tundra plants.

Tree species near tree line

Some typical Arctic and alpine tree line tree species (note the predominance of conifers):

Australia

• Snow gum (Eucalyptus pauciflora)

Eurasia

• Dahurian larch (Larix gmelinii)
• Macedonian pine (Pinus peuce)
• Swiss pine (Pinus cembra)
• Mountain pine (Pinus mugo)
• Arctic white birch (Betula pubescens subsp. tortuosa)
• Rowan (Sorbus aucuparia)

North America

• Subalpine fir (Abies lasiocarpa)^: 106 
• Subalpine larch (Larix lyallii)
• Mountain hemlock (Tsuga mertensiana)
• Alaska yellow cedar (Chaemaecyparis nootkatensis)
• Engelmann spruce (Picea engelmannii)^: 106 
• Whitebark pine (Pinus albicaulis)
• Great Basin bristlecone pine (Pinus longaeva)
• Rocky Mountains bristlecone pine (Pinus aristata)
• Foxtail pine (Pinus balfouriana)
• Limber pine (Pinus flexilis)
• Potosi pinyon (Pinus culminicola)
• Black spruce (Picea mariana)^: 53 
• White spruce (Picea glauca)
• Tamarack (Larix laricina)
• Hartweg's pine (Pinus hartwegii)

South America

• Antarctic beech (Nothofagus antarctica)
• Lenga beech (Nothofagus pumilio)
• Alder (Alnus acuminata)
• Pino del cerro (Podocarpus parlatorei)
• Polylepis (Polylepis tarapacana)
• Eucalyptus (not native to South America but grown in large amounts in the high Andes).

Worldwide distribution

Alpine tree lines

The tree line is the highest point on Earth where trees can grow. It rises as you move toward warmer areas. From 70 to 50 degrees north, the tree line goes up about 75 meters for every degree south you travel. From 50 to 30 degrees north, it rises about 130 meters per degree. Between 30 degrees north and 20 degrees south, the tree line stays between 3,500 and 4,000 meters high.

Here are some examples of tree lines from places all around the world:

Arctic tree lines

Just like in the mountains, tree lines near the poles change based on local conditions. Things like the slope of the land and how sheltered an area is can affect where trees can grow. Permafrost, soil that stays frozen, also plays a big role. When soil is too shallow, trees can fall over easily in the wind. Trees sometimes grow in valleys where rivers run, even if they can't grow in more open places. Ocean currents and other weather patterns also influence where trees can live. In parts of Scandinavia, ocean influences help keep winters milder, allowing trees to grow farther north than they normally could. Here are some typical tree lines in polar areas:

Antarctic tree lines

The world’s southernmost trees are found on Isla Hornos, at the very tip of South America. There are no trees on subantarctic islands or in Antarctica itself, so there is no tree line on the continent. The Southern Ocean acts as a boundary for tree growth.

Some subantarctic islands, like South Georgia and the Auckland Islands, have small forests of trees called Rata. These trees grow up to 370 meters high in sheltered valleys but stay very short—usually under 3 meters tall. The climate on these islands is wet and windy, with very few sunny hours, which limits how big trees can grow.