The tree line represents a distinct environmental border where atmospheric conditions become too harsh to support the growth of trees. This boundary is not set at a fixed altitude worldwide; instead, it varies significantly based on latitude, mountain size, and local climate. It marks the upper limit of arboreal life on mountains, or the poleward limit in arctic and antarctic regions. The elevation of this ecological shift provides a measurable indicator of climate and environmental conditions in a specific region.
Defining the Ecological Boundary
The term “tree line” identifies the absolute highest point where an individual tree can survive and grow. This is often distinguished from the “timberline,” which is the lower boundary where dense, closed-canopy forest gives way to a more open, transitional zone. The timberline marks the ecological end of a continuous forest ecosystem, while the tree line is the end of all tree growth.
This transition zone, known as the forest-tundra ecotone, is rarely a sharp, straight line. As altitude increases, trees become noticeably shorter, sparser, and often deformed by the harsh climate. The most characteristic growth form in this area is Krummholz, a German term meaning “crooked wood.”
Krummholz consists of stunted, wind-sheared trees, such as conifers, that grow close to the ground, sometimes appearing more like dense shrubs than actual trees. The parts of the tree exposed above the insulating snow cover in winter are often killed by desiccation and wind. This often results in a flagpole appearance where only the lower, protected branches survive.
Primary Environmental Factors Governing Altitude
The primary factor determining the tree line’s altitude is the temperature during the growing season. Trees require a specific amount of warmth over a minimum number of days to complete metabolic processes, particularly photosynthesis, and to harden new growth against winter cold. The tree line generally correlates with a point where the mean temperature of the warmest month is approximately 6 to 7 degrees Celsius.
Prolonged periods of low temperature impair a tree’s ability to produce new woody tissues and sustain necessary functions like respiration. If the growing season is too short or too cool, trees cannot invest enough energy to form a straight, tall stem, leading to the stunted growth forms seen at the tree line.
Wind desiccation is a powerful secondary factor, especially at high elevations. Strong, persistent winds strip moisture from needles and leaves, particularly during winter when the ground is frozen and trees cannot replenish lost water, a phenomenon known as physiological drought. Other limiting factors include shallow, unstable soils, high levels of ultraviolet radiation, and the mechanical damage caused by wind-blown ice and snow, often called “snow blast.”
Global Variation in Tree Line Elevation
The altitude of the tree line is highly variable across the globe, following a distinct pattern based on latitude and continentality. Near the equator, where temperatures are consistently warmer, the alpine tree line reaches its highest elevations. For instance, in the tropical Andes Mountains of South America and the mountains of East Africa, the tree line can be found at elevations between 3,500 and 4,000 meters above sea level.
Moving poleward into temperate zones, the tree line drops significantly. In the central Rocky Mountains of North America, the tree line is commonly situated around 3,500 meters, while in the Swiss Alps, it reaches about 2,200 to 2,500 meters. The drop continues until the tree line meets sea level in the Arctic, becoming the Arctic tree line, which represents the northern limit of tree growth.
Local topographic features also cause variation, a phenomenon known as the mass elevation effect. Tree lines are generally higher on large, massive mountain ranges compared to isolated peaks because the larger landmass holds heat more effectively. Additionally, the slope aspect, or the direction a slope faces, influences local height, with sunnier, south-facing slopes in the Northern Hemisphere supporting a higher tree line due to increased solar radiation and warmer soil temperatures.
Life Above the Timberline
The landscape immediately above the tree line transitions into the alpine tundra, an ecosystem defined by the absence of trees and the dominance of specialized, low-growing vegetation. This environment is characterized by extreme cold, intense winds, thin air, and a very short growing season.
Many alpine plants adopt a cushion or mat growth form, growing tightly packed and close to the ground. This diminutive size allows them to avoid the strongest winds and trap heat from the soil, which can be significantly warmer than the air just centimeters above. These plants often have thick, waxy leaves or fine hairs, which help to reduce water loss from the constant desiccation caused by high winds and intense solar radiation.
Animals living in the alpine zone, such as marmots and pikas, also possess adaptations for survival, often employing hibernation or specialized insulation.