The tree line represents a distinct boundary, marking the furthest extent, by elevation or latitude, at which trees can successfully grow and sustain themselves. This limit is determined by environmental conditions that become too harsh to support the biological demands of large, upright woody plants. It is not an abrupt division but a gradual transitional zone where the forest thins out and tree stature is noticeably reduced. This natural border is a global phenomenon, primarily controlled by temperature, separating dense forest biomes from treeless alpine or polar regions.
Defining the Tree Line and Timberline
Ecologists distinguish between the “timberline” and the “tree line” to describe different stages of this transition. The timberline defines the upper edge of a closed-canopy forest where trees are still tall and dense. Above this line, the forest structure breaks down into scattered individuals and patches, forming a diffuse zone. The true tree line is the absolute boundary where even individual, isolated trees can no longer survive the extreme conditions.
The area between the timberline and the tree line is characterized by the Krummholz zone. Krummholz, a German term meaning “crooked wood,” describes trees that are severely stunted, twisted, and deformed by wind and cold. These trees often grow as low, dense mats close to the ground. This low growth benefits them by providing insulation from snow and access to warmer surface microclimates, adapting them to an environment where vertical growth is suppressed.
Environmental Factors Restricting Tree Growth
The primary factor restricting tree growth globally is the low temperature during the growing season, which imposes a thermal budget constraint. Trees require sufficient warmth to transform photosynthetic sugars into new structural tissue for stems, branches, and roots. The global tree line correlates with a mean growing season air temperature of approximately 6 to 7 degrees Celsius. Below this threshold, trees cannot mature new wood cells before the next cold season, leading to insufficient growth and dieback.
The limitation is not a failure of photosynthesis, as cold-adapted trees can still produce sugars near freezing. However, the cold severely slows the metabolic process of investing that carbon into structural growth. Trees are disadvantaged compared to low-lying plants because their upright structure exposes temperature-sensitive growing tips (meristems) to colder air and wind. Low-growing plants benefit from warmer ground temperatures, which can be several degrees higher than the surrounding air.
Other factors modulate the tree line’s exact position, including wind and the short duration of the growing season. Strong winds at high altitudes cause mechanical damage and increase desiccation, or water loss, from needles and leaves. This is especially damaging during winter when the ground is frozen and water uptake is impossible. A short growing season limits the time available for trees to accumulate energy reserves, preventing the establishment of new seedlings and determining the upper limit of tree life.
Alpine Versus Polar Tree Lines
Tree lines manifest in two major forms: alpine and polar. The alpine tree line is defined by elevation, occurring on high mountains globally. The primary constraint is the decrease in air temperature with increasing altitude, following a predictable lapse rate. Alpine environments also feature greater solar radiation intensity and high winds, contributing to the harsh conditions.
In contrast, the polar or arctic tree line is defined by latitude, marking the northernmost limit of tree growth across vast, low-elevation areas. While the underlying thermal constraint is the same—insufficient growing season warmth—the spatial manifestation differs. The Arctic tree line is often constrained by the extremely short growing season and the presence of permafrost. Permafrost, or permanently frozen ground, limits the depth of root penetration, hindering structural stability and water uptake.
The arctic tree line transitions into the low-growing tundra environment at relatively low altitudes, sometimes near sea level. This is distinct from the alpine tree line, which can reach elevations over 4,000 meters in tropical regions. Both boundaries are fundamentally temperature-limited, but the polar line is a broad geographic transition across latitude, while the alpine line is a vertical transition across elevation.