treeline

Treeline is the upper elevational or northernmost limit at which trees can maintain a self-sustaining population. It marks the transition between forest and treeless tundra or alpine ecosystems, and its position is not a fixed line but a zone that varies with climate, species, geography, and land-use history. In mountain regions the treeline ascends with latitude and descends with increasing aridity and wind exposure, while in polar regions it lies near the limit of suitable growing-season warmth.

The primary controls on treeline position are climate and soil. Growing-season warmth, particularly the length and temperature sum of the growing season, determines the ability of tree roots and shoots to complete their annual cycle. Late or early frosts, soil depth, permafrost, and rooting space constrain establishment and survival. Snow cover can insulate seedlings, but deep, persistent snow or strong winds can delay or damage regeneration. Competition from shrubs and herbaceous plants, disturbances such as fire, pests, and human land use also influence the line. The line is often irregular, with micro-sites offering pockets of establishment that may form krummholz (wind‑pruned trees) rather than a continuous forest.

Treelines occur worldwide, including latitudinal treelines near high latitudes and altitudinal treelines on mountains. The term timberline is sometimes used interchangeably, though some definitions reserve it for the lower productive edge of a forest stand.

Climate change is affecting treelines in many regions, with observed and projected shifts poleward or upward as warming alters temperature regimes, snow dynamics, and disturbance patterns. The ecological and carbon implications of these shifts are an active area of study.