Trees generally have limits to where they can grow, particularly concerning elevation. As one ascends a mountain, a point is reached where environmental conditions become too harsh for trees to thrive, eventually leading to their disappearance. The reasons for this halt in growth involve a combination of interconnected environmental factors that challenge a tree’s ability to survive and reproduce.
The Treeline Explained
The “treeline,” also known as the “timberline,” marks the highest elevation where trees can grow. This zone is not a sharply defined border but rather a transitional area where trees become progressively shorter and more sparse. Trees found at or near this elevation often display stunted, wind-swept, and twisted forms, a phenomenon known as “krummholz,” a German term meaning “crooked wood.”
The krummholz growth habit allows trees to survive by staying low to the ground, often protected by snow cover or rock formations. Branches on the windward side may be killed or deformed, leading to a “flag tree” appearance where growth is concentrated on the sheltered side. This transitional zone reflects the increasing severity of conditions with altitude, where taller, upright trees cannot persist, giving way to more resilient, low-lying forms.
Factors Limiting Tree Growth
Low temperatures and a short growing season are the primary factors restricting tree growth at higher elevations. Plant scientists indicate that trees struggle to build cells effectively when the average growing-season temperature falls below approximately 6.7°C (44°F). Trees require a sufficiently long and warm period to accumulate energy reserves necessary for growth, reproduction, and survival. The rooting zone also needs a minimum temperature of 5-7°C for metabolic processes to occur.
Strong winds at high altitudes contribute significantly to limiting tree growth. Winds can cause physical damage, such as branch breakage and uprooting, and lead to desiccation by increasing water loss from leaves. Snow abrasion, where wind-driven snow acts like sandpaper, further damages exposed tree parts.
Persistent snow cover also plays a role in limiting tree growth. While snow can offer insulation and protection to lower-growing plants, excessive or prolonged snow cover shortens the effective growing season. The weight of heavy snow can bend and break branches, leading to physical stress and deformation. Additionally, thin soils, increased ultraviolet (UV) radiation, and lower atmospheric carbon dioxide levels can also contribute to the challenges faced by trees at high elevations.
Variations in Treeline Elevation
The specific elevation at which trees cease to grow is not uniform across the globe; it varies considerably due to a range of influences. Latitude is a significant factor, with treelines generally found at higher elevations closer to the equator and progressively lower elevations towards the poles. For example, treelines in tropical alpine regions can reach over 4,000 meters, sometimes up to 4,900 meters, while in subpolar regions, they may be only a few hundred meters above sea level. The Teton Mountains in the United States have a treeline around 3,000 meters (10,000 feet), whereas Mount Washington in New Hampshire sees trees stop at about 1,370 meters (4,500 feet).
Local environmental factors also exert considerable influence on treeline elevation. Aspect, referring to the direction a slope faces, can affect temperature and sunlight exposure; for instance, south-facing slopes in the Northern Hemisphere typically receive more sun and may support trees at higher elevations than north-facing ones. Topography, such as sheltered valleys or exposed ridges, creates microclimates that can either protect trees or expose them to harsher conditions. Proximity to large bodies of water can also moderate temperatures, potentially allowing trees to extend to slightly higher elevations. Different tree species possess varying tolerances to the cold, wind, and other high-elevation stresses.