Why are trees unable to grow in tundra ecosystems?

Trees are unable to grow in tundra ecosystems primarily due to low temperatures, which serve as the main arbiter of the tree line.12

The harsh environmental conditions of the tundra limit tree growth in several ways:

• Temperature Constraints: Tree growth is generally only possible where the mean temperature for the month of July is at least 10 °C (50 °F). Low temperatures slow biological processes, reducing the production of dry matter and shortening the growing season, which prevents trees from forming the necessary supportive tissues for annual growth and reproduction.12
• Permafrost: The presence of permafrost—a layer of permanently frozen ground—restricts root growth and limits the depth to which roots can penetrate.32
• Energy Deficits: Inadequate energy production, often exacerbated by cloudiness or short growing seasons, can lead to a carbon deficit, impairing a tree's ability to develop protective surface layers and survive environmental stresses.1
• Physical Stress: Trees in these regions are susceptible to wind damage, snow blast, and ice damage, which further inhibit their ability to maintain an erect form.1