Stone polygons, soil circles, stone or soil stripes and terraces are common to both arctic and alpine tundra. These features are created by thrusting action of repeated freezing and thawing of moist soil over a solid substrate like rock or permafrost. Polygonal patterns dominate flat surfaces. Vegetation is usually confined to the stable parts of the patterns. Permafrost is a common feature of the arctic tundra climate and biome.
Permafrost refers to permanently frozen ground.
Actually, the ground has two layers which freeze. Beneath the active layer is the inactive layer which stays frozen throughout the year. Permafrost creates a barrier to the root development. Larger trees can grow along better drained river valleys where the depth to permafrost is greater. The annual freezing and thawing disrupts root systems inhibiting the growth of very tall vegetation.