Here's why:

* Low Temperatures: Tundra biomes are characterized by extremely low temperatures, with average annual temperatures below freezing. Winters are long and harsh, with temperatures often dropping below -30°C (-22°F).

* Permafrost: The ground in tundra biomes is permanently frozen, called permafrost. This layer of ice prevents trees from growing and limits the depth of roots.

* Limited Precipitation: Tundra biomes receive very little precipitation, typically less than 25 cm (10 inches) per year. This makes them very dry, further contributing to the harsh conditions.

While the Arctic tundra is generally considered the coldest, the Antarctic tundra also experiences extremely low temperatures and is considered a tundra biome.