Most water on the tundra is frozen within the soil. The permanent frozen soil is called permafrost. Permafrost and fierce winds prevent large trees from growing deep roots. Plants in the tundra are small and grow close to the ground. Plants called cushions grow in tight clumps in the tundra, but lichens are the dominant plants. Lichens cover the rocks. The few trees that do grow on the tundra are dwarf willows and birch.