Why are trees not common in the arctic tundra? What types of plants are common in this biome?

Permafrost layer in Tundra is the layer of earth beneath the topsoil that remains frozen during the year and is usually only a few feet beneath the surface. It prevents trees from growing in the tundra because trees need to have deeper roots plus they cannot grow in the frozen ground.

Biome plants are : shrubs, sedges, mosses, lichens, grasses, and some flowering or herbaceous plants.