Tundra tend to be found near the ice covered region within northern and southern hemisphere.
The phrase "Treeless land" really comes from the fact that no trees can grow in this territory. Due to its close proximity with the ice covered region, the top part of the soil in Tundra tend to be Frozen. This make it impossible for trees root's to penetrate the soil to obtain necessary nutrients. Hence, no trees able to grow in Tundra.