Large trees cannot grow in the polar regions, because the active layer (under the surface)<u> is too thin</u>, therefore the roots aren't able to grow.
Permafrost areas have very cold air temperatures, thin topsoil, and most water is frozen during the winter. ... Tundra is found at high latitudes and at high altitudes, where the permafrost has a very thin active layer. The active layer of tundra is too thin for trees to grow, because it cannot support a tree's roots.
