Much of the soil layers of the northern half of the Northern Boreal Forest is permanently frozen, permafrost. When the glaciers retreated they left behind a sheet of water which could not drain into the underlying rock strata because of the permafrost. The layer of cold water was colonized by sphagnum moss until the habitat was a solid mass of sphagnum moss with a few small conifers for company. This type of habitat is called muskeg. The combination of the permafrost and the boggy condition of the unfrozen top layer of soil make it difficult for trees to properly root.
Trees are small, widely spaced, and often fall over if they get too tall.
In Alaska a combination of Northern Boreal Forest habitats can be found. The photograph on the left shows a glacier in Alaska. Normal Northern Boreal Forest is found in the foreground, but is replaced with muskeg half way to the glacier. Surrounding the glacier is tundra vegetation. This picture shows the Northern Boreal Forest in winter.