The tundra and taiga covering much of Alaska is a delicate yet thriving ecosystem. Many plants have evolved artfully to deal with its harsh conditions. Underlying permafrost (frozen ground) restricts the topsoil depth and limits tree growth as a result. In spite of the long harsh winter covered by snow, the summer melt is quick and plant life explodes in rapid growth under constant daylight. Many of Alaska's well know mammals live in these habitats.