Some evergreen trees live on the tundra, but in dwarfed form.
The polar tundra has few trees. The Arctic tundra is a shifting biome caught between the taiga and the ice cap, and Antarctic tundra it exists only on a few small islands. The determining factors for tree growth in polar tundra are temperature, soil and growing season. Arctic tundra can be divided into two zones, high and low, and these are separated by the treeline.
Tundra
The tundra is a relatively flat area of low-growing vegetation, mainly mosses, lichens and grasses. These support a relatively limited community of animals, mostly mammals and insects. It's too cold for reptiles and amphibians. Some migratory birds come to the tundra during the brief summer to nest and raise their chicks. Some areas of alpine tundra can be found in high mountains, but these lack the permafrost of Arctic tundra.
High Tundra
In high tundra, a thin layer of soil created by wind-blown dust and decayed plants supports a carpet of vegetation. Under this thin soil, sometimes only 3 inches deep, is a layer of permanently frozen ground called permafrost. Permafrost prevents trees from developing deep roots, so they cannot grow tall. The short growing season (50 to 60 days) means that trees cannot grow fast enough to get tall. Tree growth is also inhibited by the poor quality of the thawed soil because plants cannot decay fast in the cold to provide nutrients and animal droppings fertilize it in only some areas.
Low Tundra
In low tundra, the growing season is not much longer, but temperatures are a little warmer and the soil layer is somewhat thicker over the permafrost. Some species of trees can adapt to this environment, although they grow only to the size of bushes. Even here, the soil still freezes in the winter, making tree growth and survival problematic. The warming in global temperatures is allowing trees to move into areas where they have not been able to grow for millennia. In fact, the advance of the treeline into high tundra has been measured in feet in recent years.
Tundra Tree Species
Black spruce, white spruce, tamarack and dwarf birch are among the kinds of trees that are beginning to grow in low tundra. Black spruce (Picea mariana) in particular is tolerant of the boggy conditions that prevail on the tundra in summer. Dwarf willow (Salix herbacea), the world's smallest tree at a maximum of 4 inches tall, grows on the tundra and provides food for caribou, musk ox and Arctic hare. Native people who live at least part time on the tundra eat it because it tastes sweet and is a good source of vitamin C.
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