Summer temperature is the overriding factor determining what plants can grow in any given region in the Arctic.

Other environmental factors -- soil moisture and soil fertility being the most important -- control the vegetation within each temperature zone. The contrast between moist sites and dry sites makes for the most conspicuous differences. A much wider variety of plant species, and most of the colorful flowers, are to be found on moderately moist ground, while sedge meadows -- smooth tracts of uniform bright green -- occupy many of the wet areas. Slightly drier areas are sometime covered by seemingly endless expanses of cottongrass tussocks.

*from E.C. Pielou, A Naturalist's Guide to the Arctic, p. 95