During summer the top layer of soil may thaw long enough to let plants grow as much as they can and reproduce. There is barely vegetation in the tundra, only about 1,700 different species. They are mostly shrubs, sedges, mosses, lichens and grasses. There are about 400 varieties of flowers. The growing season is only about 50-60 days. During the off season algae is their source of carbon. There are no trees, except for some birches in lower latitudes.