Polar tundra and deserts have continuous to sparse cover of cold-tolerant mosses, liverworts, lichens, grasses, low shrubs and other flowering plants. They occur primarily in the Arctic circle, but polar desert is found in dry coastal lowlands of Antarctica. Precipitation falls as snow, with seasonal snow cover limiting the growing season. Extreme cold temperatures and short growing seasons exclude trees, as well as vascular plants in the coldest and driest locations. Permafrost substrates accumulate peat through slow decomposition rates. Migratory birds feed in distant wetlands or open oceans, and contribute nutrients to the system through guano, as well as dispersing seeds and other organisms. Migratory or hibernating mammals include seals, and, in the north, polar bears, foxes and wolves.