Of all land area of the globe (about 151 million km²), the alpine life zone covers around 3% or 4.5 million km², which is one-third less than the area of the vegetated arctic life zone of North America and Eurasia (largely tundra). The alpine life zone (just as the arctic life zone) is treeless, hence its lower boundary is the climatic, high elevation treeline. Its upper boundary is the upper limit of plant life. Both these boundaries are fragmented and represent gradual transition zones. The alpine zone covers a suite of ecosystems (e.g. grass and shrub heathlands), special microhabitats (e.g. rock crevices and springs) and includes isolated outposts of life.