Alpine refers to terrain that is at a high enough altitude that large (usually evergreen) trees cannot grow. This characteristic lack of large vegetation is usually due to the presence of harsher climate conditions at higher altitudes.
Because equatorial regions of the globe tend to have warmer climates, alpine regions are not found until substantially greater altitudes in these locales. Colorado, for instance, sports large evergreen trees well beyond the 10,000-foot level, whereas in southern Alaska, timberlines range from 2,000-3,000 feet in elevation. Trees do not grow at all in circumpolar regions of the globe - northern Russia, and the North Slope of Alaska, for instance. They do, however, grow in all but the northernmost portions of Lappland due to the relatively warm climate conditions.
Alpine regions tend to be characterised by different vegetational zones. The area just above treeline is known as the subalpine, and tends to be vegetated with good-sized deciduous trees, as well uneven patches of small, scraggly evergreen trees. At somewhat higher elevations, even deciduous trees and shrubs cannot grow; and vegetation is limited to alpine grass and mosses of various sorts. Higher still is the high alpine, where all vegetation parts, leaving nothing but barren rock and ice.