For all of these reasons, the southern Appalachians are rich in natural places for plants and animals, what ecologists call their habitat. What each species does in such habitats (such as act as predators) is called its niche. The total geographic area where plants and animals live is designated as their range, and the characteristics of the total place where they occur, including the climate, is referred to as their life zone, or biome.
A traveler who moved in a straight line northward from southern Florida to the extreme north of Canada would pass through a variety of life zones. These zones would include semitropical forests, southern pine forests, oak-hickory forests, northern pine-hardwood forests, beech-maple forests, spruce-pine-birch forests, spruce-fir forests, and, finally, in the subarctic region, muskeg or tundra shrub-grasslands.
Instead of traveling north to visit colder life zones, a visitor can experience many of these life zones in Western North Carolina by traveling altitudinally: up the mountains, from the mixed pine-hardwood forests below an elevation of 2,000 feet to red spruce–Fraser fir forests and near tundralike places called balds at elevations above 5,000 feet. From a biological standpoint, then, traveling up the mountains to more than 6,000 feet is equal to traveling a distance of 1,000 miles north, almost to the Arctic.
In and around Asheville, one would find mostly mixed pine and hardwood forests. At higher elevations, one would encounter different forest assemblages along the mountain ridges and slopes and, if rivers were encountered, still different plant assemblages. At an elevation of approximately 3,000 feet, one would find stands of pines, and upward 1,000 feet more, one would enter hemlock and heath communities. Above 4,000 feet, within a northern type of hardwood-forest life zone, one would discover habitats that are restricted to the Appalachian Mountains: cove forests and areas that ecologists have named boulderfields. Both are unique. From a canopy standpoint, cove forests are the richest woodlands in the country, containing more than 30 species of trees that uniformly reach the sunlight.
In mature forests throughout the world, stratification occurs, that is, plants survive at different levels in the forest. Dominant plants form the canopy. The subtending vegetation must be able to reproduce and thrive in the shade of these dominant species. This arrangement leads to a layered effect or stratification: mostly mosses, ferns, and other herbaceous plants occur as ground cover; above these, shrubs and small young trees reach for the canopy; and finally, the ultimate mature trees form the forest's top layer.
One who traveled to the northern area of a midwestern state such as Michigan would find a type of plant community called a beech-maple forest where only two species dominate the canopy: American beech (Fagus grandifolia) and sugar maple (Acer saccharum). In the southern Appalachians, however, occurs a plant community known as a cove hardwood forest, where a great diversity of tree species comprise the canopy. Cove forests are usually dominated by Canada hemlock (Tsuga canadensis), yellow buckeye (Aesculus flava), American basswood (Tilia americana), sugar maple (Acer saccharum), sweet birch (Betula lenta), tulip poplar (Liriodendron tulipifera), cucumbertree (Magnolia acuminata), white ash (Fraxinus americana), American beech (Fagus grandifolia), and a small tree found only in the southern Appalachians, the Carolina silverbell (Halesia carolina). Cove forest communities might support as many as 30 species of canopy trees with a great variety of shrubs and wildflowers in the understory, making it the most diverse forest type in all of North America.