THE COMPOSITION OF associations among plant species tends to vary regularly across gradients of altitude, latitude, temperature, soil types, and other variables. Making use of this observation, biogeographers are able to delineate distinct sets of conditions along such gradients that reflect changes, sometimes abrupt, in the distribution of plant associations. Where two or more such changes occur, the ecology of a region may be divided into distinct vegetation zones. The concept of the vegetation zone, alternatively called a life zone, is sometimes taken to be synonymous with the concept of the biome, and the terms are often used interchangeably in some scientific literature. The designation of biome, however, is more correctly applied at a higher level of generality. The vegetation zone concept has a long history in biogeography. Its roots are usually traced to Alexander von Humboldt, who devised the method of mapping isothermals (lines of equal temperature) and observed patterns of change in the distribution of organisms across isothermal gradients. In 1889, American naturalist C. Hart Merriam refined Humboldt’s insight over a summer of field research in the varied landscape of the southern Colorado Plateau north of Flagstaff, Arizona. There, discontinuities in the composition of plant species occur abruptly and correlate well with changes in altitude. With field notes on plants covering more than 8000 ft (2,438 m) of change in altitude—a cursory examination of the Grand Canyon extended that range, but incompletely—Merriam developed confidence in a scheme of zonation that he termed “life zones” in 1890. In addition, Merriam believed that small differences in altitude would correlate well with broad differences in latitude. Thus, the rocky, treeless area of the San Francisco Peaks above tree line, which Merriam called the “Alpine” life zone, corresponded with sub arctic regions. Merriam also noticed that boundaries between zones occur at higher altitudes on south-facing slopes than on corresponding north-facing slopes.