Introduction. In 1889 C. Hart Merriam studied the distribution patterns of plants and animals in a broad swathe from the lower elevations of the Grand Canyon to the top of Humphreys Peak (elevation 12, 760 ft) in the San Francisco Mountains near Flagstaff, Arizona. Based on his observations in the field, Merriam developed the concept of a Life Zone, a belt of vegetation and animal life that is similarly expressed with increases in altitude and increases in latitude.

The Grand Canyon	diagram
The San Francisco Peaks viewed from Flagstaff

In Arizona, where relief is high, Merriam delineated six different life zones and correlated these with latitudinal vegetation zones ranging from Sonora, Mexico, to the Arctic coast of Canada. He tried unsuccessfully to associate the boundaries of his life zones with mean annual temperatures; to match faunal distribution patterns to his zones; and to apply them on a continent-wide basis; yet the basic scheme survived and continues to be used by researchers in western North America. For descriptive purposes Merriam's life zones are a simple, straight-forward classification of readily observed vegetation patterns that work well in the area in which they were devised.

Below are brief overviews of the characteristics of the life zones as they occur in Arizona. Note that there are ranges of minimum and maximum elevations for each zone. The location of boundaries between life zones varies according to slope exposure, especially. In the northern hemisphere, zones will be higher on south-facing slopes than north-facing slopes. Note also that some life zones contain more than one biotic community, reflecting the climatic and ecological variation that so characterize mountainous areas.

Lower Sonoran Life Zone. This vegetation of this life zone corresponds with the hot deserts of the southwestern United States and northwest Mexico (the Mojave, Sonoran, and Chihuahuan deserts). Creosotebush (Larrea tridentata) and other desert shrubs and succulents occur at elevations from 100 ft to 3,500-4,000 ft above sea level. Total annual precipitation averages 10 inches or less.

Upper Sonoran Life Zone. A number of communities are characteristic of this zone that ranges from 3,500-4,000 ft to about 7,000 ft in elevation. These include a woodlands of evergreen oaks (Quercus spp.), pinyon pine (Pinus cembroides), and/or juniper (Juniperus spp.); the Arizona chaparral of leathery-leaved scrub oaks (e.g., Quercus emoryi), manzanita (Arctostaphylos spp.), buckthorn (Rhamnus spp.) and mountain mahogany (Cercocarpus spp.); grassland; and Great Basin desertscrub with its dominant sagebrush (Artemsia tridentata). Total annual precipitation varies from 8 to slightly more than 20 inches.

Transition Life Zone. An open ponderosa pine (Pinus ponderosa) forest is characteristic at elevations from 6,000 to 9,000 ft. Total annual precipitation ranges from 18 to 26 inches.

Canadian Life Zone. The fir forest of this life zone is dominated by Douglas fir (Pseudotsuga menziesi). White fir (Abies concolor) is also characteristic. In some places pines (other than Ponderosa) are also common. Deciduous broadleaf trees such as Gambel oak (Quercus gambeli), and quaking aspen (Populus tremuloides) can also be found here. The elevation of the Canadian zone in Arizona ranges from 7,500-8,000 ft to 9,000-9,500 ft; and precipitation from 25 to 30 inches.

Hudsonian Life Zone. A spruce-alpine fir community is found in this life-zone. Common species include Engelmann spruce (Picea engelmanni), blue spruce (P. glauca), Alpine fir (Abies lasiocarpa) and bristlecone pine (Pinus aristata). Trees decline in height from roughly 80 feet to dwarfed, gnarled individuals deformed by the wind near timberline (the Krummholz). The Hudsonian zone varies from 8,000-9,000 ft. to 11,500 feet and receives 30 to 35 inches of precipitation a year.

On the highest peaks of Virginia the Canadian or Boreal zone is represented by red spruce (Picea rubens) at elevations above 4000 ft. (No Virginia peak is high enough to reach timberline and have an Arctic-Alpine life-zone.) At lower elevations altitudinal zonation among species of the regional Temperate Broadleaf Deciduous Forest is apparent., but vegetation belts are not typically assigned to different life zones. Below is a schematic diagram of vegetation zonation on Mount Rogers, Virginia's highest peak (5,729 ft. above sea level). Most of the species of plants and animals restricted to the Canadian Zone in Virginia are considered relicts of the Pleistocene, a time of cooler climate. The Fraser fir (Abies fraseri), which in Virginia occurs only on the summit of Mt Rogers, is endemic to the Southern Appalachians.

Mount Rogers viewed from Elk Garden	diagram
Fraser fir near summit of Mount Rogers

Farther north in the Appalachians, in such places as the White Mountains of New Hampshire and on Mt. Katahdin at the terminus of the Appalachian Trail in Maine, an Arctic-Alpine Life Zone occurs above the boreal forest of balsam fir (Abies balsamea), black spruce (Picea mariana), and paper birch (Betula papyrifera). Many of plant species occurring at elevations greater than 4,000 ft on these mountain tops are identical to those of the Arctic tundra and are at or near their southern limits in the eastern United States on New England's highest peaks.

Mt. Washington, NH Elevation = 6,288 feet

Arctic-Alpine Life Zone on Mt. Washington