Life Zones

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.

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.

Arctic-Alpine Life Zone. Occurring above treeline, this life zone corresponds to the Arctic tundra. Two main habitat types are found on the wind-swept peaks of the highest mountains: a tundra rock field where lichens predominate, and an alpine tundra-meadow with herbs, grasses, sedges, rushes, mosses, and lichens. Tree line on Humphreys Peak, the highest summit of the San Francisco Peaks (shown to left), occurs at 11,000 to 11,400 ft. Snow-covered from late November to early April, the summit area receives from 33 to 40 inches of precipitation a year.

An interesting biogeographic note: Some 50 species of plants occur in the Arctic-Alpine Life Zone in Arizona. Forty percent of these are disjunct from the true tundra of the higher latitudes, and 15 of these are circumpolar in their distribution–meaning that they occur in Arctic areas of both Eurasia and North America. Only two plant species are endemic; one of these is a groundsel (an herb of the genus Senecio). Remember this biogeographic pattern when you look at and compare high alpine flora in the tropics.

Merriam’s Life Zones in the eastern US

When Merriam’s Life Zones are applied to altitudinal zonation patterns in the humid eastern United States, an obvious difficulty occurs in trying to equate anything with the Lower and Upper Sonoran zones. To overcome this problem, Merriam substituted the name Austral (meaning southern) for Sonoran in the broad continental expression of his vegetation belts. However, even that name is rarely used. In general it is only the uppermost three life-zones–the Canadian, Hudsonian, and Arctic-Alpine–that are applied descriptively in the East where mountains rise above the regional forest types. And often the Canadian and Hudsonian zones are combined into a Boreal Life Zone.

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.

Altitudinal zonation in the tropics

Vegetation zonation on mountains in the tropics does not
replicate the latitudinal belts of vegetation of the middle and higher latitudes
even though mountain peaks may extend well above snowline. In part this is due
to the fact that the seasonal cool and cold temperatures of the middle and high
latitudes are not experienced in the tropics. Instead, it is diurnal temperature
patterns that are important. At high elevations nocturnal frosts may occur
nearly every day throughout the year; while daytime temperatures are quite warm.
Seasons in the tropics are distinguished on the basis of precipitation patterns.

Plant species of the alpine zones of the tropics, particularly in the
southern hemisphere, are taxonomically related to the southern hemisphere
temperate zone and antarctic floras. There are interesting convergences in
lifeforms between the alpine flora of the Old World and New World tropics and,
indeed, between some animal taxa, too. For example, the hummingbirds
(Trochilidae) of the high Andes have their counterpart in East Africa in the
sunbirds (Nectarinidae).

For all the similarities, there are also distinct differences between the
mountain zones of South America and of East Africa reflecting the very different
geologic history of the two continents. The Andean Cordillera of South America
is a continuous range of mountains stretching along the entire west coast of the
continent, the result of mountain building associated with continental drift. In
east African a disjunct chain of mountains ranges from Ethiopia to South Africa.
Individual peaks and ranges have different ages, origins, and geologies. Instead
of forming a long corridor as the Andes do, Africa’s mountains are more or less
islands onto themselves.


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