Life zone
Encyclopedia
The Life Zone concept was developed by C. Hart Merriam
in 1889 as a means of describing areas with similar plant
and animal
communities. Merriam observed that the changes in these communities with an increase in latitude at a constant elevation are similar to the changes seen with an increase in elevation at a constant latitude.
The life zones Merriam identified are most applicable to western North America
, being developed on the San Francisco Peaks
, Arizona
and Cascade Range
of the northwestern USA. He tried to develop a system that is applicable across the North American continent, but that system is rarely referred to.
The life zones that Merriam identified, along with characteristic plants, are as follows:
The Canadian and Hudsonian life zones are commonly combined into a Boreal life zone.
This system has been criticized as being too imprecise. For example, the scrub oak chaparral in Arizona
shares relatively few plant and animal species with the Great Basin
sagebrush desert, yet both are classified as Upper Sonoran. However it is still sometimes referred to by biologists (and anthropologists) working in the western United States
. Much more detailed and empirically based classifications of vegetation and life zones now exist for most areas of the world.
In 1947, Leslie Holdridge published a life zone classification using indicators of:
Biotemperature refers to all temperatures above freezing, with all temperatures below freezing adjusted to 0°C, as plants are dormant at these temperatures. Holdridge's system uses biotemperature first, rather than the temperate latitude bias of Merriam
's life zones, and does not primarily use elevation. The system is considered more appropriate to the complexities of tropical vegetation than Merriam's system.
Clinton Hart Merriam
Clinton Hart Merriam was an American zoologist, ornithologist, entomologist and ethnographer.Known as "Hart" to his friends, Dr. Clinton Hart Merriam was born in New York City in 1855. His father, Clinton Levi Merriam, was a U.S. congressman. He studied biology and anatomy at Yale University and...
in 1889 as a means of describing areas with similar plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
and animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
communities. Merriam observed that the changes in these communities with an increase in latitude at a constant elevation are similar to the changes seen with an increase in elevation at a constant latitude.
The life zones Merriam identified are most applicable to western North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, being developed on the San Francisco Peaks
San Francisco Peaks
The San Francisco Peaks are a volcanic mountain range located in north central Arizona, just north of Flagstaff.The highest summit in the range, Humphreys Peak, is the highest point in the state of Arizona at in elevation. The San Francisco Peaks are the remains of an eroded stratovolcano...
, Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
and Cascade Range
Cascade Range
The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...
of the northwestern USA. He tried to develop a system that is applicable across the North American continent, but that system is rarely referred to.
The life zones that Merriam identified, along with characteristic plants, are as follows:
- Lower Sonoran (low, hot desert): Creosotebush, Joshua Tree
- Upper Sonoran (desert steppe or chaparralChaparralChaparral is a shrubland or heathland plant community found primarily in the U.S. state of California and in the northern portion of the Baja California peninsula, Mexico...
): SagebrushSagebrushSagebrush is a common name of a number of shrubby plant species in the genus Artemisia native to western North America;Or, the sagebrush steppe ecoregion, having one or more kinds of sagebrush, bunchgrasses and others;...
, Scrub OakScrub OakScrub Oak is a general name for several species of small, shrubby oaks, including the following species:*California Scrub Oak *Leather Oak *Coastal Scrub Oak...
, Colorado PinyonColorado PinyonThe Colorado Pinyon, Two-needle Pinyon, or Piñon Pine, ', is a pine in the pinyon pine group whose ancestor was a member of the Madro-Tertiary Flora and is native to the United States....
, Utah Juniper - Transition (open woodlands): Ponderosa PinePonderosa PinePinus ponderosa, commonly known as the Ponderosa Pine, Bull Pine, Blackjack Pine, or Western Yellow Pine, is a widespread and variable pine native to western North America. It was first described by David Douglas in 1826, from eastern Washington near present-day Spokane...
- Canadian (fir forest): Rocky Mountain Douglas-firRocky Mountain Douglas-firThe Rocky Mountain Douglas-fir, Pseudotsuga menziesii subsp. glauca, is an evergreen conifer native to the interior mountainous regions of western North America, from central British Columbia and southwest Alberta in Canada southward through the United States to the far north of Mexico...
, Quaking AspenAspenPopulus section Populus, of the Populus genus, includes the aspen trees and the white poplar Populus alba. The five typical aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. The White Poplar, by... - Hudsonian (spruce forest): Engelmann SpruceEngelmann SprucePicea engelmannii is a species of spruce native to western North America, from central British Columbia and southwest Alberta, southwest to northern California and southeast to Arizona and New Mexico; there are also two isolated populations in northern Mexico...
, Rocky Mountains Bristlecone PineRocky Mountains Bristlecone PinePinus aristata, the Rocky Mountain Bristlecone Pine, is a species of pine native to the United States. It appears in the Rocky Mountains in Colorado and northern New Mexico, with an isolated population in the San Francisco Peaks in Arizona... - Arctic-Alpine (alpine meadows or tundra): LichenLichenLichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
, GrassGrassGrasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
The Canadian and Hudsonian life zones are commonly combined into a Boreal life zone.
This system has been criticized as being too imprecise. For example, the scrub oak chaparral in Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
shares relatively few plant and animal species with the Great Basin
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
sagebrush desert, yet both are classified as Upper Sonoran. However it is still sometimes referred to by biologists (and anthropologists) working in the western United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Much more detailed and empirically based classifications of vegetation and life zones now exist for most areas of the world.
Holdridge
- mean annual biotemperature (logarithmic)
- annual precipitation (logarithmic)
- ratio of annual potential evapotranspiration to mean total annual precipitationPrecipitation (meteorology)In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
.
Biotemperature refers to all temperatures above freezing, with all temperatures below freezing adjusted to 0°C, as plants are dormant at these temperatures. Holdridge's system uses biotemperature first, rather than the temperate latitude bias of Merriam
Clinton Hart Merriam
Clinton Hart Merriam was an American zoologist, ornithologist, entomologist and ethnographer.Known as "Hart" to his friends, Dr. Clinton Hart Merriam was born in New York City in 1855. His father, Clinton Levi Merriam, was a U.S. congressman. He studied biology and anatomy at Yale University and...
's life zones, and does not primarily use elevation. The system is considered more appropriate to the complexities of tropical vegetation than Merriam's system.