The life zone concept was developed by C. Hart Merriam in 1889 as a means of describing areas with similar

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

and

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

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.

Merriam

The life zones Merriam identified are most applicable to western

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, being developed on the San Francisco Peaks,

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

and

Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...

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.

List

The life zones that Merriam identified, along with characteristic plants, are as follows: * Lower Sonoran (low, hot desert): creosote bush,

Joshua tree ''Yucca brevifolia'' (also known as the Joshua tree, yucca palm, tree yucca, and palm tree yucca) is a plant species belonging to the genus '' Yucca''. It is tree-like in habit, which is reflected in its common names. This monocotyledonous tre ...

* Upper Sonoran (desert steppe or

chaparral Chaparral ( ) is a shrubland plant plant community, community found primarily in California, southern Oregon, and northern Baja California. It is shaped by a Mediterranean climate (mild wet winters and hot dry summers) and infrequent, high-intens ...

sagebrush Sagebrush is the common name of several woody and herbaceous species of plants in the genus ''Artemisia (plant), Artemisia''. The best-known sagebrush is the shrub ''Artemisia tridentata''. Sagebrush is native to the western half of North Amer ...

scrub oak Scrub oak is a common name for several species of small, shrubby oaks. It may refer to: *the Chaparral plant community in California, or to one of the following species. In California *California scrub oak ('' Quercus berberidifolia''), a widesp ...

Colorado pinyon ''Pinus edulis'', the Colorado pinyon, two-needle piñon, pinyon pine, or simply piñon, is a pine in the pinyon pine group native to the Southwestern United States, used for its edible pine nuts. Description The piñon pine (''Pinus edulis'' ...

, Utah juniper * Transition (open woodlands):

ponderosa pine ''Pinus ponderosa'', commonly known as the ponderosa pine, bull pine, blackjack pine, western yellow-pine, or filipinus pine, is a very large pine tree species of variable habitat native to mountainous regions of western North America. It is t ...

* Canadian (fir forest): Rocky Mountain Douglas fir,

quaking aspen ''Populus tremuloides'' is a deciduous tree native to cooler areas of North America, one of several species referred to by the common name aspen. It is commonly called quaking aspen,

* Hudsonian (spruce forest):

Engelmann spruce ''Picea engelmannii'', with the common names Engelmann spruce, white spruce, mountain spruce, and silver spruce, is a species of spruce native to western North America. It is highly prized for producing distinctive tone wood for acoustic guitars ...

, Rocky Mountains bristlecone pine * Arctic-Alpine (alpine meadows or tundra):

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

grass Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and spe ...

The Canadian and Hudsonian life zones are commonly combined into a Boreal life zone.

Criticism

This system has been criticized as being too imprecise. For example, the scrub oak chaparral in

shares relatively few plant and animal species with the

Great Basin The Great Basin () is the largest area of contiguous endorheic watersheds, those with no outlets to the ocean, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja Californi ...

sagebrush desert, yet both are classified as Upper Sonoran. However it is still sometimes referred to by biologists (and anthropologists) working in the

western Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that id ...

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. Much more detailed and empirically based classifications of vegetation and life zones now exist for most areas of the world, such as the list of world ecoregions defined by the

World Wide Fund for Nature The World Wide Fund for Nature (WWF) is a Swiss-based international non-governmental organization founded in 1961 that works in the field of wilderness preservation and the reduction of human impact on the environment. It was formerly named th ...

, or the list of North American ecoregions defined by the

Commission for Environmental Cooperation The Commission for Environmental Cooperation (CEC; ; ) is an intergovernmental organization established by Canada, Mexico, and the United States to implement the North American Agreement on Environmental Cooperation (NAAEC), the environmental si ...

Holdridge

In 1947,

Leslie Holdridge Leslie Ransselaer Holdridge (September 29, 1907 – June 19, 1999) was an American botanist and climatologist. Career In his famous 1947 paper, he defined "life zones" using three indicators: # Mean annual biotemperature (average temperature ...

published a life zone classification using indicators of: * mean annual

biotemperature The Holdridge life zones system is a global bioclimatic scheme for the classification of land areas. It was first published by Leslie Holdridge in 1947, and updated in 1967. It is a relatively simple system based on few empirical data, giving ob ...

(logarithmic) * annual precipitation (logarithmic) * ratio of annual potential evapotranspiration to mean total annual

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

. 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 Merriam can refer to: People * Alan P. Merriam (1923–1980), American ethnomusicologist * Charles Edward Merriam (1874–1953), American political scientist *Charles W. Merriam (1877–1961), American insurance businessman and politician * Clinton ...

'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.

References

{{Authority control Biogeographic realms

Merriam

List

Criticism

Holdridge

See also

References