The life zone concept was developed by

C. Hart Merriam Clinton Hart Merriam (December 5, 1855 – March 19, 1942) was an American zoologist, mammalogist, ornithologist, entomologist, ecologist, ethnographer, geographer, naturalist and physician. He was commonly known as the 'father of mammalogy', a ...

in 1889 as a means of describing areas with similar

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

and

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

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 is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

, being developed on the

San Francisco Peaks The San Francisco Peaks (Navajo: , es, Sierra de San Francisco, Hopi: ''Nuva'tukya'ovi'', Western Apache: ''Dził Tso'', Keres: ''Tsii Bina'', Southern Paiute: ''Nuvaxatuh'', Havasupai-Hualapai: ''Hvehasahpatch''/''Huassapatch''/''Wik'hanbaj ...

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

and

Cascade Range The Cascade Range or Cascades 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, ...

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):

creosote bush ''Larrea tridentata'', called creosote bush and greasewood as a plant, chaparral as a medicinal herb, and ''gobernadora'' (Spanish for "governess") in Mexico, due to its ability to secure more water by inhibiting the growth of nearby plants. In S ...

, Joshua tree * Upper Sonoran (desert steppe or

chaparral Chaparral ( ) is a shrubland plant community and geographical feature found primarily in the U.S. state of California, in southern Oregon, and in the northern portion of the Baja California Peninsula in Mexico. It is shaped by a Mediterranea ...

): sagebrush,

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 widespr ...

, Colorado pinyon,

Utah juniper ''Juniperus osteosperma'' (Utah juniper; syn. ''J. utahensis'') is a shrub or small tree native to the southwestern United States. Description The plant reaches , rarely to 9 m, tall. The shoots are fairly thick compared to most junipers, in ...

* Transition (open woodlands): ponderosa pine * Canadian (fir forest): Rocky Mountain Douglas fir, quaking aspen * Hudsonian (spruce forest): Engelmann spruce, Rocky Mountains bristlecone pine * Arctic-Alpine (alpine meadows or tundra):

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...

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

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, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California. It is noted fo ...

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 The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

. 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 Inc. (WWF) is an 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 the W ...

, or the list of North American ecoregions defined by the Commission for Environmental Cooperation.

Holdridge

In 1947, Leslie Holdridge 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 under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

. 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.

References

{{Authority control Biogeographic realms

Holdridge

See also

References