What is "life zone"

n. any large portion of the Earth's land area which has a generally uniform climate and soil and therefore a high degree of uniformity in the composition of species; a number of different classifications have been proposed by different people

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:

• Lower Sonoran (low, hot desert): creosote bush, Joshua tree
• Upper Sonoran (desert steppe or chaparral): sagebrush, scrub oak, Colorado pinyon, Utah juniper
• 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, grass

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, such as the list of world ecoregions defined by the World Wide Fund for Nature, or the list of North American ecoregions defined by the Commission for Environmental Cooperation.