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 changes in these communities with an increase in latitude but constant elevation are similar to the changes seen with increase in elevation but constant latitude.

The life zones Merriam identified are most applicable to western North America, being developed on the San Francisco Mountains, Arizona. 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 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 commonly referred to by biologists working in the western United States.