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): 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): Eng