Question

What is a life zone?

Answer 1

C. Hart Merriam introduced the life zone definition in 1889 as a way of defining areas with similar plant and animal populations. Merriam noticed that changes in these populations with a rise in latitude at a constant elevation are equivalent to changes in elevation at a constant latitude.

Complete answer:
Merriam's life zones are most applicable to western North America, where they are being built on the San Francisco Peaks, Arizona, and the Cascade Range in the northwest. He attempted to create a method that could be used across the North American continent, but it is scarcely mentioned.
Merriam described the following life zones, along with their associated plants:
Low, hot dessert is referred to as Lower Sonoran. It has plants creosote bush, Joshua tree.
The Desert steppe is referred to as Upper Sonoran. It has sagebrush, scrub oak, Colorado pinyon, Utah juniper.
Open woodlands are characterized as Transition. The plants are ponderosa pine.
Fir forests are referred to as Canadian. The plants are Rocky Mountain Douglas fir, quaking aspen.
Spruce forest as Hudsonian. Its plants are Engelmann spruce, Rocky Mountains bristlecone pine.
Tundra is described as Arctic Alpine. Its plants are lichen, grass.

Note:
This system has been chastised for being too sloppy. The scrub oak chaparral in Arizona, for example, has few plant and animal species in common with the Great Basin sagebrush desert, although both are known as Upper Sonoran.
Leslie Holdridge published a life zone classification in 1947 based on the following indicators:
Annual precipitation,
Mean annual biotemperature, and
The ratio of annual potential evapotranspiration to mean total annual precipitation.