Life Zone

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): 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 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.

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Life Zone