Forest Garden

Life Zones


As you travel down to a lower elevation from Flagstaff, have you noticed there are fewer and smaller trees and finally, a desert of cactus and shrubs? When you travel higher up on the San Francisco Peaks, the forest changes again, from long-needled ponderosa pines to short-needled firs and spruce. You are noticing life zones: habitats with different levels of temperature, moisture, and exposure to the sun. Each life zone sustains a different community of plants and animals. 


Our Forest Garden is a sample of plants and trees found on the San Francisco Peaks, where cool air causes clouds to release rain and snow. In this garden, these high elevation species grow where the environmental education center shelters them from drying wind and intense sun. 


In 1886, U.S. Department of Agriculture scientist C. Hart Merriam wanted to help farmers decide which crops and livestock to raise in different parts of the country. Merriam applied the insights of the German naturalist, Alexander von Humboldt, who determined that climbing a mountain is like traveling from the warm south to the cool north, in terms of biodiversity. 


After researching where different wild plants and animals occur in North America, Merriam led an expedition to the San Francisco Peaks, where he confirmed that life zones with a particular group of plants and animals occur one after the other, from low, warm, and dry conditions to high, cool, and damp environments. He confirmed that some plants and animals on the Peaks are also found as far north as Canada!

Young Merriam_red
Merriam on Peaks_red
Left: Hart Merriam at age 16,                                                             Right: Dr. C. Hart Merriam camping at
upon his return from the 1872                                                            the base of the San Francisco 
Hayden Expedition.                                                                            Peaks, just north of Flagstaff, in 
Life Zones_red

Merriam's 1890 diagram of life zones on the San Francisco Peaks. The life zones are lower on the south side than on the north, because exposure to direct sunlight makes the south side warmer and drier.

For well over a century, Dr. Merriam's work has proven useful to American farmers and gardeners. His North American life zones map eventually evolved into today's USDA plant hardiness map.

Merriam Map_red

Merriam's 1893 Map of North American Life Zones.

USDA Map_red

The USDA Plant Hardiness Zone Map.