In the Tucson Mountains, only the desert scrub type is well represented, although the highest ridges support a suggestion of desert grassland. A small patch of shrub live oaks, relicts of a wetter period, huddles on the north side of Wasson Peak. Zonation is poorly developed here because the mountains are low and small in mass. (Studies have shown that mountains of smaller mass tend to have warmer and drier climates than bulkier mountains of the same height.)
Major Vegetation Types in Saguaro National Monument
| Vegetation type | Average July temp. | Elevations (feet) | Annual rainfall (inches) | Life zone |
| Prominent species | ||||
| Southwestern Desert Scrub | 94 | 2,200 to 4,000 | 7 to 13 | Lower Sonoran |
| Paloverde | ||||
| Saguaro | ||||
| Pricklypear | ||||
| Cholla | ||||
| Catclaw | ||||
| Ocotillo | ||||
| Creosotebush | ||||
| Mesquite | ||||
| Ironwood[1] | ||||
| Grassland Transition | 85 | 3,500 to 4,500 | 10 to 15 | Upper Sonoran |
| Mesquite | ||||
| Beargrass | ||||
| Gramagrasses | ||||
| Amole | ||||
| Sotol | ||||
| Oak-pine-juniper Woodland and Chaparral | 74 | 4,500 to 7,000 | 12 to 22 | Upper Sonoran |
| Emory Oak | ||||
| Mexican Blue Oak | ||||
| Shrub Live Oak | ||||
| Pinyon Pine | ||||
| Mountain-mahogany | ||||
| Manzanita | ||||
| Skunkbush | ||||
| Coniferous Forest | 68 | above 6,000 | 18 to 30 | Transition and start of Canadian |
| Ponderosa Pine | ||||
| Gambel Oak | ||||
| Buckbrush | ||||
| Mountain Muhly | ||||
| Douglas-fir | ||||
| White Fir | ||||
| Aspen | ||||
| Snowberry | ||||
| Mexican White Pine | ||||
[1]TUCSON MOUNTAIN SECTION
Biologists have developed several systems for classifying assemblages of plants and animals over broad regions, and one of the most widely known is that of C. Hart Merriam. Around the turn of the century, he conceived a system of “life zones,” named for the parts of the continent where they are best developed: Tropical, Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic. In Saguaro National Monument, the Lower Sonoran Zone corresponds to the desert scrub type; the Upper Sonoran includes desert grassland transition, oak woodland, and oak-pine woodland; the Transition Zone is equivalent to the ponderosa pine forest; and the poorly represented Canadian Zone has Douglas-fir and white fir. Because Merriam’s system is so widely used, his terms are included in the tabulation on page 20, which summarizes the main characteristics of plant zones in the monument.
The best way to appreciate the biotic changes that occur with elevation is to walk or ride a horse to the top of the Rincons. But if your time or energy is limited you can get a quick view of these sequences by driving up nearby Mount Lemmon, in the Santa Catalina Mountains.
Saguaro Forest landscape from the scenic drive.
desert plants
When you look out over the cactus forest, in either part of Saguaro National Monument, you may think there’s a sameness to it in all directions—saguaros standing amid scattered shrubs. But look harder, or walk about, and you will discover variations in the scene. First, there is a gradual change in the vegetation from the mountain foot down the bajada or pediment to the valley floor, as saguaros and paloverdes (green) become sparser and creosotebushes (smaller and brownish) take over. (The monument itself does not extend far enough from the mountain bases to include extensive creosotebush communities, but these cover large areas in the lowest parts of the valleys.) Then there is the luxuriant growth along washes, where mesquites and paloverdes grow to tree size and there are more kinds of plants. And if you are observant you may notice slight variations with each change in slope on the rolling hills—for example, more grass growing on their north sides. On another scale, you can see separate little communities of plants in special situations, as under shrubs or on rocks.