Vegetation type Average Elevations Annual Life zone
July (feet) rainfall
temp. (inches)
Prominent species
Southwestern Desert 94 2,200 to 4,000 7 to 13 Lower Sonoran
Scrub
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 74 4,500 to 7,000 12 to 22 Upper Sonoran
Woodland and
Chaparral
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.
[Illustration: 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.
These patterns are due to variations in the environment. For the desert
is hotter and drier in some places than in others. The gradual downslope
changes in vegetation reflect the decrease in the amount of soil
moisture available to plants—a condition caused by the decreasing size
of soil particles and consequent shrinkage of water-holding space
between particles. Desert washes encourage plant growth because they
channel water and cold air. Strangely enough, night temperatures are
often lower in a desert valley than farther up the slopes. This
“inversion” is due to drainage of cold air down mountainsides, forming
cold “pools” in valley bottoms, especially in winter. Cold air is
heavier than warm, and (since air flows much like water) it is channeled
down the draws—a fact that will strike you if you walk into a wash near
the mountains at night or early in the morning. Add this phenomenon to
the great amounts of moisture that lie beneath the surface of washes,
and you can see that desert drainageways are really linear oases. More
subtly, desert hills reflect in their vegetation the differences in soil
moisture from north slopes to south slopes caused by increasing exposure
to sunlight.
[Illustration: Saguaro cactus]
One controlling factor, then, is dominant in the desert: the scarcity of
water. This results not only in the unusual forms and adaptations of
desert plants, but also in a distinctive type of plant community. In
wetter climes, plants compete mostly for sunlight. They can grow close
together, as long as they receive adequate illumination. During the
regrowth of forests, several distinct sets of plants appear, each
succeeding group more tolerant of shade than the last, as the forest
canopy closes. In the desert, there is no such succession. Clear a patch
of desert vegetation, and the same species will reappear—spaced out,
with bare ground between them as before. For here sunlight is abundant
(we might say overabundant), but there is not enough water to allow
plants to cover the ground.
Dr. Forrest Shreve, who was a botanist at the Carnegie Institution’s
Desert Laboratory near Tucson and a master student of deserts, defined a
desert as “an area in which deficient and uncertain rainfall ... has
made a strong impression on the structure, functions, and behavior of
living things.” The distinctive characteristics of desert plants and
animals have evolved through millions of years, in a trial-and-error
process in which only the better-fitted forms have survived. It would be
enlightening to know how many of the species and varieties of plants
that developed during the past 60 million years or so have failed to
adapt to Sonoran Desert conditions. It is fascinating to study the
hundreds of forms that have succeeded and to try to determine what
structures they have perfected and what methods they have originated
that enabled them to maintain themselves in such a harsh environment.
Desert plants can be classified as “escapers,” “evaders,” and
“resisters,” according to their means of adaptation to water shortage.
Escapers, such as the annuals, avoid the problem entirely by waiting out
the dry periods as seeds, to sprout and reproduce only when the rains
come. Evaders, such as the ocotillo, reduce their water loss during
droughts by such methods as dropping their leaves or going into a state
of dormancy. Resisters, however, “hang in there” all year, taking the
desert’s worst. The cactuses, prime examples of this group, rapidly soak
up water from each rain and store it for use during drought; the
mesquite’s deep roots tap a more constant source of moisture.