1972
National Park Service
U.S. DEPARTMENT of the INTERIOR
For sale by the Superintendent of Documents, U.S. Government Printing
Office
Washington, D.C. 20402—Price $1.25
Stock Number 2405-0292
preface
This book is a simple account of the natural history of Saguaro National
Monument. It is intended to help you understand the relationships
between land, climate, plants, wild animals, and man in the environment
of a hot desert. While it includes brief profiles of many representative
species, it is not intended to serve as a guide to the monument. It does
indicate where the several distinctive natural communities exist, and
when and where to look for certain plants and animals. For
identification purposes, you will need field guides.
The present edition is my revision of the 1957 book by Natt Dodge, then
naturalist for the Southwest Region of the National Park Service. The
first five and the last two chapters are essentially new; the central
chapters on plants and animals remain largely as written in the first
edition.
The authors wish to thank former and present members of the monument
staff for their help and companionship, in both field and office. We are
particularly grateful to Chief Naturalist Harold T. Coss, Jr., who
devoted much time and effort to obtaining many of the photographs and,
with Park Biologist Warren F. Steenbergh, gave the manuscript a thorough
scrutiny. The cooperation and hospitality of Superintendent Harold R.
Jones have created the best possible climate for work on the revised
edition. National Park Service geologist Robert H. Rose contributed in
very great measure to the geological content. Finally, our thanks go to
the many students of desert life on whose knowledge this book has been
built, and to monument visitors who ask questions—for their concern
gives hope for better relations between man and nature.
—N.S.
contents
The Desert Scene, 1
The Sonoran Desert and the Monument, 3
Climate: The Vital Factor, 9
Rocks: Foundation and Soilmakers, 13
Plant and Animal Zones, 19
Desert Plants, 23
Succulents, 25
The Saguaro—Monarch of the Monument, 27
Other Common Cactuses, 33
Non-Succulents, 36
Perennials, 36
Ephemerals, 42
Major Plant Communities of Saguaro, 49
Plants of the Hills and Mountains, 55
Oak-Pine Woodland, 55
Ponderosa Pine Forest, 58
Animals and How They Survive, 61
Invertebrates, 62
Amphibians, 64
Reptiles, 64
Birds, 66
Mammals, 72
The Rhythms of Nature, 79
The Impact of Man, 83
Appendix, 87
Suggested Reading, 87
Common and Scientific Names of Plants, 89
Reptiles and Amphibians of the monument, 92
Birds of the monument, 94
Mammals of the monument, 97
[Illustration: Brittlebush display along Cactus Forest Drive.]
the desert scene
Scattered through the wide, lonely Sonoran Desert, isolated mountain
ranges raise jagged blue silhouettes against the sky. The high ones wear
a crown of dark pines and a speckled mantle of oaks. Lapping against
their feet is the desert sea, studded with the green masts of giant
saguaro cactuses rising above a motley crew of tough, strange, often
handsome subordinates.
On either side of the Santa Cruz Valley in southeastern Arizona, Saguaro
National Monument embraces two of these ranges, with the desert lands at
their feet. Although it was established primarily to preserve impressive
stands of saguaros, it gains wholeness by including the mountains that
shed the rocky soil on which the saguaros grow. The monument is in two
sections, some 30 miles apart. The _Rincon Mountain Section_, east of
Tucson, includes in its 99 square miles a spectrum of plantlife ranging
from saguaro communities at the low elevations to a wet fir forest on
the north slope of 8,666-foot Mica Mountain. The 24-square-mile _Tucson
Mountain Section_, about 12 miles west of the city, has a denser stand
of saguaros, on the lower slopes of a wild jumble of volcanic mountains
dominated by 4,687-foot Wasson Peak.
Within the cactus forests and upon the mountain slopes, life in myriad
forms goes on in a delicate and continuous adjustment to changing
environments. Over millennia, mountains rise and crumble, species evolve
and fade from the scene. Over days and weeks and years, populations of
plants and animals rise and fall, in response to thousands of
interactions which we are only beginning to understand. In the following
pages, we will meet the main characters in this natural drama (including
some of the most improbable in all of nature); we will investigate the
environmental stage upon which they perform; and we will try to
understand something of the play itself, including our own role in it.
[Illustration: The Saguaro Forest in the Tucson Mountain Section.]
the sonoran desert
and the monument
Saguaro National Monument is at the northeastern edge of the Sonoran
Desert. Named for the state of Sonora, Mexico, in which the greater part
of it lies, this, one of four major deserts in North America, is
distinguished by differences in climate and vegetation. The Great Basin
Desert, mainly in Nevada and Utah between the Rockies and the Sierra
Nevada, has cold winters, sparse precipitation distributed fairly evenly
throughout the year, and rather simple vegetation dominated by the low
shrubs, sagebrush and saltbrush. Immediately to the south, in southern
Nevada and southeastern California, is the Mohave Desert, with cool
winters during which most of the year’s precipitation comes, and with
plant cover consisting mostly of shrubs such as creosotebush. At higher
elevations grows the Joshua-tree, a giant yucca. The Chihuahuan Desert,
with cool winters and summer rainfall, covers the broad plateau of
north-central Mexico, extending into southern New Mexico and west Texas.
Its vegetation consists mostly of small cactuses, spiny shrubs, and
succulent-leaved plants such as yuccas.
Lying between the Mohave and Chihuahuan deserts, the Sonoran Desert has
both winter and summer rainfall, with spring and autumn droughts. Its
mild winters and bi-seasonal rainfall encourage a vegetation far
surpassing in lushness and variety that of the other deserts. From west
to east, the land rises and precipitation increases. Yuma, in
southwestern Arizona, lying at 141 feet above sea level, gets about 3
inches a year. Tucson, in southeastern Arizona, is at 2,400 feet
elevation and averages 11 inches a year. In the western part, where
plains are extensive and mountain ranges low and far apart, a very few
small-leaved species such as creosotebush dominate. Toward the east,
mountain ranges become more numerous, shedding material on which more
kinds of plants, particularly paloverde, mesquite, and cactuses, assume
leading roles in the vegetative cover. Both sections of the monument lie
within the eastern, wetter, more diversified part of the Sonoran Desert
known as the Arizona Upland. Organ Pipe Cactus National Monument, to the
southwest, is in an area transitional between the Arizona upland and
Colorado River lowland phases of the Sonoran Desert.
[Illustration: DESERT AREAS OF NORTH AMERICA]
But even between the two sections of Saguaro National Monument there are
differences. There is the obvious fact that the Tucson Mountains are
much lower and smaller in mass than the Rincons. And there is the
not-so-obvious fact that the Tucson Mountain Section extends to lower
elevations (2,200 as opposed to 2,700 feet). These conditions are
reflected in the somewhat warmer environment of the western section.
This helps to explain why certain Sonoran Desert plants and animals here
reach their northeastern limits; among them are ironwood, desert iguana,
desert horned lizard, western shovel-nosed snake, sidewinder, desert
kangaroo rat, and the Le Conte thrasher. The Rincon Mountain Section, by
virtue of its higher elevations, has plants and animals—such as
ponderosa pine, Steller’s jay, and whitetail deer—not found in the
western section.
On the geologic time scale the Sonoran Desert, like all the earth’s
present deserts, is a recent development. Some 50 million years ago
tropical forests grew here, as they did over most of southern North
America. As the Southwest gradually became drier, new species adapted to
the new conditions evolved; and the general type of vegetation
changed—with some retrogression during wetter periods—from forest to
savanna (grasslands with scattered trees) to arid subtropical scrub
(such as is now found in southern Sonora State), and finally to the
plant communities of today’s Sonoran Desert. The last stage has occurred
only during the past few million years.
The assemblage of species from which most of our present desert and
lower mountain plants were derived is known as the Madro-Tertiary Flora,
to denote its center (Sierra Madre in Mexico) and time (Tertiary
period—one million to 65 million years ago) of early development.
Reflecting these origins, a majority of the species growing below 6,000
feet elevation in the monument today are basically Mexican or Central or
South American in their distribution.
On the higher mountains of the Sonoran Desert, however, exist species
with an entirely different ancestry. Most of the plants of the pine and
fir forests of these high places, like the plants of the Great Basin
Desert, derive from the Arcto-Tertiary Flora, which dominated the
northern part of our continent during Tertiary times. As climate
changes, one or the other of these great plant assemblages will benefit
at the expense of the other. If cooler, wetter periods return, as they
did during the Pleistocene epoch, ponderosa pines, now found above 6,000
feet, may again grow nearly as low down as today’s cactus forests. But
if the present long-term drying trend continues, the northern plants
will eventually be squeezed off their mountaintops.
Animals, to some extent, reflect the same dichotomy of origins we have
seen in the plants. Thus, on the mountaintops supporting pine and fir
forests, a northern contingent of animals predominates, while on the
lower slopes and desert the fauna has a Mexican character. The canyons
and oak-pine forests of middle elevations in southeastern Arizona are
particularly exciting, because in these biological islands, isolated by
surrounding desert, live many “Mexican” species of animals found nowhere
else in the United States. Naturally, mountain ranges nearest the
border—such as the Huachucas and Chiricahuas—have the greatest numbers
of Mexican specialties; but the Rincons have their share too.
The wonderful diversity of plant and animal life in Saguaro National
Monument depends on a diversity of habitats. These in turn owe their
existence to a widely varying triumvirate of environmental
factors—climate, soil, and topography. To understand the biological
interplay that goes on here, we first must know something of the
environmental conditions that circumscribe it.
[Illustration: Plant communities on Mica Mountain.]