Saguaro National Monument, Arizona

Part 1

saguaro national monument ARIZONA

Napier Shelton based on an earlier work by Natt Dodge

NATURAL HISTORY SERIES

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

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.

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.

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.

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