SAGUARO Ecology of the Saguaro: II NPS Scientific Monograph No. 8

COVER

DEDICATION

PREFACE

ACKNOWLEDGMENTS

Chapter 1: INTRODUCTION

The Species
   Distribution
   Adaptive evolution and co-evolved species
Hypotheses on Saguaro Population Change
   The grazing-plus-rodents hypothesis
   The climatic change hypothesis
   The "bacterial necrosis disease" hypothesis
Limiting Factors
Ecological Perspective

Chapter 2: REPRODUCTIVE GROWTH

Phenology
Freeze and Drought Limitations
Reproductive Potential
Summary and Conclusions

Chapter 3: THE FATE OF THE SEED: DISPERSAL, ATTRITION AND GERMINATION

Seed Dispersal
Seed Consumption and Loss
Germination
Summary and Conclusions

Chapter 4: ESTABLISHMENT AND SURViVAL

Biotic Factors
   Rodents and birds
   Insects
   Experimental exclosures
Abiotic Factors
   Drought
   Freezing
   Topography and slope exposure
   Soil moisture
Factor Interactions
Summary and Conclusions

Chapter 5: GROWTH

The Start of Growth
Growth and Growth Rates of Young Saguaros at Saguaro National Monument, Arizona
Age and Height
Effects of Flower-Bud Removal on Stem Growth
Freezing Effects on Growth
Geographic Variation in Growth
   Organ Pipe Cactus National Monument
   Tempe, Arizona
   Tucson, Arizona
Summary and Conclusions

Chapter 6: OVERVIEW AND OUTLOOK

Commerce and Conservation: The Human Factor
   Cactus Pirates
   Woodcutting
   Grazing
   Fire control
   Predator control
   Other decimating factors
   The "bacterial necrosis disease" as an historical factor
History in Perspective—Ecology or Pathology?
   Responses of an abused environment
   Requiem for a myth
Diverse Records: Populations in Response to Climatic Events
   Populations in trouble
   Catastrophic freezes
The Outlook
   Climatic trends
   Population trends

LITERATURE CITED

APPENDIX I: Management Recommendations

APPENDIX II: Bibliography

INDEX (omitted from the online edition)

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Library of Congress Cataloging in Publication Data

Steenbergh, Warren F
Ecology of the saguaro.

(National Park Service scientific monograph series: no. 8)
The second part of a two-part work; part 1 was a paper presented at Research in the Parks, a conference held in Philadelphia in 1971.
"The saguaro giant cactus: a bibliography. Warren F. Steenbergh and Lupe P. Hendrickson": p.
Bibliography: p.
Supt. of Docs. no.: I 29.80:8
1. Saguaro. 2. Desert flora—Sonoran Desert.
I. Lowe, Charles H., joint author. II. Title.
III. Series: United States. National Park Service.
Scientific monograph series; no. 8.
QK495.C115S74   581.5'265   74-600189

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1. Geographic distribution of the saguaro (Cereus giganteus) in the Sonoran Desert region (Map).

2. Vicinity map of Tucson and Saguaro National Monument.

3A. Dr. Homer L. Shantz and the Cactus Forest as it appeared in 1930.

3B. View of the site shown in Fig. 3A photographed 38 years later.

4A. Overview of the Cactus Forest, Saguaro National Monument (east).

4B. The die-off of saguaros within the Cactus Forest continues in response to recurring catastrophic freezes.

5A. A dense and vigorous saguaro population occupies the rocky footslopes and upper bajadas of the Tucson Mountains at Saguaro National Monument (west).

5B. January 1971 freeze-killed saguaros on the lower bajada at Saguaro National Monument (west).

6A. A young saguaro population on the eastern footslopes of the Tucson Mountains.

6B. An old saguaro population on the lower bajada at Saguaro National Monument (west).

7A. Senita cactus (Cereus schotti) reaches the absolute northern limits of its distribution in Organ Pipe Cactus National Monument in southwestern Arizona.

7B. The organpipe cactus (Cereus thurberi) occurs north of the U.S.-Mexico border almost entirely within and a few miles north of Organ Pipe Cactus National Monument.

8A. The southernmost saguaros grow on the slopes of Cerro Masiaca, 48 km (30 miles) south of Navojoa, Sonora, Mexico.

8B. Cerro Masiaca and the southernmost saguaro population.

9A. The saguaro occurs with cardon (Cereus pringlei), senita (Cereus schotti), and organpipe (Cereus thurberi) cacti in the vicinity of Puerto Libertad, Sonora, Mexico.

9B. Sparsely scattered saguaros growing with "desert riparian species" at the western base of the Mohawk Mountains in Yuma County, southwestern Arizona.

10A. At the cold-limited northern, eastern and upper elevational extremes of its distribution in Arizona and northern Sonora, the saguaro grows on south-facing slopes in close association with boulders and rock outcrops.

10B. Vertical distribution of saguaros on Tanque Verde Ridge (Rincon Mountains), Saguaro National Monument (east).

11A. Saguaros growing on a south-facing rocky slope 72 km (45 miles) west of Tucson, Arizona.

11B. Saguaro population west of Quijotoa (elevation 794 m; 2600 ft) in south-central Pima County, Arizona.

12A. An isolated population of saguaros growing on a south-facing rocky slope near Ruby, Santa Cruz County, Arizona.

12B. Disjunct saguaro population at Cerro Masiaca in southern Sonora, the southern limit of the saguaro's range.

13A. Approximately 100 ripening fruits on a 4.5 m (14.8 ft) saguaro in mid-June.

13B. Reproductive growth of a "dead" saguaro completely severed from its roots by lightning in early September 1969.

14A. The effect of freeze-caused injury on saguaro fruit development at Saguaro National Monument (east).

14B. Reproductive growth of a senescent saguaro.

15A. Reproductive growth of a saguaro begins when the plant is approximately 2.2 m (7.2 ft) tall.

15B. Five small arms produced 113 of the total annual crop of 182 fruits on this 5 m (16.4 ft) saguaro.

16. Heights and percentage of saguaros with reproductive structures at Saguaro National Monument (west).

17A. Ripe saguaro fruit; seed mass drying in open receptacle.

17B. Seeds from a single saguaro fruit.

18A. Accumulation of ripe fruits beneath mature saguaros in flat habitat at the west Monument.

18B. Absence of fruit at the base of mature saguaro in flat habitat at the east Monument.

19A. Empty receptacles of saguaro fruits surround the entrance to the den of a round-tailed ground squirrel (Spermophilus tereticaudis).

19B. Exclosure used in experiments to determine the effect of vertebrate animals on the survival of saguaro seeds and young plants.

20A. Harvester ant (Pogonomyrmex barbatus) nest in the Cactus Forest at Saguaro National Monument (east).

20B. Harvester ants (Pogonomyrmex barbatus) at the nest entrance.

21. Natural field germination of saguaro seeds and accompanying precipitation and soil moisture.

22. Hygroscopic moisture uptake by saguaro seeds at high relative humidities.

23. Effect of pre-exposure to high relative humidity on the rate of saguaro seed germination after contact with free water.

24A. A newly sprouted saguaro seedling in the Cactus Forest at Saguaro National Monument (east).

24B. Yearling saguaro, Cactus Forest, Saguaro National Monument (east).

25A. Juvenile saguaro, globose stage, height approximately 2.5 cm (1 inch), in rock outcrop.

25B. Juvenile saguaro, globose stage, height approximately 2.5 cm (1 inch) in nonrocky habitat.

26A. Juvenile saguaro, columnar stage, ht 50 cm (20 inch), in rocky habitat.

26B. Juvenile saguaro columnar stage, ht 50 cm (20 inch), in a low shrub (triangle bursage [Ambrosia deltoidea]).

27A. Juvenile saguaro severely damaged by white-throated woodrat (Neotoma albigula) and subsequent freezing.

27B. Juvenile saguaro on the Cabeza Prieta Game Range (west of Organ Pipe Cactus National Monument).

28. Post-germination, first-year survivorship of saguaro seedlings germinated July 1967 at Saguaro National Monument.

29. First-year, post-germination survivorship in open and protected field plots of saguaro seedlings germinated July 1968 at Saguaro National Monument (west).

30. First-year, post-germination survivorship in open and protected plots of saguaro seedlings germinated July 1968 at Saguaro National Monument (east).

31A. Sub-marginal habitat for saguaros—a north-facing slope over-looking the Cactus Forest at Saguaro National Monument (east).

31B. South-facing slope of the ridge shown in Fig. 31A.

32A. Saguaro at the upper limits of the elevational distribution of the species (approximately 1524 m; 5000 ft) in Saguaro National Monument (east).

32B. Freeze-killed 1.2 m (3.9 ft) juvenile saguaro on northeast-facing slope at Saguaro National Monument (east).

33A. A forest of saguaros growing 53 km (33 miles) west of Sonoyta in northwestern Sonora, Mexico.

33B. A singularly outstanding example of the association of young saguaros with "nurse-trees" in the Altar Valley, northern Sonora, Mexico.

34A. The saguaro is co-dominant with foothill paloverde (Cercidium microphyllum) on this rocky southwest-facing slope in the Tucson Mountains, Saguaro National Monument.

34B. Creosotebush community in the Avra Valley northwest of Tucson, Arizona.

35A. Young saguaro established in a fractured basalt boulder at Cerro Masiaca in extreme southern Sonora.

35B. Coastal thornscrub vegetation on the plains of northern Sinaloa.

36. Net first-year survivorship (from seeds) for 497 saguaro seedlings at Saguaro National Monument

37A. Height gauge used to measure apical growth of juvenile saguaros.

37B. Height gauge used to measure apical growth of adult saguaros.

38A. Year-to-year variation in length of saguaro spines appear as distinctive bands encircling the plant stem.

38B. Saguaros in a permanent plot established in 1941 at Alamo Canyon, Organ Pipe Cactus National Monument, Arizona.

39. Regression on semi-logarithmic coordinates of stem height on age of young saguaros at Saguaro National Monument (east).

40. Regression on logarithmic coordinates of apical growth on stem height of young saguaros at Saguaro National Monument (east).

41. Regression on semi-logarithmic coordinates of stem height on age for healthy young saguaros at Saguaro National Monument (east)

42. Effect of freezing on apical growth of 17 young saguaros at Saguaro National Monument (east).

43. Apical growth on stem height (1941) of 93 saguaros at Organ Pipe Cactus National Monument, Arizona.

44. Regression on logarithmic coordinates of 26-year apical growth on 1941 stem height for 93 saguaros at Organ Pipe Cactus National Monument, Arizona.

45. Regression on logarithmic coordinates of 26-year apical growth on height for young saguaros at Saguaro National Monument (east) and Organ Pipe Cactus National Monument.

46A. Stump of mesquite (Prosopis juliflora) in the Cactus Forest at Saguaro National Monument (east).

46B. Mesquite tree (Prosopis juliflora) resprouted from cut stump in the Cactus Forest, Saguaro National Monument (east).

47A. Actively grazed paloverde-saguaro (Cercidium microphyllum-Cereus giganteus) community adjacent to Saguaro National Monument (east).

47B. Paloverde-saguaro (Cercidium microphyllum-Cereus giganteus) community after 11 years' protection from cattle grazing.

48A. Closely cropped desert zinnia (Zinnia pumila) and desert fluff-grass (Tridens pulchellus) at the actively grazed site shown in Fig. 47A.

48B. Natural regeneration of desert zinnia (Zinnia pumila) and fluff-grass (Tridens pulchellus) at Saguaro National Monument (east).

49A. Intensively grazed rocky footslope of the Rincon Mountains, Saguaro National Monument (east).

49B. Natural regeneration of vegetation on rocky footslopes following exclusion of cattle in Saguaro National Monument.

50A. Area denuded by grazing and shade-seeking cattle at Organ Pipe Cactus National Monument.

50B. Seven juvenile saguaros growing among the cast limbs beneath foothill paloverde (Cercidium microphyllum) a short distance from the site shown in Fig. 50A.

51A. Fire-killed saguaros and foothill paloverde trees (Cercidium microphyllum) in Tonto National Forest northeast of Phoenix, Arizona.

51B. A fire-scarred saguaro, on the slopes of the Santa Catalina Mountains, Coronado National Forest northeast of Tucson, Arizona.

52A. Dead, roadside saguaro, Tucson Mountains, Saguaro National Monument (west).

52B. Closeup showing saguaro roots, severed by roadgrader, of saguaro shown in Fig. 52A.

53A. Concentrated visitor use—as in this picnic area—hastens the death of living saguaros and precludes the establishment of new individuals.

53B. Vandalized young saguaro adjacent to picnic area shown in Fig. 53A.

54A. Constricted upper stem of this young saguaro at Saguaro National Monument (east) is a result of a January 1971 freeze-caused crown-kill.

54B. Constrictions near the base of the stem of saguaros provide a record of previous catastrophic freezes.

55A. Recent and historic evidence of a saguaro population in trouble.

55B. Road-associated 1971 freeze-caused saguaro deaths, Saguaro National Monument (west).

56A. Jumping cholla (chain-fruit cholla, Opuntia fulgida) photographed immediately following critical subfreezing temperatures in January 1971.

56B. Rephotograph of the cholla shown in Fig. 56A taken approximately 3 years after lethal freeze-caused injury.

57A. Freeze-caused injury (1971) to organpipe cactus (Cereus thurberi) at Organ Pipe Cactus National Monument

57B. Freeze-caused injury (1971) to senita cactus (Cereus schotti) at Organ Pipe Cactus National Monument.

58A. Freeze-killed (1971) barrel cactus (Ferocactus wislizeni) at Saguaro National Monument (east).

58B. Freeze-killed (1971) foothill paloverde (Cercidium microphyllum) at Saguaro National Monument (east).

1. Locality, elevation, topographic association, and slope exposure of four habitats sampled to obtain saguaro flowering phenology data.

2. Effect of freezing on fruit production of two unbranched saguaros at Saguaro National Monument (east).

3. Number of fruits maturing on each of four unbranched saguaros at Saguaro National Monument (east).

4. Reproductive status of young saguaros at Saguaro National Monument (west).

5. Size, weight, and seed content of ripe saguaro fruits collected at Saguaro National Monument (west).

6. Viable saguaro seed content of native mammal feces.

7. Viable saguaro seed content of native bird feces.

8. The effect of predation on saguaro seed germination success in protected and exposed experimental plots.

9. Summary of natural germination in experimental plots from broadcast saguaro seeds, 1965-68.

10. Percent soil moisture in shaded and unshaded saguaro-paloverde habitat at Saguaro National Monument (east), July 1967.

11. Hygroscopic imbibition by saguaro seeds in the laboratory at high relative humidities.

12. Number and percent germination of humidified (pretreated) and of air-dry (untreated) saguaro seeds in the laboratory.

13. Consumption of saguaro seedlings by species of rodents occurring commonly in saguaro habitats at Saguaro National Monument.

14. Native rodents and lagomorphs occurring in saguaro habitats in the Mohave Desert, Sonoran Desert, and in Thornscrub in southwestern Sonora, Mexico.

15. Insect consumers of young saguaro cacti, collected in saguaro habitats at Saguaro National Monument (east).

16. First-year survivorship of 231 saguaro seedlings naturally germinated July 1967 in seven experimental plots at Saguaro National Monument.

17. Regression equations for post-germination survival of saguaro seed lings germinated July 1967.

18. Post-germination survival and establishment rates of saguaro seedlings (N = 231) naturally germinated July 1967 in experimental plots at Saguaro National Monument.

19. First-year survivorship in protected (exclosure) plots of 1018 saguaro seedlings naturally germinated July 1968 from seeds broadcast within five experimental plots.

20. First-year survivorship in unprotected (open) plots of 250 saguaro seedlings naturally germinated July 1968 from seeds broadcast within five experimental plots.

21. Comparison of post-germination survival and establishment rates of saguaro seedlings (N = 1268) in paired protected (exclosure) and unprotected (open) experimental plots for seedlings naturally germinated July 1968.

22. Regression equations for post-germination survival of saguaro seed lings germinated July 1968.

23. Statistical comparison of soil characteristics of the two major climax vegetation types of the Sonoran Desert in Arizona (from Yang and Lowe 1956).

24. Terms for the Saguaro Seasons (Southern Arizona).

25. Summary of natural germination from broadcast saguaro seeds, Saguaro National Monument.

26. Cumulative first-year natural mortality and net survivorship of saguaro seedlings at Saguaro National Monument germinated from seeds broadcast during July 1965, 1966, and 1967.

27. Regression equations for net first-year survivorship (from seeds) for saguaro seedlings during three consecutive years at Saguaro National Monument.

28. Stem height-age relationship and subsequent 1-year apical growth of healthy young saguaros in flat (nonrocky) habitat at Saguaro National Monument (east).

29. Effect of flower bud removal on apical growth of unbranched saguaros at Saguaro National Monument (east).

30. Effect of freezing on apical growth of young saguaros.

31. Regression equations for 1-year apical growth on stem height for young saguaros (N = 17) at Saguaro National Monument (east).

32. Six-year on-site record of summer precipitation at Saguaro National Monument.

33. Summary of climatic parameters for Tucson, Ajo, and Yuma Citrus Station, Arizona.

34. Saguaro stem height and 26-year apical growth at Organ Pipe Cactus National Monument, Arizona.

35. Height measurements and subsequent 1-year apical growth increments for 14 young saguaros at Tempe, Arizona.

36. Original height measurements and subsequent 1-year apical growth increments for eight saguaros on Tumamoc Hill, Tucson, Arizona.

37. Height, estimated age, and 1-year apical growth for saguaros on Tumamoc Hill, Tucson, Arizona.

38. Mean annual apical growth of saguaros in USDA sample at Saguaro National Monument 1951-59.