Ecology of the Saguaro: II
NPS Scientific Monograph No. 8
Observe constantly that all things take place by
change, and accustom thyself to consider that the nature of the Universe
loves nothing so much as to change the things which are, and to make new
things like them.Marcus Aurelius Antoninus, "Meditations,"
Ours is a constantly changing environment and we most
easily relate to and understand those responses that are immediate and
that occur over a short span of time. As the time span of those changes
increases it becomes increasingly difficult for us to relate to them,
more so to understand them. We have particular difficulty understanding
and accepting the observed effects of causal events that long pre-date
our association with such results.
So it is with the saguaro, that we regard as a
"problem" the dramatic fluctuations in some populations that we have
witnessed during our brief temporal contact with the continuing process
of evolution. With a maximum life span nearly twice our own, living
saguaros have survived climatic events that long pre-date our
recollection or knowledge. The species has survived through continuing
environmental changes and has evolved during millions of years in
environments that we have not experienced, and can but vaguely
comprehend. However, examination of the question of what has happened,
what is happening, and what will happen to the saguaroat Saguaro
National Monument and elsewhere throughout the range of its distribution
in the Sonoran Desertrequires that perspective (Fig. 1). It is
within the framework and perspective of the evolutionary process that we
explore the ecology of the saguaro.1
1Investigations on the saguaro giant
cactus (Cereus giganteus Engelm., Carnegiea gigantea
[Engelm.] Britt. and Rose) reported here and elsewhere were
independently initiated in 1951 by co-author C. H. Lowe. Subsequently,
the work was continued with the support of the National Park Service
(and others) in recognition of the need for basic knowledge of the
biology of the species to provide information essential for ecologically
sound management of the natural resources of Saguaro National Monument
and other National Park System areas in southern Arizona. Vernacular and
scientific names now most commonly used are saguaro and sahuaro, and
Cereus giganteus or Carnegiea gigantea.
Fig. 1. Geographic distribution of the
saguaro (Cereus giganteus) in the Sonoran Desert region.
Triangles indicate saguaro observations. In addition to our own
observations, these include distributions reported by Benson (1940,
1950, 1969), Shreve (1951), Soule and Lowe (1970), and Hastings, et al.
(1972). Boundary of the Sonoran Desert from Shreve (1951). Some major
islands are not shown. (click on image for an enlargement in a new
The National Park Service long has been concerned
with the nature and cause of dramatic fluctuations that have altered
grossly the structure of saguaro populations in certain portions of
Saguaro National Monument (Fig. 2).2 In the
Cactus Forest area of the original Saguaro National Monument, the once
spectacularly dense concentration of giant old saguaros has dwindled in
a few decades to an unimpressive population of sparsely scattered and
dying old individuals (Fig. 3).
2Saguaro National Monument is comprised of
two separate units situated on opposite sides of the Tucson basin. The
original, Rincon Mountain Section (east unit), is located approximately
15 miles (24.2 km) east of the Tucson city center. The more recently
established Tucson Mountain Section (west unit) is located about 15
miles (24.2 km) northwest of Tucson. Shelton (1972) provided a general
description and account of the natural and human history of the Saguaro
Fig. 2. Vicinity map of Tucson and
Saguaro National Monument. (click on image for an enlargement in a
More important to the present and future condition of
this population, however, is the relative sparsity of younger plants.
That conditionthe lack of sufficient younger saguaros to replace
dead and dying older saguarosinsures absolutely that the number
of large saguaros in this population will continue to decline.
The problem long pre-dates the establishment of the
original Saguaro National Monument in 1933. The predominance of older
saguaros and the lack of young individuals in photographs taken more
than 40 years ago clearly reveal a population already many years in
trouble (Fig. 3A). To a greater or lesser degree, other saguaro
populations within Saguaro National Monument and elsewhere have
undergone similar recent fluctuations (Figs. 4-6).
Fig. 3A. Dr. Homer L. Shantz and the
Cactus Forest as it appeared in 1930, immediately prior to establishment
of the original Saguaro National Monument. The superlative quality of
this standthe abundance of large, old saguarostogether with
the near absence of smaller, young saguaros clearly indicates a
population already in trouble. Homer L. Shantz photograph collection,
University of Arizona Herbarium. Photographed 22 Feb. 1930.
Fig. 3B. Dramatic changes are evident in
this view of the site shown in Fig. 3A, photographed 38 years later. In
less than 4 decades, the forst of giant saguaros has been reduced to a
few sparsely scattered individuals. No less significant is the death of
all the chain-fruit cholla (Opuntia fulgida) present in the 1930
photograph. Photographed 19 Feb. 1968.
Fig. 4A. Overview of the Cactus Forest,
Saguaro National Monument (east). The Castus Forest is situated in a
gently sloping north-draining basin between the north-facing slopes of
the Rincon Mountains (behind) and the south-facing slopes of the Santa
Catalina Mountains on the horizon. Photographed 21 Feb. 1969.
Fig. 4B. Within the Cactus Forest, the
die-off of saguaros continues in response to recurring catastrophic
freezes. The abnormally slender stems and contorted form of moribund
saguaros in the background, and the oozing black rot of the saguaro in
the foreground are typical stages in the delayed collapse of
freeze-damaged saguaros. As many as 9 years can elapse between lethal
injury and the final collapse of a freeze-damaged saguaro. Photographed
28 May 1969.
Fig. 5A. At Saguaro National Monument
(west), a dense and vigorous saguaro population occupies the rocky
footslopes and upper bajadas of the Tuscon Mountains. Rock outcrops and
the southwestern slope exposure mitigate extremes of winter cold.
Photographed 28 April 1974.
Fig. 5B. January 1971 freeze-killed
saguaros on the lower bajada at Saguaro National Monument (west). Here,
in the Cactus Forest of Saguaro National Monument (east) and in other
topographically similar situations, cold air drainage contributes to the
severity of recurring catastrophic freezes. Photographed 21 Nov.
Fig. 6A. A young saguaro population on
the eastern footslopes of the Tucson Mountains. A total of 48 juvenile
and unbranched young adult saguaros is visible within the photograph.
There are no living individuals of the parent generation on this
southwest-facing slope. Photographed 2 April 1970.
Fig. 6B. An old saguaro population on
the4 lower bajada at Saguaro National Monument (west). Compare with the
nearby young population shown in Fig. 6A. Fluctuation in density and
age-structure is characteristic of saguaro populations in the
cold-limiting parts of the range of this subtropical species.
Photographed 16 April 1971.
In recognition of the needs of the National Park
Service, our continuing investigations on the ecology of the saguaro are
designed to provide definitive knowledge of the problem and to develop
information essential for interpretive and resource management programs.
Those portions of our investigations we report here concern the second
aspect of the problemthe establishment, growth, and survival of
the young saguaro. Our experimental designs and the hypotheses that they
test are directed specifically toward obtaining information on the
operation of natural selection through climatic and other physical and
biotic factors that are critically limiting on saguaro populations.
More amazing perhaps than any aspect of its biology
is man's emotional involvement with the saguarothe saguaro is a
"hero" among plants. He has endowed it with human attributes and
bestowed upon it affection and concern for its "problems." Moreover, he
has embraced myths, half-truths, and even eulogies generated by
reporters and feature writers concerned more with the production of
sensational and emotionally appealing "doomsday" stories than with the
effective communication of accurate information. Cast in the role of a
"dying hero," the saguaro has been accorded affection and credited with
traits of character that have seriously beclouded general understanding
of the true nature of its biology.
The truth in its great complexity lacks such direct
emotional appeal. In place of such appeal, the truth offers a far more
satisfying and significant understanding of a fascinating scheme for
survivalthe continuing process of evolution through millennia of
natural selection in a rigorous and ever-changing environment. It is for
those who find satisfaction in such understanding that we offer the
results of these investigations.
While the concerns of the National Park Service are
necessarily provincial, the answers to the question of the fate of
saguaros in Saguaro National Monument transcend political boundaries to
include the species population and the environmental factors that
control the limits of its distribution. Thus we have examined and report
here not simply upon our specific observations and experiments conducted
within Saguaro National Monument and at the University of Arizona, but
also upon our investigations and those of others made elsewhere
throughout the range of the saguaro.
The question of what has happened, what is happening,
and what will happen to saguaros at Saguaro National Monument is a
long-standing one of primary concern to the National Park Service.
Further, the answers to that question directly relate to the condition
and management of saguaro populations at Tonto National Monument and
Organ Pipe Cactus National Monument (Fig. 1).
The applicability of these findings likewise is not
limited by political boundaries; neither is the operation of factors
that control the fate of the saguaro in the northern portions of its
distribution in Arizona and northern Sonora limited in its controlling
effects only to the saguaro. To a greater or lesser degree, the same
factors are involved in controlling the northern limits of distribution
and population dynamics of a large number of the tropically derived
Sonoran Desert species in Arizona and Sonora. Particularly, many similar
relationships occur in similarly evolved species of giant columnar cacti
that reach the northern limits of their distributions either in Sonora
or in adjacent southern Arizona: organpipe, senita, cardon, and
hecho3 (Figs. 7-9).
3Organpipe or pitahaya (Cereus
thurberi, Lemaireocereus thurberi); senita or old-man cactus
(Cereus schotti, Lophocereus schotti); cardon (Cereus
pringlei, Pachycereus pringlei); hecho (Cereus pecten-aboriginum,
Pachycereus pecten aboriginum). See Benson (1940, 1950, 1969) and
review by Felger (1970); Shreve and Wiggins (1964).
Fig. 7A. The cold-intolerant senita
cactus (Cereus schotti) reaches the absolute northern limits of
its distribution in Organ Pipe Cactus National Monument in southwestern
Arizona. Truncated and prostrate stems are the result of freeze-caused
injuries. Photographed 7 May 1971.
Fig. 7B. The organpipe cactus (Cereus
thurberi), only slightly more cold-tolerant than the senita cactus
(Cereus schotti), occurs north of the United States-Mexico border
almost entirely within and a few miles north of Organ Piple Cactus
National Monument. Nearly every large individual in this northernmost
part of its range bears a series of scars from recurring freezes.
Photographed 12 July 1974.
Fig. 8A. The southernmost saguaros grow
on the slopes of Cerro Masiaca, 48 km (30 miles) south of Navojoa,
Sonora, Mexcio. On the slopes, it occurs with the closely related
organpipe cactus (Cereus thurberi), left foreground) and hecho
(Cereus pecten-aboriginum, right foreground). Photographed 7
Fig. 8B. Cerro Masiaca and the
southernmost saguaro population. The saguaros occur among basalt
boulders and only on the south-facing slope of the hillthey do not
occur on the adjoining plain. Subtropical deciduous thorn-forest, as
seen in Fig. 8A, blankets the plains of southern Sonora and continues
into northern Sinaloa on the distant horizon. Photographed 27 Dec.
Fig. 9A. The saguaro (left foreground
and right background, arrow A) occurs with cardon (Cereus
pringlei, arrow B), senita (Cereus schotti, arrow C), and
organpipe (Cereus thurberi, arrow D) cacti in the vicinity of
Puerto Libertad, and elsewhere along the Sonora coast of the Gulf of
California. Teddybear cholla (Opuntia bigelovi) and ocotillo
(Fouquieria splendens) are conspicuous in right foreground. Most
of the associated plant speciesbut not the saguaroalso occur
in Baja California, on the west side of the gulf. Photographed 19 April
Fig. 9B. Sparsely scattered saguaros
grow with "desert riparian species," desert ironwood (Olneya
tesota) and foothill paloverde (Cercidium microphyllum), at
the western base of the Mohawk Mountains in Yuma County, southwestern
Arizona. Along the moisture-limited westernmost boundaries of
its distribution in the United States, the saguaro is associated with
moisture-concentrating drainage channels. The nonriparian desert
species conspicuous in the foreground are creosotebush (Larrea
divaricata) and white bursage (Ambrosia dumosa).
Photographed 13 April 1968.
Fig. 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.|
The northernmost saguaros (shown) occur at an elevation of approximately
1524 m (5000 ft) in Cottonwood Canyon approximately 3.2 km (2 miles)
northwest of Hualpai Peak, Mojave County, Arizona. All of the 11 adult
saguaros at this site grow against the base of the vertical south-facing
cliff; 10 saguaros are visible in the photograph. Associated plant
species on the rocky slope include Canotia holacantha
(crucifixion thorn), Eriogonum fasciculatum (shrubby buckwheat),
Larrea divaricata (creosotebush), Fouquieria splendens
(ocotillo), Opuntia acanthocarpa (buckthorn cholla), Opuntia
basilaris (beavertail cactus), Yucca baccata (banana yucca),
Aplopappus laricifloius (turpentinebush), and Gutierrezia
sarothrae (snakeweed). Photographed 4 May 1975.
Fig. 10B. Vertical distribution of
saguaros on Tanque Verde Ridge (Rincon Mountains), Saguaro National
Monument (east). On this northwest exposure, the uppermost saguaros
grow at an elevation of approximately 1350 m (4400 ft). On the southern
slopes of these mountains, a few individuals grow in a rocky
south-facing canyon at approximately 1524 m (5000 ft) elevation.|
The saguaro species boundary line runs through the
monumentthe saguaro population at Saguaro National Monument (east)
is a marginal one in every sense of the wood. Photographed 14 Nov.
Last Updated: 21-Oct-2005