Saguaro, giant cactus of the Sonoran Desert

A bobcat (Lynx rufus) seems to stare in wonder and fascination at a middle-aged saguaro.
We know many of the answers to your questions, Mr. Cat. Photo by David Dettmann


The saguaro is a weird surprise to the newcomer and a cherished symbol of home to the Arizonan. Who can look at Tumamoc Hill without being impressed by the abundance and beauty of her iconic giant cacti? Indeed, Tumamoc's first scientists were impressed and got to work on them almost immediately.

We have never quit. Most of what the world knows about saguaros (Carnegia gigantea) comes from our research, and research done on our cherished daughter, Saguaro National Park.

William Cannon, The Desert Botanical Laboratory's first resident director, began the tradition. He decided to dig up some saguaros to study their roots. He discovered that they have remarkably shallow, widespread root systems (1909, The root system of Cereus giganteus, pp 59-66 in Spalding, Distribution and movements of desert plants). Shallow roots that grow over a large area allow saguaros to grab the water that falls in torrents during thunderstorms, and then runs off quickly without penetrating deeply. Other plant species, such as palo verde & mesquite, send their roots deep into the ground, tapping into water that originates from gentler rains.

At the same time, Effie Spalding studied saguaro ribs. She wanted to know what they are for. Her idea was that the ribs are like pleats, allowing the cactus to expand as they fill up with water and then contract as they use that water up and their waist shrinks. So she measured the response of saguaro ribs to rainfall, and she was right. Mrs. Spalding published her results in a paper that is one of the earliest works to be done on the Hill. (1905. Mechanical adjustment of the sahuaro to varying quantities of stored water. Bull. Torrey Botanical Club, 32: 57-68).

Effie Spalding's saguaro-rib investigation was fascinating but not as important as her work to measure the height and growth of sixteen saguaros for as long as four years (1905-1909). She used precise repeat photography to accomplish it. The data she gathered allowed Volney Spalding's protege, Forrest Shreve to figure out the speed of saguaro growth, the height of saguaros at different ages, and the age at which saguaros branch (1910. The rate of establishment of the giant cactus. The Plant World 13: 235-240)

Mrs. Spalding, calipers in hand, measuring the width of a saguaro rib.

 HeightAge (years)
 10cm  8

To estimate the growth of the larger saguaros, Shreve also used repeat photography. He used cactus landmarks, prominent distinctive features of the ribs of a few saguaros that could be seen in earlier photos. The larger saguaros grow about 1.5 cm per year. (Other sites around Tucson show rates only half as large.)

Shreve's results (Shreve, 1910 — to the left) will help you to guess the age of the saguaros that you see on the Hill.

Saguaros at Saguaro National Park East and Tumamoc generally began producing branches reliably at a height of 3.75m, and in all other study plots between 4 and 5m (Pierson,Turner,Betancourt,2013).

A new branch emerges

Of course, saguaros do not stop growing at 5m (17 feet). They often reach 10m (33 feet) and some extraordinary giants grow to 15m (50 feet). And saguaros typically live for 150 years.

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Try as they might, Shreve and all the other scientists could not find a single saguaro less than 10 cm (four inches) tall on the Hill. Meanwhile, Shreve was surveying 175 saguaros and their heights off the Hill and reconstructing their age structure. His findings were ominous.

Forrest Shreve (1910) censuses saguaros by age group. They were in steady decline.

Shreve found 20 individuals in his sample that were 60 years old and 14 that were about 45 years old. During the 60-year period from 1850 on, successful saguaro reproduction (called recruitment by population biologists) fell steadily. In the ten years from 1885 to 1895, only two saguaros got established, and from 1900 to 1910, none did. Shreve became quite concerned. It appeared that the spiny wonder was becoming extinct — at least in the vicinity of Tucson. He published his work and his concerns in The Plant World (1910). It actually led to the establishment of Saguaro National Park (link). But it turns out that the Tumamoc Hill population of saguaros was never in trouble at all. Here's the story.

In 1908, Effie's husband, Volney, took on the job of mapping every individual saguaro on the reservation. But there were too many saguaros and not enough scientists to keep up the research. After only a year, the saguaro plots were largely abandoned.

Saguaros being measured in one of the subplots in 2011


Then, in 1964, J. R. Hastings & Ray Turner revived the study by setting out four large subplots. These plots are still monitored. After 85 years, they gave us a real look at what is going on (Pierson & Turner. 1998. An 85-year study of saguaro (Carnegiea gigantea) demography. Ecology 79:2676-2693). It is remarkable.

Times of saguaro recruitment are clustered together in episodes. On the order of once or twice a century, a local population has a boom period. Their land overflows with young saguaros, all about the same age. This is called a cohort. But in between these episodes, saguaros experience extended periods of failure.

The figure above shows saguaro recruitment in the four Tumamoc Hill subplots from 1850 to 1993. There is a sharp peak in recruitment in the East Plot from 1940 through 1970; there is one in the West Plot from 1920 through 1960; there is one in the North Plot from 1910 through 1970; and there is one in the South Plot from 1940 through 1970.

After germinating, the members of the cohort grow; they flower; they even set their seeds. But it largely comes to naught. The cohort dwindles as more and more individuals die without replacement. It looks like curtains for them. And then the magic happens again. We are not sure why. We do know that the answer is not rainfall. Good rains come at far more than 75 or 100 year intervals! But not good saguaro recruitment years. Over a century and we have not done enough research!

The same pattern shows up in the land set aside for Saguaro National Park, but the times are different. Look above at the repeat photographs taken in the eastern part of the park, the part that Homer Shantz selected in 1928 because of its abundant supply of vigorous saguaros. From 1935 to 2010, saguaros dwindled almost to the vanishing point. Meanwhile the Tumamoc population was burgeoning. It does seem fitting that such a majestic creature should be characterized by a stately rhythm of reproduction.

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Long-nosed bat arriving for some nectar.

In the Tucson area, saguaros flower in May-June. Each flower lasts but a single day. It opens at night and remains open for part of the next day. It cannot pollinate itself, so it must get pollen from another individual. Pollen transfer is accomplished by long-nosed bats and white-winged doves and bees which visit the flowers to obtain nectar. In the Sonoran Desert, the two vertebrate pollinators migrate to our area in synchrony with the sagauro flowering season. Other species such as gilded flickers also help to pollinate the flowers.

Each saguaro fruit produces about 2000 tiny black seeds per year. But most years, most seeds are destroyed by granivores. Packrats, doves, quail, etc. Even if a seed survives the packrats, it must get lucky to keep going. It has to be in a good spot to germinate. And it has the best chance of survival if that spot happens to be in the shade of a nurse plant like a foothills palo verde. Oh yes, and it must again escape the packrats and other small mammals which tend to make a salad out of it while it is small. Over a very, very long period time (decades) each average saguaro adult has only one successful baby.

Bahidaj, ripe saguaro fruit

The bright red fruit of saguaro is also a target of consumers. Some species, such as coyotes and cactus wrens, eat the fruit but help to disperse the seeds as they pass through unharmed. But dove and quail guts destroy the seeds. Consult the magisterial works of Steenbergh & Lowe for much more information (1977. Ecology of the saguaro. II. Reproduction, germination, establishment, growth, and survival of the young plant. Scientific Monograph Series 8 and 1983. Ecology of the saguaro. III. Growth and demography. Scientific Monograph Series 17 National Park Service).

Gathering bahidaj

People are a minor but culturally significant source of seed mortality. We consume the fruit of saguaro because its pulp is sweet. The Tohono O'odham harvest the fruit (called bahidaj) and families process it into a syrupy food. Bahidaj matures (turns red) in the hot season just before the rains begin and must be harvested right away because rain spoils it. The families each give about a tithe to the community as whole. Those who receive it, ferment it for four or five days. Just before the monsoon begins, the result is used as a libation in communal prayers for rain (Thackery & Leding. 1929. The giant cactus of Arizona: The use of its fruits and other cactus fruits by the Indians. Journal of Heredity 20:400-414).

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Adult saguaro die mostly of exposure to frost (Steenbergh & Lowe. 1976. Ecology of the saguaro. I. The role of freezing weather on a warm-desert plant population, pp 49-92 in Research in the National Parks). Not brief periods of frost, but frosts that last a day or two. One rule of thumb: saguaro cannot even establish a population where there are days in which temps fall to freezing and remain there all day long. Frost kills because ice crystals are hard and sharp and they cut up saguaro tissues from the inside.

The problem of frost explains why, in our area, saguaro grows on slopes. Slopes shed water better. Too much water and there could be too much ice formation. It also explains why saguaro grows to higher elevations on slopes that face south — they are warmer. Saguaro can be found well up into the hills, even beyond 5000 feet elevation if it is on a warm, south facing slope. Yet many species cannot withstand ANY frost. In places where there is none, saguaro fails because it is outgrown by species that are super-sensitive to frost.

So saguaro needs to solve a goldilocks problem to sustain a healthy population: just the right amount of frost — some but not too much! Saguaros find the right conditions in the geographical area indicated on the map — and nowhere else.

  Map of the Southwest showing the extent of saguaro distribution (shaded area).

Besides frost, other quite significant sources of adult saguaro mortality are lightning (Steenbergh, 1972. Lightning-caused destruction in a desert plant community. Southwestern Naturalist. 16:419-429) and windthrow.

You can easily tell when lightning has killed a saguaro. The main body of the cactus is fried to its skeleton but its green arms simply fall off and remain green for a long while before they die. (They do not take root however.) Look for examples on the Hill.

Windthrow of a saguaro happens when a good soaking rain is followed by unusually high winds. The soft, water-soaked soil simply cannot hold the tall, heavy, shallow rooted cactus. Down it goes.

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Saguaro Spines as Records of Past Climate

If you are very clever working with isotopes, you can tell past climate conditions by testing saguaro spines. They take carbon from the atmosphere and grow out of the tip of a saguaro stem or arm. Most of their carbon is ordinary carbon-12, but some is the rare, radioactive carbon-14 isotope. Only one carbon-14 atom exists for every 1,000,000,000,000 carbon-12 atoms. Carbon-14 is the isotope that has given us the ability to date material in such disparate fields as archaeology and palaeoclimatology.

  Actively growing tip of a saguaro. Photo by David Dettman.
Velvety white circular shapes are new areoles with their developing spines.

The growth cells forming new spines die in a few days. But they keep the carbon they used while building the spine. The spine, itself, containing those cells, lasts for many decades.

The carbon isotopes available for spine growth vary regularly during the year and between years, too. When growing conditions are relatively good, a smaller proportion of the new tissue of a spine is the carbon-14 isotope. Meanwhile, as the stem lengthens year after year, the spine holds its postion and new spines grow at the tip. Thus the farther away a spine is from the tip, the older it is. And by looking at the proportion of carbon-14 in the parade of spines along a saguaro rib from bottom to top, we get a sequential record of growing conditions encountered by the cactus during its lifetime (English, Dettman, Sandquist & Williams. 2007. Past climate changes and ecophysiological responses recorded in the isotope ratios of saguaro cactus spines. Oecologia 154:247-258. DOI 10.1007/s00442-007-0832-x).

  Williams carefully harvests spines for isotope analysis. Photo by David Dettman.

It's a breakthrough. As big as they are, giant cacti have no growth rings, and the desert lacks the trees that could reveal past desert climate. But now, although they have no growth rings, saguaros will be able to do the job.

The Saguaro Genome

Plants, just like animals, have genes and genomes. Only in recent years have we had the technology to read those genomes.

As befits the world's oldest desert research lab, one of its veterans, a century-old Tumamoc saguaro that grows along one of the paths near the old laboratory building, was the very first cactus to have its genome deciphered. It was biopsied in 2013.

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We don't know what other secrets are waiting for our persistent efforts to uncover them. Tumamoc and its saguaro story are an object lesson in the importance of long term research. Very long term research. The Spaldings would be proud to know it. So would Forrest Shreve.

So Tumamoc keeps up its saguaro research. The plots that Hastings and Turner trimmed out of the old Spalding effort were very recently restudied by Ray Turner, Bob Webb and Susana Rodriguez. They used modern methods of global positioning and digitization, which improve our ability to analyze and compare individual plants and different years.

Tumamoc is the National Historic Landmark that is still making history.

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